The extrathyroidal conversion rate of thyroxine to triiodothyronine in normal man.

Eight normal subjects were administered tracer amounts of a (14)C-labeled thyroxine, L-[tyrosyl-(14)C] T(4), by multiple injections. Then serial blood samples were collected for isolation of the thyroxine, triiodothyronine, and tetraiodothyroacetic acid fractions by a combination of column and paper chromatographies. The chromatographic artifacts were corrected by adding to the sera a purified (3)H-labeled thyroxine, D,L-[alpha,beta-(3)H] T(4) immediately after the separation of sera from blood. 1-2% of the serum (14)C radioactivity was observed in the triiodothyronine fraction and 2-4% of the serum (14)C radioactivity was observed in the tetraiodothyroacetic acid fraction. Complete kinetic studies of thyroxine and triiodothyronine were compared in the same individual in four of the subjects. The extrathyroidal conversion rates of thyroxine to triiodothyronine were calculated from data obtained during both the injection and the postinjection periods as functions of the (14)C-labeled thyroxine and triiodothyronine remaining in the body at time t and their fractional turnover rates. The average daily rate of the extrathyroidal conversion of thyroxine to triiodothyronine was 4% of the extrathyroidal thyroxine pool or 33% of the total thyroxine production. The amount of triiodothyronine generated by this pathway (22 mug/day) was found to contribute 31% of the extrathyroidal triiodothyronine pool or 41% of the daily triiodothyronine production. This pathway is a major source of triiodothyronine production. The extrathyroidal conversions of thyroxine to triiodothyronine and tetraiodothyroacetic acid are major metabolic pathways of thyroxine in normal man.

Urinary metabolites of 14 C-labeled thyroxine in man.

Studies were carried out to determine the chemical structures of thyroxine metabolites after total deiodination. Normal subjects were given thyroxine labeled with (14)C on the nonphenolic ring and the alanine side chain, 8-11 mug/day for 10 days. By paper chromatography of fresh urine, six or more (14)C-labeled compounds were separated. The (14)C-labeled metabolites were concentrated by passing the urine through a nonionic polymeric adsorbent. Two major thyroxine metabolites were identified. The identification was made by three different methods: (a) chromatography, (b) synthesis of derivatives, and (c) recrystallization to constant specific activity. One (14)C-labeled metabolite was identified as thyroacetic acid or 4-phenoxy-(4'-hydroxy) phenyl-acetic acid. Another one was identified as thyronine. Of the total urinary (14)C radioactivity, 43.7% was recovered as thyroacetic acid and 19.8% was recovered as thyronine. Approximately one-fifth of each of these metabolites was present in the urine in bound form which released the free metabolites during acid hydrolysis. The average daily excretion of thyroacetic acid was 13.7% of the renal disposal rate of thyroxine, or approximately 7.5 mug/day. The average daily excretion of thyronine was 6.5% of the renal disposal rate of thyroxine or approximately 3.9 mug/day while the urinary iodide made up 64.7% of the renal disposal rate of thyroxine. Our findings provide the needed proof that the major metabolic pathways of thyroxine remove the iodine atoms by substituting hydrogen for iodine and leave the diphenyl ether nucleus intact.

Reduced peripheral conversion of thyroxine to triiodothyronine in patients with hepatic cirrhosis.

