Metabolic implications of low-dose triiodothyronine administration in rats: relevance to the adjunctive use of triiodothyronine in the treatment of depression.

The adjunctive use of triiodothyronine (T3) with tricyclic antidepressants is generally believed to augment the efficacy of the tricyclic medications in unipolar, bipolar, and treatment-resistant depression. In the small subset of depressed patients with evidence for overt or subclinical hypothyroidism, the efficacy of supplementary T3 is logically presumed to derive from the amelioration of the hypothyroidism. It is, however, uncertain why adjunctive T3 therapy is often effective in the initially euthyroid depressed patient and if such therapy induces subclinical hyperthyroidism. To determine the metabolic state induced by low-dose T3 treatment, rats were administered nonpulsatile, submaintenance doses of T3 to achieve marked but incomplete suppression of the serum thyroxine (T4) (to 25%-50% of control levels) and serum thyrotropin (TSH) concentrations over a 10-week interval. No statistically significant change in the serum T3 was observed. At sacrifice, multiple parameters of peripheral metabolic status (growth rate, heart rate, organ weights, and tissue alpha-glycerophosphate dehydrogenase activities) in cerebrum, liver, kidney, spleen, and testes were consistent with euthyroidism. Thus, in a centrally regulated T3-predominant environment such as accompanies treatment with submaintenance doses of T3, originally euthyroid animals appear to remain in a euthyroid metabolic state.

The appearance, distribution, and longevity of receptor-[125I]T3 complexes within the nuclei of isolated rat hepatocytes.

The nuclei of isolated rat hepatocytes were separable into three receptor compartments based upon their differential salt extractabilities: nucleoplasmic receptors (NP) extractable with 0.15 M KCl, high-salt extractable receptors (HSE) extractable with 0.4 M KCl, and salt-resistant receptors (SR) extractable with 0.4 M KCl/5 mM dithiothreitol. The receptor distribution among the three compartments was approximately NP, 45%; HSE, 30%; SR, 25%. The mean percent occupancy with endogenous T3 of the SR receptors (86%) was higher than the occupancies of the NP receptors (68%) and the HSE receptors (63%). When hepatocytes were pulsed with 3 nM [125I]T3 at 37 degrees C for brief intervals, receptor-[125I]T3 complexes were detectable in all three nuclear compartments within 15 sec. With increasing pulse intervals up to 120 sec, the receptor content of each nuclear compartment increased progressively and without evidence of preferential accumulation in any of the three compartments. To determine the life span and intercompartmental "migration" pattern of nuclear receptors, hepatocytes were pulsed with 3 nM [125I]T3 at 37 degrees C for 2.5 min or 5 min, followed by a chase with a 500-fold excess of nonlabeled T3. The population of receptor-[125I]T3 complexes generated during the pulse was serially recovered at increasing intervals after the chase. The complexes of each compartment dissociated with a half-life of approximately 3 min and manifested no predilection to accumulate in any of the compartments. Exposure of isolated hepatocytes to 3 nM T3 for 5 min or 10 min at 37 degrees C induced no change in the gross intercompartmental distribution of receptors compared to control hepatocytes incubated without T3.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of human prolactin administration on gonadotropin and thyrotropin secretion in normal men.

To test the hypothesis that PRL is able to feedback negatively on its own secretion (short-loop feedback) in humans via augmentation of the turnover of tuberoinfundibular dopamine (TIDA), the effects of the administration of purified hPRL on endogenous LH, FSH and TSH were assessed. Purified hPRL, given in an i.v. loading dose of 90 micrograms followed by a continuous infusion of 1.39 micrograms/min to 4 normal male volunteers resulted in a tripling of PRL levels (10.5 +/- 1.9 micrograms/L increasing to 30.9 +/- 3.6 micrograms/L) at the end of 90 min. There were no changes in LH, FSH or TSH levels, however, during or following the infusion. Purified hPRL was also given in 1 and 8 micrograms/kg doses IM to 5 normal male volunteers. Although PRL levels did not rise significantly with the 1 microgram/kg dose, levels almost doubled with the 8 micrograms/kg dose (9.5 +/- 2.2 micrograms/L increasing to 17.4 +/- 1.5 micrograms/L). Again, LH, FSH and TSH levels did not change significantly over the three hour period of sampling with either dose. In conclusion, in this study we found that a 2-3 fold increase of circulating PRL levels maintained for 1.5-3 h exerted no apparent effects on the secretion of endogenous LH, FSH and TSH. This study provides direct evidence against the existence of a short-loop feedback occurring via TIDA activation in humans over this time interval but does not rule out the possibility that such feedback may occur with more prolonged states of hyperprolactinemia or via other mechanisms or the possibility of an effect on the hypothalamic pulse generator.

