Regulation of thyroid hormone metabolism in rat liver fractions

The nature of the conversion of thyroxine (T₄) to triiodothyronine (T₃) and reverse triiodothyronine (rT₃) was investigated in rat liver homogenate and microsomes. A 6-fold rise of T₃ and 2.5-fold rise of rT₃ levels determined by specific radioimmunoassays was observed over 6 h after the addition of T₄. An enzymic process is suggested that converts T₄ to T₃ and rT₃. For T₃ the optimal pH is 6 and for rT₃, 9.5. The converting activity for both T₃ and rT₃ is temperature dependent and can be suppressed by heat, H₂O₂, merthiolate and by 5-propyl-2-thiouracil. rT₃ and to a lesser degree iodide, were able to inhibit the production of T₃ in a dose related fashion. Therefore the pH dependendy, rT₃ and iodide may regulate the availability of T₃ or rT₃ depending on the metabolic requirements of thyroid hormones.     

A study of hepatic metabolism of thyroxine in BB/W rats treated with L-thyroxine

The effect of thyroxine (T4) on T4 conversion to triiodothyronine (T3) and reverse T3 (rT3) was studied in BB/W rats. A colony of 38 BB/W rats was obtained and half were treated with thyroxine (T4), 1 mg per liter of drinking water. At 106 days of age the following groups were identified: nondiabetic, no T4 treatment, 8 rats; nondiabetic, T4 treated, 8 rats; diabetic, no T4 treatment, 10 rats; diabetic, T4 treated, 7 rats. All animals with diabetes were treated with insulin. T4 conversion to T3 and rT3 was assessed in liver homogenates in 0.1 M Tris-HCl buffer, pH 7.4, with or without 5 mM dithiothreitol (DDT). Serum T4 and rT3 were significantly elevated in both T4-treated groups (P less than 0.001), while serum T3 was not affected in either. Basal T4 deiodination to T3 by the liver homogenate did not change on treatment with T4; the addition of DTT increased T3 production in the homogenate from T4 treated nondiabetic animals (P less than 0.05). In both nondiabetic and insulin-treated diabetic rats there was no effect of T4 on the rate of rT3 production. Since, in the rat, 30-40% of circulating T3 is a direct contribution of thyroid gland secretion, and that would be absent in our T4-suppressed animals, the normal serum T3 may reflect increased absolute peripheral T3 production from the greater concentration of circulating T4.     

Increase of biliary excretion of reverse triiodothyronine in rats during the infusion of neurotensin possibly resulting from the inhibition of iodothyronine 5'-monodeiodination.

Male rats weighing about 350 g were inserted polyethylene tubes into the bile duct and femoral vein under pentobarbital anaesthesia. After taking the first (control) 2-h bile sample the control group (n = 24) was infused saline for 4 h and the other group (n = 14) was infused neurotensin in a dose of 27 micrograms per animal per 4 h. The concentration of thyroxine (T4), triiodothyronine (T3) and reverse triiodothyronine (rT3) in the bile was estimated by radioimmunoassay. No significant differences between groups were found in the biliary excretion of T4 and T3, while the excretion of rT3 after the infusion of neurotensin was significantly increased which was not the case in controls. Since neurotensin is known to increase glycemia which effect might be or might not be mediated by glucagon, it may be suggested that these results bring an additional support for the previously reported coincidence between a prevailing effect of gluconeogenetic hormones and inhibition of iodothyronine 5'-deiodination in the liver.     

Phenolic and nonphenolic ring deiodinations of iodothyronines in cultured hepatocarcinoma cell homogenate from monkey.

