Extrathyroidal conversion of thyroxine to 3,3',5'-triiodothyronine (reverse-T3) and to 3,5,3'-triiodothyronine (T3) in humans

In order to estimate the relative magnitude of the two alternative pathways of monodeiodination of thyroxine (T4) in adult humans, the metabolic clearance rates (MCR) and production rates (PR) of 3,3',5'-triiodothyronine (reverse-T3,rT3) and of 3,5,3'-triiodothyronine (T3) were determined in six euthyroid control subjects (C) and in five hypothyroid patients (H) receiving L-T4 as replacement therapy (0.15-0.3 mg/day). MCR was computed by a non-compartmental method of analysis from the plasma disappearance of 125I rT3 and 131I T3 during 72 h following simultaneous injection of tracers. PR was calculated from MCR and the serum concentration of rT3 and T3, respectively, determined by radioimmunoassay. In the H subjects, rT3 MCR averaged 97.1 +/- 12.8 (SD) 1/day and rT3 PR, 34.3 +/- 12.8 microng/day; T3 MCR was 28.7 +/- 6.1 1/day and T3 PR, 20.3 +/- 6.6 microng/day (all corrected to 70 kg body weight). These results were not significantly different from those in the control group; rT3 MCR 104 +/- 24 1/day, rT3 PR 33.0 +/- 9.2 microng/day; T3 MCR 24.0 +/- 5.9, T3 PR 24.2 +/- 4.1. The proportionof total triiodothyronine (rT3 averaged 62% in H patients and was similar (57%) in the C group. The results obtained in the H subjects indicate that the production of rT3 is a major route of T4 metabolism, equal to or exceeding that of T3. From the close agreement between the mean values for rT3 PR in the C and H groups it is concluded that most, if not all of the rT3 produced in normal humans is derived by extrathyroidal conversion from T4.     

Short-term effects of thyroid hormones on the Na/H antiport in L-6 myoblasts: high molecular specificity for 3,3',5-triiodo-L-thyronine

The thyroid hormones L-T3 and L-T4 were shown to activate the Na/H antiport in L-6 cells from rat skeletal muscle by a rapid, nongenomic mechanism. Under pH equilibrium conditions, a significant rise in the intracellular pH, measured by the fluorescent pH indicator 2',7'-bis-(carboxyethyl)-5(6)-carboxyfluorescein was observed after the addition of physiological concentrations (10(-10) M) of either L-T3 or L-T4, but with different time courses. L-T3 at all concentrations increased the pH after a delay of 2 min, whereas L-T4 showed a concentration-dependent lag time, going from 11 min at 10(-11) M down to 5 min for a hormone concentration of 10(-6) M. The effect of L-T4 was blocked in the presence of the 5'-deiodinase inhibitor 6-n-propyl-2-thiouracil, suggesting that the difference in lag time between L-T3 and L-T4 was due to the 5'-deiodination process that transforms L-T4 into the bioactive L-T3. In short term studies (<5 min), a high molecular specificity for L-T3 was found, as L-T4, rT3, the D-isomer of T3, and the deaminated analogues were ineffective at physiological concentrations. In analogy with the results found at equilibrium, intracellular pH recovery from an acid load and set-point were increased after 2 min for L-T3 (10(-9) M) and after 10 min for L-T4 (10(-9) M). The effect of the hormones on the intracellular pH was completely blocked by the specific antiport inhibitor 5-(ethyl-N-isopropyl)amiloride. These findings suggest that thyroid hormones may play an active role in the recovery from muscular acidosis through direct stimulation of the Na/H antiport.     