Endocrine disorders of occupational and environmental origin.

Many compounds used and produced in industry are potentially toxic to the endocrine system. The importance of medical surveillance in the workplace is emphasized in light of the increasing numbers of women in industry and the associated potential reproductive risks. Endocrine-toxic compounds also can gain wider access to entire communities through designed release to the environment (as with pesticides), accident, or disposal of toxic waste. The mechanisms of various industrial toxins in causing disease of the thyroid, testes and ovary, and pancreas are reviewed.

The hepatocellular thyroid status of uremic rats estimated from the abundances of thyroid hormone-dependent nucleoplasmic proteins.

Although the serum thyroxine (T4) and triiodothyronine (T3) concentrations of uremic rats are commensurate with moderate hypothyroidism, their thyroid status at the tissue level remains controversial. To help establish the hepatocellular thyroid status of uremic rats, a novel tissue marker (nuclear protein abundances) was evaluated in uremic rats (U), hypothyroid rats (H), and hypothyroid uremic rats (HU). Uremia was established by five-sixths nephrectomy. Moderate hypothyroidism was established by partial thyroidectomy or by provision of drinking water supplemented with propylthiouracil and T4. Normal rats (N) and pair-fed, sham-operated rats (1 to 3 weeks after surgery) served as controls. Animals were killed 1 to 5 weeks postoperatively. The following values were obtained 5 weeks after surgery, at which time the total and free serum T4 and T3 levels of the hypothyroid rats (H) were equivalent to those of the uremic rats (U). Total T4 (micrograms/dL +/- 1 SD): N, 5.4 +/- 1.7; H, 2.2 +/- 0.5; U, 1.9 +/- 1.5; HU, 0.5 +/- 0.0. Free T4 (ng/dL +/- 1 SD): N, 535 +/- 165; H, 126 +/- 37; U, 135 +/- 89; HU, 26 +/- 1. Total T3 (ng/dL +/- 1 SD): N, 63 +/- 20; H, 39 +/- 14; U, 38 +/- 18; HU, 13 +/- 4. Free T3 (ng/dL +/- 1 SD): N, 7.83 +/- 3.00; H, 3.87 +/- 1.05; U, 3.47 +/- 1.73; HU, 0.94 +/- 0.47. Hepatocellular thyroid status was estimated from the relative abundances of two nucleoplasmic proteins on polyacrylamide gel electrophoregrams.(ABSTRACT TRUNCATED AT 250 WORDS)

Tissue-specific differences in the compartmentalization of rat nuclear triiodothyronine receptors.

In addition to the recognized rat liver nuclear T3 receptor extractable with hypertonic salt, recent studies have described nucleoplasmic receptors extractable with isotonic KCl and salt-resistant receptors localized to the nuclear matrix. A method was developed for the determination of intra-nuclear receptor distribution in small samples of nuclei dispersed within glass wool matrices. After in vitro labelling with 6 nmol/l [125I]T3, dispersed nuclei were sequentially extracted with 0.15 mol/l KCl (yielding nucleoplasmic receptors), 0.4 mol/l KCl. and 2 mol/l KCl (the latter two concentrations yielding hypertonic salt-extractable receptors). The salt-resistant receptors were retained within the glass wool columns. The intra-nuclear distribution of in vivo labelled receptors was very similar to that obtained by in vitro labelling. The equilibrium association constants for L-T3 binding among the receptor pools ranged from 0.6 X 10(9) to 1.0 X 10(9) l/mol. The distribution of total nuclear receptors within each nuclear compartment was (percentage of nucleoplasmic, hypertonic salt-extractable, and salt-resistant receptors): Cerebrum: 23.6, 52.2, 24.2; Liver: 25.2, 57.2, 17.5; Kidney: 45.9, 33.5, 20.6; Testis: 65.5, 14.7, 19.7; and Spleen: 66.7, 18.7, 14.6. The rank order of percentage of hypertonic salt-extractable receptors approximates the rank order of thyroid hormone-responsiveness by traditional criteria. The inverse is true for the percentage of nucleoplasmic receptors. The percentage of salt-resistant receptors was very similar in all of the tissues.