Iodothyronine monodeiodinase activities in homogenates of cultured monkey hepatocarcinoma cells were measured by the deiodination of [3.5-(125)I]-diiodo-L-thyronine or 3-[3',5'-(125)I]triiodo-L-thyronine (phenolic ring-labeled 'reverse' triiodothyronine). The assay system utilized a small ion-exchange column (AG50W-X4, O.9 X approximately 1 cm) to measure 125I-. Both deiodinases were destroyed by boiling for 1 min. Maximal nonphenolic ring deiodination was observed at pH 7.9 whereas maximal phenolic ring deiodination was at pH 6.3. Both reactions were enhanced strongly by dithiothreitol (0.1-5mM), and slightly by 5 mM beta-mercaptoethanol. Phenolic ring deiodination was strongly inhibited by 0.1 mM propylthiouracil. Nonphenolic ring deiodination was accelerated by EDTA (1.2 MM) and inhibited by Mg(2+) (5mM). Methylmercaptoimidazol and Mg(2+), Ca(2+) and Mn(2+) (0.1-1.0 mM) had little or no effect on either reaction, but Zn(2+) (0.1 mM) strongly inhibited both. Both reactions were inhibited by excess iodothyronine analogues at 10 mM to 10 micron M, and thyroxine was shown to be a competitive inhibitor in both cases. On the basis of relative affinities and inhibitory effects, it appears that the order of affinity for the phenolic ring deiodinase is 3,3',5'-triiodo-L-thyronine(rT3) greater than L-thyroxine(T4) greater than 3,5,3'-triiodo-L-thyronine(T3), whereas for the nonphenolic ring deiodinase the order is T3 greater than T4 greater than rT3. Diiodotyrosine did not affect their deiodination.     

Decline of T3 and elevation in reverse T3 induced by hyperglucagonemia: changes in thyroid hormone metabolism, not altered release of thyroid hormones

Recently we reported that hyperglucagonemia induced by glucagon infusion causes a decline in serum Triiodothyronine (T3) and a rise in reverse T3 (rT3) in euthyroid healthy volunteers. These changes in T3 and rT3 levels were attributed to altered T4 metabolism in peripheral tissues. However, the contribution of altered release of thyroid hormones by the thyroid gland could not be excluded. Since the release of thyroid hormones is suppressed by exogenous administration of L-thyroxine (L-T4) in appropriate dosage, we studied thyroid hormone levels for up to 6 hours after intravenous administration of glucagon in euthyroid healthy subjects after administration of L-T4 for 12 weeks. A control study was conducted using normal saline infusion. Plasma glucose rose promptly following glucagon administration demonstrating its physiologic effect. Serum T4, Free T4 and T3 resin uptake were not altered during both studies. Glucagon infusion induced a significant decline in serum T3 (P less than 0.01) and a marked rise in rT3 (P less than 0.01) whereas saline administration caused no alterations in T3 or rT3 levels. Thus the changes in T3 and rT3 were significantly different during glucagon study when compared to saline infusion. (P less than 0.01 for both comparisons). Therefore, this study demonstrates that changes in serum T3 and rT3 caused by hyperglucagonemia may be secondary to altered thyroid hormone metabolism in peripheral tissues and not due to altered release by the thyroid gland, since the release of thyroid hormones is suppressed by exogenous L-T4 administration.     

Monodeiodination of thyroxine to 3, 5, 3'-triiodothyronine and to 3, 3', 5'-triiodothyronine in isolated dog renal cortical tubuli

Monodeiodination of T4 to T3 and rT3 in the intact cells of dog renal tubuli and glomeruli was investigated. The tubuli and glomeruli were obtained by a sieve method. T4 (2 micrograms/ml) was incubated in Tris-HCl buffer, pH 7.5, with renal cells (180 micrograms protein/ml) and 5 mM DTT for 1 h at 37 degrees C and the T3 and rT3 generated during incubation were measured by specific radioimmunoassays. In order of decreasing activity, dog renal cortical tubuli, cortical homogenate, glomeruli and medullary tubuli were capable of converting T4 to T3. Net rT3 production from T4 in cortical tubuli was also greater than that in cortical homogenate. The conversion of T4 to T3 and also to rT3 in cortical tubuli was enzymatic in nature, since the reactions showed dependence on time and protein concentration; instability to heating; temperature and pH optimum. The production of T3 and rT3 from T4 was maximum at pH 6.5 and at pH 9.5, respectively, indicating that two different enzymic systems, a 5- and a 5'-monodeiodinase, might be involved in the deiodination of the tyrosyl and the phenolic ring of T4 in dog kidney.     

pH-dependency of iodothyronine metabolism in isolated perfused rat liver.