The role of liver in the peripheral conversion of thyroxine (T4) to triiodothyronine (T3) was studied in normal subjects and patients with alcoholic liver disease by measurement of thyrotrophin (TSH) and total and free T4 and T3 in randomand serial serum samples. Also, T4 to T3 conversion rates and T3 disposal rates were compared by noncompartmental analysis. While the mean total serum T4 values were similar for the two groups, 8.6 and 8.1 mug/kl, the mean free T4 value was significantly higher in the cirrhotic patients (3.3 ng/dl) than in the normal subjects (2.1 ng/dl, P less than 0.001). The mean serum T3 value, 85 ng/dl, was significantly reduced in the hepatic patients as compared to a mean serum T3 value of 126 ng/dl in the normal subjects (P less than 0.001), while the free T3 value was 0.28 ng/dl in both groups. The reduction of the serum total and free T3 values were closely correlated with the degree of liver damage, as indicated by elevation of serum bilirubin (r equal -0.547) and reduction of serum albumin (r equal 0.471). The mean serum TSH level was 3.1 muU/ml in the normals and 7.1 muU/ml in the cirrhotic aptients ( less than 0.001). 15% of the hepatic patients had serum TSH values above 10 muU/ml, which, however, did not correlate with any of the four liver function tests studied. Serial blood sampling from two convalescing patients with alcoholic hepatitis showed a gradual normalization of serum TSH and T3 levels as the liver function improved. After oral T4 administration, 0.25 mg/day for 10 days, three of four cirrhotic patients studied failed to raise their serum T3 values. The mean T4 to T3 conversion rate of seven normal subjects was 35.7%. The mean T4 to T3 conversion rate of four cirrhotic patients studied was significantly reduced to 15.6% (P less than 0.001). The mean disposal rates of T4 and T3 of the normal subjects were 114 and 34 mug/day, respectively. The ratio of T4 disposal to T3 disposal was 3.5. In contrast, the mean T4 disposal rate, 82 mug/day, and the mean T3 disposal rate, 10 mug/day, were both reduced in the cirrhotic patients. Their ratio of T4 disposal to T3 disposal was 7.9. These findings suggest that impairment of T4 conversion in patients with advanced hepatic cirrhosis may lead to reduced T3 production and lowered serum T3 level. Therefore, the liver is one of the major sites of T4 conversion to T3.

The integrity of the ether linkage during thyroxine metabolism in man.

The structures of thyroxine metabolites after total deiodination bear on the mode of thyroxine (T(4)) action in vivo. The present study was undertaken to determine the integrity of the ether linkage during thyroxine metabolism in man. Normal volunteers were given simultaneous intravenous injections of two thyroxines labeled with either (14)C or (3)H on the opposite sides of the ether linkage, D,L-[alpha,beta-(3)H]T(4) and D,L-[phenolic ring-(14)C]T(4). The ratio of alanine side chain to phenolic ring which was measured as (3)H:(14)C ratio was found to remain constant in the serum, urine, and feces during the subsequent 3 wk. The disappearance rates of the (3)H and (14)C radioactivity from blood were similar. The values of half-life were in the ranges of 4.2-6.7 days. 51-63% of the (3)H and 50-57% of the (14)C doses were recovered from urine and 13-20% of the (3)H and 15-20% of the (14)C doses were recovered from feces. Chromatography of the urinary metabolites confirmed that the phenolic ring and the nonphenolic ring including at least part of the side chain remained linked together.

Production and partial characterization of a monoclonal antibody to rat anterior pituitary somatotrophs.

A cell type-specific monoclonal antibody (Mab) against a cell surface antigen of rat anterior pituitary somatotrophs has been generated by fusion of a nonsecreting mouse myeloma line with spleen cells from a mouse immunized with enzymatically dispersed anterior pituitary cells of adult random cycling female rats. Hybridomas were initially screened for antibodies to cell surface antigens by an enzyme-linked immunosorbant assay using rat anterior pituitary cells and smooth muscle cells of aorta as positive and negative controls, respectively. Positive clones were further checked for cell type specificity by immunofluorescence. Mab WHC-1 is an immunoglobulin M (IgM) with kappa-light chains and is cytotoxic in the presence of complement. Based on double immunofluorescence, this Mab reacted with 22.5 +/- 2.0% (+/- SEM) of the anterior pituitary cells of adult random cycling female rats. Among them, about 93.5 +/- 1.4% were somatotrophs, and only 4.1 +/- 1.2% were mammotrophs. Approximately two thirds of the somatotrophs were Mab WHC-1-positive. The reaction of this Mab with gonadotrophs, thyrotrophs, or corticotrophs were negligible. The percentage of Mab WHC-1-positive cells derived from immunoperoxidase staining was significantly greater than that from immunofluorescence. The cell surface antigen defined by Mab WHC-1 is expressed heavily on GH3 cells, but not on smooth muscle cells. It is resistant to trypsin digestion, but sensitive to ethanol treatment, and exhibits the solubility property of a glycolipid. Mab WHC-1 cross-reacts with the anterior pituitary cell of rabbits, but not mice. These results provide the immunological evidence for heterogeneity among somatotrophs and demonstrate the feasibility of making pituitary cell type-specific Mabs.