Role of protein degradation in the growth of livers after a nutritional shift

Four patients with idiopathic pituitary dwarfism were shown to have growth hormone (GH), adrenocorticotropin (ACTH), and luteinizing hormone (LH) deficiencies. Basal levels of thyrotropin (TSH) were within normal range in three patients and slightly elevated in one. Exaggerated and delayed responses were obtained after TSH-releasing hormone (TRH) stimulation. Serum thyroxine (T4) values were low (2.3 +/- 0.4 mug/100 ml), while triiodothyronine (T3) levels were in the normal range (1.22 +/- 0.25 ng/ml), both rising substantially after exogenous TSH and consecutive TRH administration. Their hypothyroid state was, therefore, probably due to TRH deficiency. To examine the dose of L-T4 necessary to produce inhibition of the TSH response to TRH, 50 mug/m2/day of L-T4 was administered to these patients. At the end of 4 weeks of replacement, serum T4 rose to 5.2 +/- 0.5 mug/100 ml, whereas T3 was unchanged from the previous levels, after which TSH responses to TRH were completely suppressed in all patients. As a control group, six patients with primary hypothyroidism received gradually increasing doses of L-T4 for 4-week periods, and TSH response to TRH was tested at the end of each dosage of L-T4, until complete inhibition of TSH release was obtained. The primary hypothyroid patients required approximately 150 mug/m2/day of L-T4 for suppression of TSH response to TRH. At this dosage, serum T4 and T3 levels were 8.5 +/- 0.9 mug/100 ml and 2.34 +/- 0.5 ng/ml respectively, which were significantly higher than those levels in the pituitary dwarfs (P less than 0.001 for T4 and P less than 0.01 for T3). These observations indicate that the set point of TSH release in feedback inhibition by throxine is low in idiopathic hypopituitarism with TRH deficiency, and TRH seems to control the pituitary sensitivity to feedback regulation of thyroid hormones.     

Changes in thyroid hormone metabolism in exertional heat stroke with or without acute renal failure

The effects of exertional heat stroke (ExHS), with or without acute renal failure (ARF), on thyroid hormone metabolism were investigated. Eighteen ExHS patients were recruited and divided into two groups based on the presence or absence of ARF. Eleven age-matched healthy subjects served as a control group. Serum values of T3, T4, TSH, free T4 (FT4), rT3, and sulfated T3 (T3S) were measured in these groups during the acute and recovery stages of ExHS. Serum T3, T4, and FT4 levels were reduced, with reciprocal increases in rT3 and T3S levels as the severity of ExHS increased. The following mean levels of thyroid hormones were found (controls vs. ExHS without ARF vs. with ARF): T3, 1514 vs. 1164 vs. 393 pmol/L (P < 0.05 each); T4, 97 vs. 79 vs. 49 nmol/L (P = NS and P < 0.05, respectively); FT4, 20.5 vs. 19.5 vs. 19.0 pmol/L (P = NS each); rT3, 371 vs. 617 vs. 805 pmol/L (P < 0.05 and P = NS, respectively); and T3S, 30.1 vs. 34.2 vs. 71.1 pmol/L (P = NS and P < 0.05, respectively). The serum TSH levels were not significantly different among the three groups. Significantly negative correlations were found between serum creatinine and T3 (r = -0.75; P < 0.001) and T4 levels (r = -0.65; P < 0.001), whereas no relationship was noted between serum creatinine and rT3 values (r = 0.11; P < 0.05). In contrast, a correlation was observed between serum glutamic pyruvic transaminase and rT3 (r = 0.45; P < 0.01). Thyroid function tests returned to normal after patients recovered. In conclusion, our results show that patients suffering from ExHS, with or without ARF, displayed altered serum thyroid function in proportion to the severity of their condition. No significant changes in serum levels of rT3 were observed between the two groups, whereas a positive relationship was observed between serum rT3 and serum glutamic pyruvic transaminase values, suggesting that the changes in serum rT3 levels were more dependent on extrarenal illness than on renal disease per se. The moderate increase in serum T3S levels found in patients suffering from both ExHS and ARF may represent a decrease in tissue 5'-monodeiodinase activity as found in other nonthyroidal illnesses. A return of serum thyroid function tests to normal values after recovery from ExHS suggests that the low T3 state may play a protective role to prevent undesirable catabolic effects. Replacement therapy is thus not recommended.     