1. Isolated livers from fed male rats were perfused for 2 h with T4 (L-thyroxine), T3 (L-3,3',5-tri-iodothyronine) or rT3 (L-3,3',5'-tri-iodothyronine) at different pH values (7.1--7.6) in a fully synthetic medium, whereby normal metabolic functions were maintained without addition of rat blood constituents or albumin. 2. T3 output into the medium and net T3 production reached a maximum at a pH of the medium of 7.2 and significantly decreased with alteration of the pH when livers were perfused with T4 as a substrate. 3. However, the net T4 and T3 uptake by the liver, as well as the hepatic T4 and T3 content after perfusion, were not dependent on the pH of the perfusion when livers were offered T4 or T3 as substrates respectively. 4. Determination of intracellular pH by the analysis of the distribution of the weak acid dimethyloxazolidinedione allows the conclusion that the pH optimum of iodothyronine 5'-deiodinase in the intact perfused liver corresponds to the maximum determined in vitro for the membrane-bound enzyme localized in the endoplasmic reticulum. 5. The rapid 5'-deiodination of rT3 to 3,3'-T2 (L-3,3'-di-iodothyronine), the fast disappearance of 3,3'-T2, and the fact that no net rT3 production from T4 could be detected, supports the hypothesis that in rat liver iodothyronine 5'-deiodinase activity seems to predominate over iodothyronine 5-deiodinase activity. 6. Thus the rat liver can be considered in normal physiological situations as an organ forming T3 from T4 and deiodinating rT3 originating from extrahepatic tissues, whereby the cellular iodothyronine 5'-deiodination rate is controlled by the intracellular pH.     

Potentiation of thyroxine 5-deiodination by aminotriazole

Aminotriazole, a goitrogen, in addition to its known inhibitory effects on the thyroid, demonstrated a unique effect on peripheral deiodination of thyroxine (T4). In contrast to the well-known peripheral effects of goitrogens such as propylthiouracil in inhibiting 5'-deiodinase activity, i.e., to effect a decrease in T4 to triiodothyronine (T3) conversion, aminotriazole had no effect on the 5'-deiodinative pathway. Rather, this goitrogen appeared to stimulate the alternative pathway, viz. T4 5-deiodination, resulting in an increased reverse triiodothyronine (rT3) serum concentration. This was shown in comparisons of serum T4, T3 and rT3 concentrations and serum T3/T4 and rT3/T4 ratios between rats treated with aminotriazole and T4, and rats treated with T4 alone. The finding that aminotriazole may specifically enhance T4 5-deiodination, independently of T4 5'-deiodination, is novel, as this has not been observed in the case of other goitrogens. It is of interest that this goitrogen is devoid of sulphur, which is a prominent constituent of thiourylene compounds which have been noted to affect 5'-deiodination. The potentiating effect of aminotriazole on 5-deiodination of T4 was not attributable to dietary factors.     

Plasma iodothyronines in the domestic fowl: newly hatched to early adult stages, with special reference to reverse triiodothyronine (rT3)

Circulating concentrations of thyroxine (T4), triiodothyronine (T3) and reverse triiodothyronine (rT3) were measured in chicks before, during, and after hatching, up to 9 weeks of age. T4 decreased prior to hatching, rose after emergence, and was variable in the immature domestic fowl. T3 increased prior to emergence, decreased until 5 days after hatching, and increased again by 1 week of age, after which the levels declined. Plasma rT3 declined prior to hatching, remained low until 5 days after emergence, and then increased, again, to 0.14-0.19 ng/ml between 1-9 weeks of age.     

Apparent increase in type I 5'-deiodinase activity induced by antiepileptic medication in mentally retarded subjects

BACKGROUND/OBJECTIVES: Thyroid function measurements in 3 mentally retarded patients treated with antiepileptic drugs (phenytoin or carbamazepine) showed normal thyroid-stimulating hormone (TSH) responses in spite of markedly low levels of total thyroxine (T(4)), triiodothyronine (T(3)), and free thyroxine (FT(4)) concentrations; free triiodothyronine (FT(3)), as well as mean thyroxine-binding globulin (TBG) concentrations were normal. The objective of the present investigations was to determine if antiepileptic medication in these patients contributed to the disparate TSH and thyroid hormone (TH) levels. METHODS: Thyroid tests and other laboratory parameters were measured by conventional techniques. RESULTS: Circulating TH changes noted in retarded patients were similar to those observed in control subjects receiving carbamazepine alone. Reverse T(3) (rT(3)) levels in all patients were either undetectable or below the normal range. CONCLUSIONS: As type I 5'-deiodinase has a higher affinity for rT(3) than T(4), an increased activity of this enzyme would enhance rT(3) deiodination and reduce serum rT(3) concentration whereas enhanced T(4) deiodination would aid in normalizing intracellular FT(3) concentration. The finding of normal serum FT(3) concentration was consistent with normal TSH response and clinical euthyroidism in both retarded and control subjects. While phenytoin-induced increase in type I 5'-deiodinase has been previously noted, the present studies demonstrate a similar effect of carbamazepine on 5'-deiodinase.     