Leukocyte alkaline phosphatase in hypothyroidism and hyperthyroidism. Response to initiation of thyroxine replacement therapy.

Leukocyte alkaline phosphatase (LAP) activity was determined in normal subjects, and in untreated, symptomatic patients with primary hypothyroidism or thyrotoxicosis. The means +/- 1 SD of (n) subjects were, respectively: 61.7 +/- 27.5 (16), 149.9 +/- 56.3 (9) and 96.9 +/- 27.7 (9). The mean LAP values of the hypothyroid and thyrotoxic groups were significantly different from that of the normal group (P less than .01). Values were above the normal range (20 to 120) in seven of the nine hypothyroid patients. LAP values were in the upper half of the normal range in eight of the nine thyrotoxic patients. In the two hypothyroid patients studied at 24-hour intervals, LAP activity was altered markedly within 48 hours of initiation of thyroxine therapy, 25 micrograms daily. In five hypothyroid patients followed for several months after initiating thyroxine replacement, LAP levels were essentially normal within 1 to 2 months. In the thyrotoxic patients, LAP values declined within the first month of medical management, but tended to remain within the normal range.

Immunoradiometric assay for basal thyroid-stimulating hormone levels: strategy for the management of thyroxine replacement.

Basal levels of thyroid-stimulating hormone (TSH) as determined by a highly sensitive immunoradiometric assay (IRMA) were evaluated in 30 clinically euthyroid patients receiving levothyroxine (T4) replacement therapy for primary hypothyroidism. In each patient, the serum TSH level had been normal as determined by conventional TSH radioimmunoassay while the patient had been receiving a constant dosage of T4 for at least three months before the study. Correlation of the TSH levels by IRMA with the concurrent concentrations of serum T4 and triiodothyronine (T3) showed that basal TSH levels by IRMA were not predictable from the serum T4 and/or T3 levels. We observed a relatively high frequency of suppression of basal TSH levels (8/30, 27%), a finding compatible with, though not necessarily indicative of, overmedication with T4. In view of the reasonable suspicion that modest but protracted overmedication with T4 may be detrimental and that suppressed TSH levels are commonly observed in conventionally managed patients, and since suppression of TSH levels is now identifiable, it seems prudent to revise the guidelines of T4 replacement therapy accordingly. In addition to maintaining clinical euthyroidism, we propose that titrating the T4 dosage to establish lower normal TSH concentrations without suppression (eg, 0.1 to 3.0 microU/ml) would assure the resolution of frank hypothyroidism, reduce the likelihood of persistent goiter, and minimize the potential for subtle but chronic overmedication.

The effects of postoperative factors on serum thyroid hormones and rat liver nuclear 3,5,3'-triiodothyronine receptor concentrations in surgical models of uremia and regenerating liver.