Inhibition in the senso-motor cortex of cats

The clinical course from birth and serial measurements of serum T3, T4 and TSH in an infant with untreated neonatal thyrotoxicosis are reported. The thyroid hormone levels fell exponentially with time at rates very much slower than those previously reported for the maternally-transmitted thyroid stimulating antibody generally thought to cause the disorder. Steady physiological levels of thyroid hormones were achieved after 110 days (serum T3 = 3.4 NMOL/L, T4 = 118 nmol/l). TSH first rose to a measurable level after about 90 days.     

Annual variations in serum thyroid-stimulating hormone and thyroid hormones and in their responses to thyrotrophin-releasing hormone in the reindeer

The reindeer in its natural habitat is subject to great annual variations in ambient temperature, illumination and nutrition. To ascertain the effect of these environmental factors on thyroid function, serum thyroid-stimulating hormone (TSH), thyroxine (T4), tri-iodothyronine (T3) and reverse T3 (rT3) concentrations were measured four times a year (2 June, 8 October, 21 November, and 24 February) in 14 animals housed outdoors at latitude 69 degrees 10'N. They all showed statistically significant (P < 0.05) seasonal changes. Serum TSH and T4 were highest in February (623 +/- 30 ng/ml and 287 +/- 19 nmol/l respectively). TSH was lowest in October (318 +/- 47 ng/ml) and T4 in November (199 +/- 19 nmol/l). The T3 concentration was highest in November (3.0 +/- 0.3 nmol/l) and lowest in June (1.8 +/- 0.2 nmol/l). In contrast, rT3 was highest in June (3.6 +/- 1.2 nmol/l) and lowest in November (1.9 +/- 0.6 nmol/l). Thus, there was an inverse relationship between T3 and rT3 (linear regression r = -0.406, P < 0.01). TSH, T4, T3 and rT3 responses to exogenous thyrotrophin-releasing hormone (synthetic TRH; 500 micrograms i.m.) were determined in ten animals. The magnitude of their response to TRH was significantly (P < 0.05) dependent on the time of year. When compared with the control level all the parameters rose significantly (P < 0.05). The greatest rise in serum TSH occurred in October (219 +/- 151%) and the smallest in February (66 +/- 53%).(ABSTRACT TRUNCATED AT 250 WORDS)     

Augmented hepatic and skeletal thyromimetic effects of tiratricol in comparison with levothyroxine

A thyroid hormone analog with organ-selective effects could have therapeutic application for disorders such as hyperlipidemia and osteoporosis. We performed a randomized clinical trial to determine the specific thyromimetic effects of tiratricol. Twenty-four athyreotic patients underwent detailed metabolic and physiological evaluation after a 2-month baseline period, taking TSH-suppressive doses of L-T4. They were then randomized to blinded treatment with either tiratricol (24 micrograms/kg twice daily) or L-T4 (1.9 micrograms/kg daily). The dose of hormone was increased until the TSH level was less than 0.1 mU/L, and the metabolic and physiological testing was repeated. Comparing the change from baseline to the study drug periods, when serum TSH levels were equivalently suppressed, there were no significant differences between the two groups in resting metabolic rate, weight, urea nitrogen excretion, or symptom score. Plasma total and low density lipoprotein cholesterol levels declined 13 +/- 4% and 23 +/- 6% in the tiratricol group compared with 2 +/- 2% and 5 +/- 3% in the L-T4 group (P = 0.015 and P = 0.0066, respectively). Serum sex hormone-binding globulin levels increased 55 +/- 13% with tiratricol compared with a 1.7 +/- 4% decline with L-T4 (P = 0.0006), indicating an augmented hepatic response to tiratricol. Skeletal metabolic activity was enhanced, with increased levels of serum osteocalcin and urinary excretion of calcium and pyridinium cross-links. Tiratricol and L-T4 had comparable effects on cardiovascular function. Tiratricol has distinct augmented hepatic and skeletal thyromimetic actions of potential therapeutic value.     