Central stimulatory effect of leptin on T3 production is mediated by brown adipose tissue type II deiodinase

To assess whether intracerebroventricular leptin administration affects monodeiodinase type II (D2) activity in the tissues where it is expressed [cerebral cortex, hypothalamus, pituitary, and brown adipose tissue (BAT)], hepatic monodeiodinase type I (D1) activity was inhibited with propylthiouracil (PTU), and small doses of thyroxine (T4; 0.6 nmol. 100 g body wt(-1). day(-1)) were supplemented to compensate for the PTU-induced hypothyroidism. Two groups of rats were infused with leptin for 6 days, one of them being additionally treated with reverse triiodothyronine (rT3), an inhibitor of D2. Control rats were infused with vehicle and pair-fed the amount of food consumed by leptin-infused animals. Central leptin administration produced marked increases in D2 mRNA expression and activity in BAT, changes that were likely responsible for increased plasma T3 and decreased plasma T4 levels. Indeed, plasma T3 and T4 concentrations were unaltered by central leptin administration in the presence of rT3. The additional observation of a leptin-induced increased mRNA expression of BAT uncoupling protein-1 suggested that the effect on BAT D2 may be mediated by the sympathetic nervous system.     

The effect of cortisol on thyroid hormone kinetics in the ovine fetus

The mechanism underlying the association of rising concentrations of circulating triiodothyronine (T3) with the prepartum surge in the concentration of cortisol was investigated in 11 fetal sheep. The concentrations and metabolic clearance rates of T3 and thyroxine (T4) were measured prior to and following a continuous intravascular infusion of cortisol (1 mg/h for 84 h). Mean plasma T3 concentrations increased 10-fold following cortisol infusion whereas the concentrations of T4 either remained stable or exhibited a variable decline. Cortisol induced a 5-fold decrease in the metabolic clearance rate of T3 and a 6-fold increase in that of T4. The corresponding mean production rates of T3 and T4 increased significantly although the magnitude of the change varied between fetuses. We conclude that the prepartum rise in plasma T3 concentrations is likely to be a consequence of both a decreased metabolic clearance of T3 and increased peripheral conversion of T4 to T3 caused by rising concentrations of cortisol in fetal plasma.     

Lowering of T3 and rise in reverse T3 induced by hyperglucagonemia: altered thyroid hormone metabolism, not altered release of thyroid hormones

Recently we reported that hyperglucagonemia induced by glucagon infusion causes a decline in serum T3 and a rise in reverse T3 in euthyroid healthy volunteers. These changes in T3 and rT3 levels were attributed to altered T4 metabolism in peripheral tissues. However, the contribution of altered release of thyroid hormones by the thyroid gland could not be excluded. Since the release of thyroid hormones is inhibited in primary hypothyroidism and is almost totally suppressed following L-thyroxine replacement therapy, we studied thyroid hormone levels for up to 6 hours after intravenous administration of glucagon in subjects with primary hypothyroidism who were rendered euthyroid by appropriate L-thyroxine replacement therapy for several years. A control study was conducted using normal saline infusion. Plasma glucose rose promptly following glucagon administration demonstrating its physiologic effect. Serum T4, Free T4, and T3 resin uptake were not altered during both studies. Glucagon infusion induced a significant decline in serum T3 (P less than 0.05) and a marked rise in rT3 (P less than 0.05) whereas saline administration caused no alterations in T3 or rT3 levels. Thus the changes in T3 and rT3 were significantly different during glucagon study when compared to saline infusion. (P less than 0.01 for both comparisons). Since, the release of thyroid hormones is suppressed by exogenous LT4 administration in these subjects; we conclude that changes in serum T3 and rT3 observed following glucagon administration reflect altered thyroid hormone metabolism in peripheral tissues and not altered release by the thyroid gland.     