We have determined the individual effects of postoperative fasting, surgical/anesthetic factors, acute uremia (AU), and regenerating liver (RL) on nucleoplasmic (NP; 0.15 M KCl-extractable) and chromatin-bound (CB; 0.4 M KCl-extractable) rat liver T3 receptors. AU and RL rats were studied 24 h after bilateral nephrectomy (blood urea nitrogen, 128 +/- 13 mg/dl) or two thirds hepatectomy, respectively. The effects of postoperative fasting were assessed by comparison of normal rats (N) with control rats (N6) pair-fed to match the caloric intake of the AU and RL rats. Surgical/anesthetic effects were determined by comparison of N6 rats with sham-operated pair-fed rats (S6). The effects of AU or RL were obtained by comparison with S6 controls. Changes in mean body weight attributable to fasting (N6-N), surgical/anesthetic effects (S6-N6), acute uremia (AU-S6), and regenerating liver (RL-S6) were: -17.3 (P less than 0.001), -4.0 (P = NS), -4.5 (P less than 0.05), and -1.0 g/24 h (P = NS), respectively. Changes in mean serum T4 (N, 5.3 +/- 1.3 micrograms/dl) were: -1.0 (P = NS), -0.6 (P = NS), -0.9 (P less than 0.05), and -1.0 micrograms/dl (P less than 0.05), respectively. Changes in mean serum T3 (N, 53 +/- 23 ng/dl) were: -8 (P = NS), -18 (P less than 0.05), -10 (P = NS), and -14 ng/dl (P less than 0.05), respectively. The NP and CB receptor pools of the AU and RL rats were not significantly different from those of age-matched N rats (NP, 25 +/- 5 fmol/mg DNA; CB, 405 +/- 134 fmol/mg DNA). Chronically uremic (CU) rats 2 weeks after five sixths nephrectomy (blood urea nitrogen, 36 +/- 2 mg/dl) did not exhibit significant change in their extractable receptor pools. Complete starvation for 24 h (NO) or 72 h (NOO) generally resulted in marked reductions in receptor concentrations compared to those in age-matched N rats fed ad libitum: NP pool (N, 31 +/- 17 fmol/mg DNA): NO-N, -40% (P = NS); NOO-N, -59% (P less than 0.01); CB pool (N, 303 +/- 105 fmol/mg DNA): NO-N, -19% (P less than 0.05); NOO-N, -41% (P less than 0.001). These studies indicate that severe AU, moderate CU, and LR have relatively little effect on solubilized rat liver nuclear receptor concentrations. In contrast, complete starvation is a potent depressant of both nuclear receptor pools. In the surgical models of AU and LR, postoperative fasting was the primary cause of weight loss.(ABSTRACT TRUNCATED AT 400 WORDS)

A specific growth hormone-binding protein in human plasma: initial characterization.

Human (h) GH in plasma exists as a series of size isomers, which are in part explained by the presence of hGH oligomers. However, certain aspects of circulating large mol wt hGH, such as its high relative proportion compared to that in the pituitary, are poorly understood. To explore whether binding of hGH to plasma protein(s) could contribute to the phenomenon of large mol wt hGH, we incubated freshly prepared monomeric [125I]hGH or biosynthesized [3H]hGH with normal human plasma or serum at pH 7.4 for various time periods at 22 and 37 C. Plasma radioactive hGH patterns were then analyzed simultaneously with unincubated tracer hGH by Sephadex G-100 and G-200 chromatography. We found that part of the radioactivity was converted to a component with an apparent mol wt of 85,000, suggesting binding to a plasma protein(s). This phenomenon was inhibited in a dose-dependent fashion by unlabeled hGH. Saturation/Scatchard analysis indicated an association constant (Ka) of 2-3 X 10(8) M-1 and a maximum binding capacity of 20 ng hGH/ml plasma. Binding was rapid, reversible, and specific. A number of polypeptide hormones, including human placental lactogen, hPRL and rat GH, did not inhibit hGH binding. However, the 20K variant of hGH interacted weakly with the plasma binding component (Ka, 1.2 X 10(7) M-1; maximum binding capacity, 450 ng/ml). The binding component was heat labile and could be partially purified by gel permeation chromatography and affinity chromatography on a hGH-Sepharose column. Its estimated mol wt is 60,000-65,000, and it appears to bind one molecule of hGH to form a complex of 80,000-85,000 mol wt. The binding component is neither albumin nor an immunoglobulin. Under physiological conditions, a minimum of 15-18% of circulating hGH is presumably bound to this plasma component. We conclude that a specific, high affinity, low capacity binding protein for hGH with mol wt of 60,000-65,000 exists in normal and hypopituitary human plasma. hGH complexed with this protein forms part of big-big hGH. The biological significance of this binding protein remains to be investigated.

Serum thyroglobulin in the management of patients with thyroid cancer.