Triiodothyronine (T3) and thyroxine (T4) levels in Rana curtipes during development and metamorphosis

During premetamorphosis, levels of circulating triiodothyronine (T3) and thyroxine (T4) were below the limits of detection of RIA. They became detectable in late prometamorphic stages. A gradual increase in T3 and T4 was observed during this period. A sharp rise in hormone levels was apparent at the onset of metamorphic climax. Peak levels of both hormones were found at Taylor-Kollros stage XXI. The T3 reached a peak level of 101.4 ng/dl, about 3 fold increase over the level at prometamorphosis. Thereafter the circulating hormones (particularly T4) decreased rapidly and reached levels similar to prometamorphic stages. Significant high levels of thyroid hormones (T3 and T4) in metamorphic climax suggest that like other anurans, elevation of these hormones is required for normal metamorphosis in R. curtipes tadpoles.     

Cardiovascular effects of long-term L-thyroxine therapy for Hashimoto's thyroiditis in children and adolescents

Morphology and function of the left ventricle were evaluated by echo and Doppler examination in 16 females affected by Hashimoto's thyroiditis, aged 13.3 (4.5) years (range 7.9-24.6). They were on L-thyroxine (L-T4) treatment for a period of 2.8 (2.8) years (range 0.8-7.6) with a mean daily dose of 77 (18) micrograms/m2. Left ventricular mass, systolic and diastolic function, cardiac output and systemic vascular resistance did not differ from a control group matched for age, sex and body size. A further analysis of the patients according to thyrotrophin serum levels (less or more than 0.1 mU/l) gave similar results. Moreover, no relationship was found between echocardiographic findings and age, L-T4 daily doses, duration of treatment and serum level of thyroid hormones. We can therefore conclude that chronic L-T4 treatment for Hashimoto's thyroiditis at the given doses did not affect cardiac function and morphology in children and adolescents; however, a longer follow-up is needed before confirming the safety of this therapy in the long term.     

Thyroid hormone loss and replacement during resuscitation from cardiac arrest in dogs

Circulating concentrations of thyroxine (T4), triiodothyronine (T3), and reverse triiodothyronine (rT3) were followed in dogs subjected to 9 min of normothermic ventricular fibrillation. Significant decreases were detected 12 h post-arrest when compared to pre-arrest levels in total T4 (P < 0.0005), free T4 (P < 0.0005), total T3 (P < 0.003), and free T3 (P < 0.003), and levels of reverse T3 were significantly elevated (P = 0.0001). Similar changes occurred with only 30 s of arrest. Post-arrest replacement therapy with 7.5 micrograms/kg per h (Rx-7.5) and 15 micrograms/kg per h (Rx-15) levothyroxine sodium (L-T4) increased total T4, free T4, and total T3 (P < 0.01). Free T3 decreased in the Rx-7.5 group (P < 0.01) and did not fall in the Rx-15 group (P = 0.16). Reverse T3 increased with either treatment (P < 0.005). Both treatment groups had higher levels of all five hormones than non-treated animals (P < 0.001). Neurologic function, assessed with a standardized scoring system, showed significant improvement in the treated groups by 6 h (P < 0.05, compared to non-treated group) and remained significant through 24 h post-arrest (P < 0.05). The documentation of rapid and dramatic changes in thyroid hormones immediately following cardiac arrest and resuscitation indicates a significant acute hypothyroid state that may potentially benefit from replacement therapy.     