Comparison of peripheral thyroid hormone metabolism in normal rats and in rats receiving prolonged glucagon infusion.

An elevated plasma glucagon concentration and reduced T3 production from T4 have both been observed in several clinical disorders, including hepatic cirrhosis, uremia, diabetes mellitus, and starvation. The question of whether glucagon has a direct effect on T3 production was studied in normal rats infused iv with [125I]T4 of [125I]T3 and 3 micrograms T4/day, using implanted minipumps. The blood [125I]T4 and [125I]T3 levels maintained a plateau between the fifth and ninth days of infusion. Each animal also received a second minipump, implanted ip, that infused either a diluant solution or 30 micrograms glucagon/100 g BW . day. After 7 days of continuous infusion, the glucagon-treated animals showed a 20% increase in plasma glucose and a 4-fold increase in plasma glucagon from baseline. However, the levels of insulin, T4, and T3 remained unchanged. The MCRs and the disposal rates of T4 and T3, calculated by the constant infusion method, showed T4 and T3 MCRs to be 0.99 +/- 0.18 and 11.25 +/- 2.52 ml/h . 100 g, respectively, and T4 and T3 disposal rates to be 68 +/- 10 and 9 +/- 2 ng/h . 100 g; there was no difference between the control animals and the glucagon-infused animals. T3 production was also determined in vitro from T4 added to a liver homogenate. Compared to control animals, the liver homogenate prepared from glucagon-infused animals showed a modestly higher T3 production rate throughout the 60-min incubation period (P = 0.025--0.05). However, the concentration of nonprotein-bound sulfhydryls was similar in the liver, kidney, brain, muscle, and heart of the two animal groups. In conclusion, glucagon does not have an important regulating role on the peripheral metabolism of thyroid hormone and T3 production in rats.

The nondeiodinative pathways of thyroxine metabolism: 3,5,3',5-tetraiodothyroacetic acid turnover in normal and fasting human subjects.

Complete turnover studies of T4, T3, rT3, and 3,5,3',5-tetriodothyroacetic acid (TA4) were carried out in normal subjects given T4 (0.2 mg, by mouth daily) by the integration method. When compared to the five fed controls, the four fasting subjects showed a decrease of mean T3 disposal from 41 to 18 micrograms/day, an increase of mean rT3 disposal from 49 to 61 micrograms/day. The mean serum TA4 concentration rose from 53 to 112 ng/dl, while the TA4 metabolic clearance remained unchanged. The fraction of T4 metabolized by deiodination changed from 79.0% to 77.5% in the fasting subjects as the fraction of T4 metabolized by deamination changed from 1.1% to 2.2%. Therefore, fasting induces a significant shunting of T4 away from T3 production into rT3 and TA4 production. However, oxidative deamination remains a minor metabolic pathway of T4 in man during both normal and fasting conditions. Given its low disposal rate and low biological potency, the increased TA4 production during fasting is probably not the inhibitory factor of TSH response to the lowered T3 production during fasting.

Effect of estrogen on the expression of a cell-surface antigen associated with rat anterior pituitary somatotrophs.

The long-term in vivo effect of diethylstilbestrol (DES) on the expression of a cell-surface antigen associated with the anterior pituitary somatotroph was studied in two strains of female rats using double immunofluorescence techniques. Mab WHC-1, a recently generated and characterized monoclonal antibody, was used to detect the antigen associated with somatotrophs, whereas rabbit anti-rat prolactin (rPRL) and anti-human growth hormone (hGH) antisera were used to identify mammotrophs and somatotrophs, respectively. In F344 rats, Mab WHC-1-positive cells increased from 13.8 +/- 0.5% of total pituitary cells in normal anterior pituitaries to 34.2 +/- 4.0% in DES-induced pituitary tumors. The number of mammotrophs also increased significantly from 58.0 +/- 3.2% in controls to 75.9 +/- 2.2% in tumors. On the other hand, somatotrophs decreased significantly in number following ovariectomy (OVX) and DES implantation (19.7 +/- 0.5% vs. 6.1 +/- 1.2%). Based on double immunofluorescence, the percentage of Mab WHC-1-positive cells, which were somatotrophs, decreased from 85.5 +/- 2.7% in normal controls to 6.7 +/- 1.5% in DES-induced tumors. On the other hand, the percentage of Mab WHC-1-positive cells which were mammotrophs increased significantly from 14.0 +/- 1.4% to 86.1 +/- 1.8% following OVX and DES implantation. A similar change was found in the number of somatotrophs and mammotrophs following the same treatment in Sprague-Dawley (SD) rats which did not develop pituitary tumors. In contrast to F344 rats, the number of Mab WHC-1-positive cells in SD rats decreased significantly from 32.4 +/- 2.8% in sham-operated controls to 19.3 +/- 2.9% in OVX + DES-implanted rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Lithium treatment of hyperthyroidism and mania.