The substrate specificity, tissue specificity and regulation of the 5' deiodination systems in rat liver and kidney tissues

Studies were carried out to compare the 5' deiodination reactions of thyroxine (T4) and 3, 3', 5'-triiodothyronine (rT3) in rat liver and kidney homogenates. The 5'-deiodinase activity was assayed by the 3, 5, 3'-triiodothyronine (T3) produced from T4 or by the 125I-iodide released from 125I-rT3. The two 5' deiodination reactions had similar ranges of optimal pH, incubation temperature, and apparent Km, T4 1.1 and rT3 1.3 microM. However, the apparent Vmax values for T4 and rT3 deiodination reactions were 0.9 and 220 pmol/mg protein/min, respectively. Both reactions were stimulated by thiol reagent but only rT3 deiodination showed complete thiol dependence. The inhibitory effect of 6-propyl-2-thiouracil (PTU) on the 5' deiodination of rT3 was 50 times as great as that of T4. Only the 5' deiodination of rT3 was inhibited by low concentrations of calcium and magnesium. The 5' deiodination reactions in the liver and kidney tissues showed very similar substrate specificity. However, only the hepatic deiodinase activity was reduced to 60-65% of the control value after fasting, whereas the renal 5'-deiodinase activity was unaffected or even enhanced by fasting up to 72 hours. The results showed the existence of a diverse and complex 5' deiodination system in the rat tissues which is comprised of multiple similar but distinct 5'-deiodinase enzymes with respect to their substrate specificity, tissue specificity and regulation.     

Thyroid hormone response to thyrotrophin releasing hormone (TRH) in the sex-linked dwarf chicken

The effect of an injection of thyrotrophin releasing hormone (TRH) on plasma levels of thyroid hormones was studied in dwarf and normal Rhode Island Red chickens with similar genotypes other than for the sex-linked dwarf gene dw. The sex-linked dwarf chickens had different plasma iodothyronine levels from control normal chickens: high thyroxine (T4), low triiodothyronine (T3) and similar reverse T3 (rT3) levels. The injection of TRH (10 micrograms/kg) in 5-day- and 5-week-old normal chickens increased the plasma T4 within 30 min without a significant increase in T3, whereas the injection of TRH in 11-and 26-week-old normal chickens increased plasma T3 60 min later. In dwarfs the response of T4 to TRH was the same as that in normals but no increased T3 response was observed. The plasma level of rT3 was not influenced by the TRH injection in either strain. These results suggest that although in the sex-linked dwarfs thyroidal response to exogenous TRH is similar to that of normals, the dwarf gene dw inhibits the conversion of T4 to T3 in peripheral tissues without any inhibitory effect on rT3 production.     

Thyroid hormone binding to the rat heart cytosol proteins

The properties of the thyroid hormone binding to rat heart cytosol were studied. Cytosol proteins were found to bind specifically T4 with high affinity (Ka approximately equal to 10(8)M-1) and rT3 with lower affinity (Ka approximately equal to 10(7)M-1), but they do not bind T3. The binding of both T4 and rT3 was pH dependent, however, while T4 binding had the highest values between pH 7.0 and 10, rT3 binding increased from pH 6.0 to 10.7. Divalent ions also stimulated T4 and rT3 binding. Sulfhydryl groups blocking agents such as N-ethylmaleimide (NEM) and iodoacetamide significantly decreased rT3 binding and had less profound effect on binding of T4 to cytosol proteins. The importance of free -SH groups remains unclear as dithiothreitol was found to diminish the binding of T4 and rT3.     

Thyroid function tests in children with congenital hypothyroidism on L-thyroxine treatment

The plasma levels of thyroxine (T4), triiodothyronine (T3), free T4 (FT4), free T3 (FT3), reverse T3 (rT3) and immunoradiometrically assayed thyrotropin (IRMA TSH) have been measured in 28 L-T4-treated children with congenital hypothyroidism as well as in a control group (group C). The patients were subdivided into 2 groups according to the nonsuppressed (group A) or suppressed (group B) TSH response to TSH-releasing hormone (TRH). Basal IRMA TSH correlated with the TSH increment after TRH and it was significantly lower in group B vs. groups A and C, while no difference was present between groups A and B in regard to T4, FT4 and rT3, all higher than in group C. FT3 levels were similar in the 3 groups. In children, as in adults, basal IRMA TSH seems to be a reliable index in monitoring overtreatment.     