Membrane fusion in organelle biogenesis

Enteric bacteria have been postulated to have a role in thyroid economy by promoting the hydrolysis of thyroid hormone conjugates of biliary origin, thus permitting the absorption and recycling of thyroxine (T4) and triiodothyronine (T3). An enterohepatic circulation of T3 might be more pronounced under conditions in which type I iodothyronine deiodinase activity (5'D-I) is inhibited, because this augments the accumulation of T3 sulfate conjugates in bile. This potential of increased gut reabsorption of T3 might explain, at least in part, the failure of serum T3 values to decrease appreciably when marked reductions in peripheral 5'D-I activity are induced by selenium deficiency or 6-anilino-2-thiouracil (ATU) administration. Thus, studies were performed to determine the effect of intestinal decontamination, in the absence and in the presence of 5'D-I inhibition, on plasma T4 and T3 concentrations. Groups of adult male rats received either enteric antibiotics or no antibiotics for 12 days and then, in half of the rats in each group, treatment for 10 days with ATU, a 5'D-I inhibitor that does not affect thyroid hormone synthesis. The activity of intestinal arylsulfatase and arylsulfotransferase, enzymes that catalyze hydrolysis of thyroid hormone conjugates, was reduced markedly by approximately 87% in rats that received antibiotics, regardless of whether or not they also received ATU. The ATU treatment markedly inhibited liver 5'D-I activity in antibiotic-treated as well as in non-antibiotic-treated rats (control = 399 +/- 32 U/mg protein (mean +/- SEM); ATU = 152 +/- 17: antibiotics = 351 +/- 29; antibiotics + ATU = 130 +/- 10; p < 0.01) and significantly increased plasma T4 and T3 sulfate (T4S, T3S) concentrations (control: T4S = 2.8 +/- 0.4 and T3S = 6.7 +/- 1.3 ng/dl; ATU: T4S = 6.2 +/- 1.4 and T3S = 10.6 +/- 2.1 ng/dl; antibiotics: T4S = 1.8 +/- 0.2 and T3S = 3.6 +/- 1.0 ng/dl; antibiotics + ATU: T4S = 6.8 +/- 0.7 and T3S = 9.7 +/- 1.8 ng/dl; p < 0.05). The ATU treatment was associated with a significant increase in plasma T4 and rT3 concentrations but did not affect plasma T3 concentrations, and intestinal decontamination did not alter these ATU-associated effects on circulating thyroid hormones. These results suggest that anaerobic enteric bacteria in the rat do not have an important role in recycling of thyroid hormones, either under normal conditions or in circumstances where 5'D-I activity is markedly reduced, and that increased gut absorption of T3 from T3S cannot explain the near-normal serum T3 values found when peripheral 5'D-I activity is markedly decreased.     

Treatment of hypothyroidism with once weekly thyroxine

We compared daily T4 therapy with 7 times the normal daily dose administered once weekly in 12 hypothyroid subjects in a randomized cross-over trial. At the end of each treatment we measured serum free T4 (FT4), free T3 (FT3), rT3, and TSH levels and multiple markers of thyroid hormone effects at the tissue level repeatedly for 24 h. Compared with daily administration, the mean serum TSH before the administration of weekly T4 was higher (weekly, 6.61; daily, 3.92 microIU/mL; P < 0.0001), and the mean FT4 (weekly, 0.98; daily, 1.35 ng/dL; P < 0.01) and FT3 (weekly, 208, daily, 242 pg/dL; P < 0.01) were lower. A minimally elevated serum total cholesterol during weekly administration (weekly, 246.8; daily, 232.6 mg/dL; P < 0.03) was the only evidence of hypothyroidism at the tissue level. Compared with daily administration, the mean peak FT4 following weekly administration of T4 was significantly higher (weekly, 2.71; daily, 1.59 ng/dL; P < 0.0001), as was the mean peak FT3 level (weekly, 285; daily, 246 pg/dL; P < 0.01). None of the tissue markers of thyroid hormone effect changed compared to daily T4, and there was no evidence of treatment toxicity, including cardiac toxicity. During weekly T4 administration, autoregulatory mechanisms maintain near-euthyroidism. For complete biochemical euthyroidism a slightly larger dose than 7 times the normal daily dose may be required.     

Time course of changes in free thyroid indices, rT3, TSH, cortisol and ACTH following exposure to sulfur mustard

In order to evaluate the time course of changes in serum concentration of thyroid hormones, cortisol and ACTH in patients exposed to chemical weapons containing sulfur mustard, we measured serum concentrations of hormones on the first, third and fifth week following injury in 13 soldiers and compared them to the results obtained from 34 control men. Free T4 and T3 indices were decreased and rT3, cortisol and ACTH were increased in the first week following exposure. There was a subnormal TSH response to TRH in 2 of 3 men tested. Except for an increase in FT4I and a decrease in TSH by the third week, and a steady decline in serum cortisol, serum hormone concentrations were unchanged until the fifth week after injury. The decline in serum cortisol occurred despite a constant increase in serum ACTH. By the fifth week only 1 of 13 men had serum cortisol levels > 10 micrograms/dl. We conclude that exposure to chemical weapons containing sulfur mustard results in alterations in serum concentrations of thyroid and adrenal hormones and ACTH, resembling changes seen in burn trauma. Some evidence of direct effects of mustard on endocrine glands exist.     