We have reviewed our experience with the management of patients with thyroid cancer to assess the potential benefits of employing the serum thyroglobulin assay in patient management programs and to determine the optimal conditions for this application. Serum thyroglobulin levels were found to be more reliable when obtained from hypothyroid patients. Levels of thyroglobulin greater than 10 ng/mL appeared to be abnormally elevated in both thyroidectomized patients prior to radioactive iodine therapy (group 1) and in thyroidectomized patients after radioactive iodine therapy (group 2). Elevated thyroglobulin levels were found to be useful indicators of the presence of metastatic disease, whereas normal thyroglobulin levels were reliable indicators of the absence of metastases. In group 1 patients, elevated thyroglobulin levels reliably predicted the presence of important total body scan uptake. In group 2 patients, normal thyroglobulin levels reliably predicted the absence of total body scan uptake. The serum thyroglobulin assay can substantially reduce the need for repetitive total body scanning in the follow-up of group 2 patients with thyroid cancer.

Environmental factors altering thyroid function and their assessment.

Chronic ingestion of modest doses of dietary iodine, radiation, and polyhalogenated biphenyls (PCB's and PBB's) are environmental factors with known or suspected adverse effects on the human thyroid. Iodine consumption in the United States is approaching 1 mg daily for a large segment of the population. Data are reviewed which support the need for concern regarding the long-term adverse effects of dietary iodine on thyroid function, particularly in certain susceptible individuals. Environmental sources of radiation pose a significant risk of thyroid cancer and hypothyroidism under certain circumstances which may be intentional, inadvertent, or accidental. Exposure to polyhalogenated biphenyls during manufacture or as industrial pollutants are hazardous to man and to wildlife in moderate or large quantities and perhaps also in small amounts. The need to investigate the potential harm posed by these factors in the quantities commonly encountered is emphasized.

Description and partial characterization of thyroid hormone-specific and thyroid hormone-dependent rat liver nuclear proteins.

A fraction of readily elutable rat liver nuclear proteins (nuclear globulins), analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, contains two proteins which reflect the thyroid status of the animal. The larger (n-band) protein is abundant in normal rat nuclei but diminished in hypothyroidism by thyroidectomy or hypophysectomy and restored with T3 treatment. It is also diminished in starvation, during which time T3 treatment of concurrent hypothyroxinemia is ineffective in tis restoration. Refeeding restores the n-band protein in starved normal rats but not in starved thyroidectomized (Tx) rats. GH does not restore this protein in either Tx or hypophysectomized (Hx) rats. The dual requirements of euthyroidism and adequate nutrition imply that the n-band protein is thyroid hormone dependent but not thyroid hormone specific. The smaller of the two nuclear globulins, the t-band protein, is prominent in Tx or Hx rats and is not altered by additional hypometabolic factors (starvation) or by nonthyroid hormone related hypermetabolic stimuli (refeeding or GH treatment of Hx rats). It is reduced in normal or T3-treated Tx or Hx rats regardless of simultaneous hypometabolic states (starvation or nonthyroid hormone-related deficiencies of hypopituitarism) or hypermetabolic states (refeeding, liver regeneration, or hyperthyroidism). The t-band protein thus appears to be inversely thyroid hormone specific in its concentration. Electrophoretic and chromatographic analysis suggest that the n-band protein is a 125,000 mol wt subunit of a 290,000 mol wt holoprotein. The t-band protein is a 70,000 mol wt peptide which exists, in part, as a monomer but largely as a subunit of a protein complex with a mol wt greater than 100,000.

The effect of thyroid hormone on in vitro rat liver mitochondrial RNA synthesis.

Liver mitochondrial preparations from normal, thyroidectomized, and triiodothyronine-treated thyroidectomized rats were assayed for in vitro RNA synthetic activity. Thyroidectomized rat mitochondrial preparations incorporated UTP into RNA at 70% the rate of normal control preparations. Mitochondrial preparations from triiodothyronine-treated thyroidectomized rats incorporated UTP at rates 35%-45% greater than those of sham-injected thyroidectomized rats. These differences were statistically significant and could not be attributed to inequalities in mitochondrial sampling, dilution of labeled precursor specific activity, nucleotide substrate concentrations, or differences in ribonuclease activities.