A proton and fluorine-19 nuclear magnetic resonance and fluorescence study of the binding of some natural and synthetic thyromimetics to prealbumin (transthyretin)

Interactions between prealbumin and several thyromimetic compounds have been examined by proton magnetic resonance spectroscopy. One equivalent of thyroxine (T4) or reverse triiodothyronine (rT3) selectively broadens a number of protein signals, while addition of a second equivalent induces much less widespread changes. One equivalent of triiodothyronine (T3), however, produces much less dramatic changes, and effects comparable with T4 and rT3 are only apparent when a second equivalent binds. The broadening is ascribed to immobilization of flexible residues. The non-halogenated analogue 3,5-dimethyl-3'-isopropylthyronine induces qualitatively different changes suggesting incomplete entry into the thyromimetic binding channel. The fluorinated analogue SK&F 95049 (3,5-bis-thiotrifluoromethyl-3'-isopropylthyronine) induces very similar changes to T3. A fluorine-19 NMR signal with a half-height line width of approximately 150 Hz can be observed from the bound ligand. Finally, a spin-labeled T4 analogue, with a nitroxyl on the alanyl moiety, induces changes identical to those induced by T4 itself, and additionally broadens some signals corresponding to residues at the opening of the ligand binding channel. The natural tryptophan fluorescence of the protein is shown to be a sensitive indicator of binding. The possible influence of the dynamic restrictions induced by binding the first molecule of T4 or rT3 on the protein's affinity for a second hormone is discussed. It is suggested that the first interaction confers rigidity on the second site and reduces its ability to flex open and accommodate a second thyromimetic, which results in the marked negative co-operativity associated with the occupancy of this site.     

Substrate specificity of iodothyronine 5'-deiodinase in rat liver homogenates and its requirements of divalent cations in vitro

Studies were carried out to compare the 5'-deiodination reactions of thyroxine (T4) and 3,3'-5'-triiodothyronine (rT3) in 2.5% rat liver homogenates. The 5'-deiodinase activity was assayed by the 3,5,3'-triiodothyronine (T3) produced from T4 or by 125I-rT3. Under our experimental conditions, the two 5'-monodeiodination reactions resulted in similar apparent KMs: 1.5 microM for T4 and 1.1 microM for rT3. However, the apparent Vmax values of T4 and rT3 deiodination reactions were, respectively, 0.91 and 222 pmol/mg protein/min. Both reactions were stimulated by thiol reagents but only rT3 deiodination showed complete thiol dependence. The inhibitory effect of 6-propyl-2-thiouracil on the 5'-deiodination of rT3 was at least 50 fold greater than that of T4. The divalent ion requirement of the deiodination system was tested with CaCl2, MgCl2, and ZnCl2 at a range of concentrations. Zinc ion appeared to be a potent inhibitor in both T4 and rT3 deiodination systems. Only the 5'-deiodination of rT3 was inhibited slightly by low concentrations of calcium and magnesium ions. Our results suggest that based on their apparently distinct regulation mechanisms, the 5'-monodiodination of T4 and rT3 in rat liver homogenates is likely mediated by more than one enzyme, despite the similarity of observed KMs.     

Serum TSH, T4, T3, FT4, FT3, rT3, and TBG in youngsters with non-ketotic insulin-dependent diabetes mellitus

Several parameters of thyroid function were studied in 112 non-ketoacidotic youngsters with insulin-dependent diabetes mellitus (IDDM). Levels of thyroxine (T4), reverse triiodothyronine (rT3), thyroxine-binding globulin (TBG) and T3 were lower than in controls, whereas FT4, and FT3 were normal. T4 levels in IDDM patients were positively related to T3, rT3 and TBG, and inversely related to haemoglobin A1 (HbA1). However, only 4 patients showed biochemical hypothyroidism (T4 less than 5 micrograms/100 ml), whereas their FT4, FT3 and thyroid-stimulating hormone (TSH) levels were normal. Concurrent variations of T3 and rT3 levels were found in IDDM patients; thus, their T3/rT3 ratios were stable or higher than in controls, indicating that peripheral deiodination of T4 is preferentially oriented to production of rT3 only during ketoacidosis. Although changes in thyroid function may reflect the degree of metabolic control of diabetes in a large population, the clinical usefulness of serum thyroid hormone measurements in an individual case still appears to be limited.