Thyroid function during a prolonged stay in Antarctica

Adaptation of the thyroid gland to the Antarctic environment was studied in nine healthy euthyroid tropical men of the Sixth Indian Antarctic Expedition during 1 year of their residence at polar latitudes. Circulatory concentrations of thyroid hormones, total T4 (TT4), total T3 (TT3), free T4 (FT4), free T3 (FT3), reverse T3 (rT3), thyroxine binding globulin (TBG), T3 uptake and thyroid stimulating hormone (TSH) were estimated in New Delhi and during the first week of each month of the stay in Antarctica. At the end of the Austral summer in March, the TT3 concentrations were found to be significantly lower (P < 0.01) compared to values recorded in New Delhi and showed a significant increase (P < 0.05) during the Austral winter in August. The mean TT3 concentrations from May to December were found to be significantly higher than the March or April values. Plasma TT4 and rT3 concentrations tended to decline in March but remained unaltered during the entire period in Antarctica. The FT4, FT3, TBG and T3 uptake did not show any appreciable change. Though, the TT3:TT4 ratio tended to decline in March and April suggesting decreased peripheral conversion of T4 to T3 as the possible mechanism for a decline in TT3 in March. physical exertion and prolonged exposure to extreme cold appeared to be the major contributory factors. The TSH concentration in March, April, November and December were found to be significantly higher than the New Delhi values. The morning as well as evening cortisol concentrations during the Austral winter were higher than the March values suggesting that cortisol rhythmicity was well maintained in Antarctica, albeit at a higher level. These observations indicated that the subtle changes in thyroid hormones during a prolonged stay at polar latitudes are related not only to the extreme cold but also to other factors such as physical activity, polar days and polar nights.     

Inhibitory effect of unlabeled iodothyronines on the deiodination of labeled thyroid hormones by cultured hepatocarcinoma cells

Inhibitory effects of unlabeled iodothyronines on the metabolism of thyroxine (T4), 3,3',5-triiodothyronine (T3) and 3,3',5'-triiodothyronine (reverse T3, rT3) were investigated in continuously cultured monkey hepatocarcinoma cells which showed a rapid metabolism of the thyroid hormones. Nonphenolic ring deiodination of [3',5'-125I]-T4 and [3'-125I]-T3 was strongly inhibited by excess T3, 3,5-diiodothyronine (3,5-T2) and T4, whereas rT3 was the least effective inhibitor. Phenolic ring deiodination of [3,5'-125I]-rT3 was strongly affected by excess unlabeled rT3. However, the inhibitory effect of T4, T3 and 3,5-T3 was much weaker than that of rT3. It was concluded that rT3 is apparently the most effective inhibitor of phenolic ring deiodination but the least effective inhibitor of nonphenolic ring deiodination.     

Effect of axial bulbus length and preoperative squint angle on the effect of horizontal combined squint operations

OBJECTIVE: We examined zinc (Zn) status in relation to thyroid function in disabled persons, because the association between Zn deficiency and thyroid function remains controversial. METHODS: After measuring serum free 3,5,3'-triiodothyronine (T3) and free thyroxine (T4) in 134 persons, TSH-releasing hormone (TRH) injection test and estimation of Zn status were conducted in persons with low free T3. RESULTS: Thirteen had low levels of serum free T3 and normal T4. Patients with elevated levels of serum 3,3',5'-triiodothyronine (rT3) showed an enhanced reaction of serum thyrotropin (TSH) after TRH injection. Nine of 13 patients had mild to moderate Zn deficiency evaluated by body Zn clearance and increased urinary Zn excretion. After oral supplementation of Zn sulphate (4-10 mg/kg body weight) for 12 months, levels of serum free T3 and T3 normalized, serum rT3 decreased, and the TRH-induced TSH reaction normalized. Serum selenium concentration (Type 1 T4 deionidase contains selenium in the rat) was unchanged by Zn supplementation. CONCLUSION: Zn may play a role in thyroid hormone metabolism in low T3 patients and may in part contribute to conversion of T4 to T3 in humans.     