A case of a man who was admitted to the hospital with both mania and hyperthyroidism is presented to illustrate the interactions between affective disorder and thyroid function. In addition, the methods in which lithium carbonate affects thyroid function, thus making diagnosis more difficult, are discussed. The authors suggest guidelines for evaluation and management of similar patients.

Carbohydrate metabolism in hypothyroid myopathy.

The carbohydrate metabolism in hypothyroid patients was investigated. After an overnight fast, the blood glucose level was 24% lower and the blood lactate level was 35% lower in the untreated hypothyroid patients than that observed in the treated hypothyroid patients or in the normal subjects. There was no difference in the blood alanine or plasma free fatty acid values between the subject groups. Skeletal muscle biopsied from two hypothyroid patients with marked myopathy showed normal glycogen content, 0.83%-0.86% (normal 1.06%), but reduced activity of acid maltase, 32-50 nmoles/min/g (normal 97). Forearm ischemic stimulation applied to hypothyroid patients failed to elevate the level of lactate. The results are compatible with impaired glycogenolysis from the skeletal muscle, which may be a contributory factor in the myopathy in hypothyroidism.

The effects of propranolol on thyroxine metabolism and triiodothyronines production in man.

The effects of propranolol on the turnover of thyroxine (T4), 3,5,3'-triiodothyronine (T3), and 3,3',5'-triiodothyronine (rT3) were determined by a noncompartmental analysis in seven normal men. Fourteen normal subjects were treated with 0.2 mg T4 daily, and half of this group (seven) received in addition 80 mg propranolol daily. Fifteen days of propranolol treatment did not alter serum T4 concentration or T4 turnover. However, it lowered serum T3 concentration from 173 to 102 ng/dl (P = 0.001); T3 clearance was unchanged. Propranolol treatment elevated serum rT3 concentration from 54 to 69 ng/dl (P = 0.05); rT3 metabolic clearance rate fell from 105 to 90 liters/day but the difference did not reach statistical significance. The rT3 disposal rate was unchanged by propranolol. The fractional T4 disposal which was degraded via the deiodinative pathways was reduced from 82.0 per cent in the control subjects to 65.5 per cent in the propranolol treated subjects. Therefore, propranolol appears to be a potent inhibitor of 5'-deiodination. The interpretation of serum T3 measurements in patients treated with propranolol requires caution.

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.

Corticotropin production by tumors of the autonomic nervous system.

Cushing's syndrome in association with a tumor of the autonomic nervous system (ANS) has been reported occasionally. We studied a patient who had an intra-adrenal paraganglioma (pheochromocytoma), and whose plasma corticotropin level was elevated prior to surgery but dropped to a low value following removal of the tumor. Catecholamine levels were elevated preoperatively and catecholamines were extracted from the tumor tissue. Corticotropin was identified in the tumor by immunoperoxidase staining. We also compared the endocrine data of 16 previously reported cases of Cushing's syndrome secondary to the release of ectopic corticotropin from ANS tumors. We concluded that in these patients, the plasma corticotropin level is only modestly elevated but indexes of steroid production frequently are markedly elevated. Also, discrepant responses to dexamethasone suppression tests occur, perhaps via sporadic release of corticotropin. These factors complicate evaluation of the cause of Cushing's syndrome in these patients.