The hamstring index

Hyperthyroidism is characterized by increased bone turnover and resorptive activity. Similar changes in remodeling are seen in osteoporosis. To study the pathogenetic role of thyroid hormone in osteoporosis, we measured concentrations of free and total thyroid hormones and investigated the sensitivity of the skeleton toward thyroid hormones in 14 osteoporotic, 16 estrogen-treated, and 15 normal postmenopausal women with comparable thyroid status. Triiodothyronine (T3, 60 microg/day for 7 days) was administered to the three groups. The skeletal response was assessed by monitoring bone alkaline phosphatase (BAP), osteocalcin (BGP), and pyridinium cross-linked telopeptide domain of type I collagen (ICTP) in serum and urinary excretion of hydroxyproline (OHP), pyridinoline (PYR), and deoxypyridinoline (DPR) at days 0, 8, 15, and 57. Women on estrogen replacement therapy exhibited lower bone turnover than the normal postmenopausal women. Markers of bone formation were reduced by 19-43% and markers of resorption by 22-48%. The osteoporotic women displayed lower bone mass at the lumbar spine and the distal forearm (p < 0.01-0.001), but the levels of biochemical markers of bone formation and resorption were comparable to values obtained in the normal postmenopausal women. T3 stimulation caused significant increases (p values ranging between 0.05-0.001) in all three groups of the resorptive markers: ICTP (47%, 47%, 45%), OHP (29%, 30%, 33%), PYR (43%, 27%, 51%), and DPR (42%, 24%, 59%). Of the formative markers, only BGP increased significantly (32%, 40%, 47%) (p < 0.001). At day 57, however, all three formative markers increased compared with day 15 (p < 0.05-0.001). No significant differences in bone markers were demonstrated between groups. In the osteoporotic group, as the only group, serum calcium increased (p < 0.05) and serum PTH fell (p < 0.05). In conclusion, osteoporosis and estrogen substitution are not characterized by altered concentrations of thyroid hormones or responsiveness to thyroid hormones at the level of individual bone cells; however, altered responses pertaining to PTH and calcium were detected.     

Evidence for two tissue-specific pathways for in vivo thyroxine 5'-deiodination in the rat

Propylthiouracil (PTU) is a well known inhibitor of thyroxine (T(4)) to triiodothyronine (T(3)) conversion as evidenced by its effect in several in vitro systems and by the decrease in serum T(3) caused by this drug in either rats or man receiving T(4) replacement. However, the failure of PTU to decrease the intrapituitary T(3) concentration and to completely blunt the serum T(3) concentration in T(4)-replaced athyreotic rats suggest that there may be a PTU-insensitive pathway of T(4) to T(3) conversion in some tissues. To address this question, we have studied the in vivo effect of PTU treatment on the generation of [(125)I]T(3) from [(125)I]T(4) in the serum and cerebral cortex (Cx), cerebellum (Cm), liver (L), and anterior pituitary (P) of euthyroid rats. Whereas PTU decreased the concentration of [(125)I]T(3) in the serum, L homogenates, and L nuclei after [(125)I]T(4), it did not affect the concentration of [(125)I]T(3) in homogenates or nuclei of Cx, Cm, or P. Iopanoic acid pretreatment significantly reduced the [(125)I]T(3) concentration in serum, homogenates, and cell nuclei of all these organs. Neither agent affected the metabolism or tissue distribution of simultaneously injected [(131)I]T(3). The presence of PTU in these tissues was evaluated by in vitro assessment of iodothyronine 5'-deiodinating activity using both [(125)I]rT(3) and [(125)I]T(4) as substrates. In agreement with the in vivo findings, generation of [(125)I]T(3) from T(4) in vitro was not affected by PTU in Cx, Cm, P but it was inhibited by 76% in L. However, rT(3) 5'-deiodination, known to be sensitive to PTU in these tissues, was inhibited in all four indicating that the PTU given in vivo was present in significant amounts. These results demonstrate that in rat Cx, Cm, and P unlike liver, PTU does not inhibit T(4) to T(3) conversion in vivo despite the presence of the drug in the tissues in amounts that significantly inhibit reverse T(3) 5'-deiodination. These results show that in vivo 5'-deiodination of T(4) proceeds via a PTU-insensitive pathway in the central nervous system and pituitary, while this pathway is not quantitatively important in the L. This mechanism accounts for the "locally generated" T(3) in central nervous system and pituitary and could also provide the approximately one-third of extrathyroidally produced T(3) not blocked by PTU administration in athyreotic T(4)-replaced rat.     

Direct measurement of the contributions of type I and type II 5'-deiodinases to whole body steady state 3,5,3'-triiodothyronine production from thyroxine in the rat

Production of T3 from T4 in tissues is catalyzed by two 5'-deiodinases, type I (D1) and type II (D2), but the quantitative contribution of each pathway to whole body T3 production is not well established. In the presence of propylthiouracil (PTU), D1, but not D2, can be effectively blocked, providing an experimental probe for addressing this problem. Decades ago, this approach provided indirect estimates ranging from 23-44% contribution by D2, based on plasma T3 appearance rate comparisons (PAR3 = PCR3 [T3]p) in periodically T4-injected athyreotic rats vs. controls. Two, more recent studies, using constant infusions of T4 for replacement, achieved 22% and 65% estimates, respectively, from PAR3 comparisons. We have revisited this problem more directly and precisely, with two major differences in experiment design. We used direct whole body steady state measurements of T3 production, instead of indirect plasma-only data (PAR3). We also used (euthyroid) physiological doses of both T4 (0.9 microg/day x 100 g BW) and T3 (0.15 microg/day x 100 g BW) for replacement in two thyroidectomized rat groups, instead of T4 only, in a 7-day constant steady state, dual tracer infusion protocol. The first group also had chronically implanted 150-mg PTU pellets (TXR-PTU); the other had implanted 0.1 N NaOH placebo pellets (TXR-EU); each delivered their product at constant rates. A third euthyroid intact group was used as the controls. The completeness of D1 inhibition was ascertained in a fourth group, identically treated with 150-mg PTU pellets, in which negligible D1 activity was found in liver and kidney using labeled rT3 as substrate for the 5'-D assays and minimal (1 mM) dithiothreitol as cofactor. In the TXR-PTU group, the percentage of T4 converted to T3 was 11.8%, compared with 23.4% (P < 0.0005) in the TXR-EU group, and 22.7% (P = NS) in controls. Thus, in euthyroid steady state, D2 contributes about half of the T3 produced from T4.     

Thyroxine administration to infants of less than 30 weeks gestational age decreases plasma tri-iodothyronine concentrations

OBJECTIVE: To investigate the effect on thyroid hormone metabolism of the administration of thyroxine to very preterm infants. DESIGN AND METHODS: Two hundred infants of less than 30 weeks gestation were enrolled into a randomized, double-blind, placebo-controlled trial. Thyroxine (T4) (at a fixed daily dose of 8 microg/kg birthweight) or placebo was started 12-24h after birth and discontinued 6 weeks later. Plasma concentrations of T4, tri-iodothyronine (T3), reverse T3 (rT3), TSH, and thyroxine-binding globulin were measured weekly during trial medication and 2 weeks thereafter. RESULTS: The T4 and the placebo group each comprised 100 infants. Antenatal, perinatal, and postnatal clinical characteristics were comparable in both groups. T4 and rT3 were significantly increased in the T4 group. TSH concentrations were depressed in the T4 group and T3 was significantly decreased, probably as a result of TSH depression. The T4/T3 and T4/rT3 ratios differed significantly between the two study groups. CONCLUSIONS: Daily T4 administration during the first 6 weeks after birth to infants of less than 30 weeks gestation prevents hypothyroxinemia, but decreases plasma T3 concentrations. Our finding possibly implies that very preterm infants should receive supplements of both T4 and T3.