T3 Production Research Articles

Diphenylhydantoin stimulates the intrapituitary conversion of thyroxine to triiodothyronine in the rat.

Treatment of normal rats with diphenylhydantoin (DPH) decreases serum thyroxine (T4) and triiodothyronine (T3) levels without the anticipated rise in serum thyrotropin (TSH). The present work has studied the intrapituitary conversion of T4 to T3 in male Wistar rats, 200-250 g body weight (BW), treated with DPH 5 mg/100 g BW/day for 8 days. A tracer dose of 3',5'-[125I]T4 (150 microCi) was injected intravenously, and 2 h later hypophyses were removed and homogenized individually at 4 degrees C in ice-cold PBS buffer (pH 7.4). T4 and T3 were extracted in 400 microliters n-butanol:2 N HCl (9:1) and chromatographed in tertiary amyl alcohol:hexane: 1 N ammonia (5:1:6). In 11 untreated control rats, [125I]T3 generated from [125I]T4 deiodination was 35 +/- 6% and intact [125I]T4 was 49 +/- 9% of total chromatographic radioactivity. In 11 DPH-treated rats [125I]T3 increased (p < 0.001) and [125I]T4 decreased (p < 0.02). The DPH effect was blocked in rats treated for 2 days with iopanoic acid 10 mg/100 g BW, though blocking was not seen in rats treated with half the dose of iopanoic acid. In normal rats receiving supplemental doses of T4 (2 micrograms/100 g BW/day for 8 days), DPH similarly increased pituitary 5'-deiodination. Administration of propylthiouracil (PTU) to T4-supplemented rats had no effect on pituitary 5'-deiodination of T4, whereas the addition of DPH to PTU treatment increased [125I]T3 production (p < 0.01). Serum T4 (p < 0.001) and T3 (p < 0.01) were decreased after DPH therapy, while serum and pituitary TSH were not altered.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of epidermal growth factor, phorbol ester, and retinoic acid on hormone synthesis and morphology in porcine thyroid follicles cultured in collagen gel.

Epidermal growth factor (EGF), phorbol esters (PEs), and retinoic acid (RA) inhibit differentiated functions of thyrocytes. In the present study the inhibitory effects of these growth-promoting factors on hormone synthesis were studied in thyroid follicles cultured in type-I collagen gel, and morphologic alteration by these factors was examined by light and electron microscopy (EM). Porcine open thyroid follicles obtained by treatment with 0.1% collagenase were embedded in collagen gel and cultured in Ham's F12 medium supplemented with 6H (insulin, hydrocortisone, somatostatin, transferrin, glycyl-his-lys, and thyrotropin) + 0.5% fetal bovine serum (FBS). After 1 week these open follicles developed to closed follicles, and the medium was changed to one containing 6H + 0.5% FBS + 0.1 microM sodium iodide (NaI). Some media were supplemented with either EGF, phorbol 12-myristate 13-acetate (PMA), or all-trans RA. The closed follicles retained ability for hormone synthesis for 2 weeks after the medium change in the presence of 6H + FBS + NaI. The amounts of T4 and T3 secreted into the culture medium from day 9 to day 12 after the medium change were 60% and 45% of those from day 0 to day 4, respectively. EGF reduced production of T4 and T3 by 61% and 69%, respectively; PMA, by 87% and 99%; and RA, by 55% and 44%. In the medium supplemented with 6H + 0.5% FBS, the follicles exhibited intact polarity. Apical surfaces with microvilli were oriented to the follicular lumen and tight junctions were on the apical side of cell-to-cell contacts. Desmosomes were found on both the apical and basal halves of the cell contacts.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of hypothyroidism on hypothalamic release of thyrotropin-releasing hormone in rats.

The aim of this study was to investigate whether the severity and duration of primary hypothyroidism influence hypothalamic TRH release. Hypothyroidism was induced in male Wistar rats by treatment with different thyrostatic drugs or by thyroidectomy. Serum TSH in rats treated for up to 3 weeks with methimazole (MMI; 0.05% in drinking water) increased 20-fold, but TRH release into hypophyseal portal blood (HPB) did not change. Treatment with propylthiouracil (PTU; 0.1% in drinking water), which inhibits thyroidal T4 production and peripheral conversion of T4 to T3, resulted in a more rapid reduction in serum T3 levels and increase in serum TSH than those in rats treated with 0.1% MMI. Although these differences were no longer observed after 3 weeks of treatment, TRH release into HPB of rats treated with PTU was 34-49% higher than that in MMI-treated rats. Combined treatment with MMI (0.05-0.1% in drinking water) and iopanoic acid (IOP; 4 mg/100 g BW.day, ip), an inhibitor of both peripheral and central T4 to T3 conversion, also tended to produce a more rapid decrease in serum T3 and increase in serum TSH. After 3 weeks of treatment, serum T4, T3, and TSH were not different in the two groups, but TRH release into HPB was 48-65% increased by MMI plus IOP vs. MMI alone. Three to 10 weeks after thyroidectomy, TRH release into HPB was 58-72% higher than that in untreated controls. In vitro incubation of hypothalami isolated from rats treated for 3 weeks with MMI, MMI plus IOP, or PTU, as described above, showed that basal and 56 mM K(+)-induced TRH release were not influenced by the different drugs. Also, the total hypothalamic TRH content was not changed by any of these treatments. However, in rats treated for 1 or 2 weeks with MMI or PTU, the TRH content of the median eminence was decreased by 17-25%. These findings indicate that, depending on severity and duration, experimental hypothyroidism may cause a significant increase in hypothalamic TRH release in rats. The magnitude of these changes compared with the much larger increases in serum TSH suggests that the feedback of thyroid hormone on TSH secretion is mainly exerted at the pituitary level.

Rapid alteration in circulating free thyroxine modulates pituitary type II 5' deiodinase and basal thyrotropin secretion in the rat.

TSH secretion is decreased by both T4 and T3. This negative feedback control of TSH secretion has been correlated with an increase in pituitary nuclear T3 content, and it is not clear whether T4 exerts its effect directly on the thyrotroph or after its deiodination to T3. However, levels of the pituitary enzyme catalyzing T4 to T3 conversion, 5'D-II, are decreased in the presence of an increased amount of T4. Thus, it is unclear why the thyrotroph would have a mechanism for modulating the production of T3, if T3 is, in fact, the sole bioactive signal providing negative feedback inhibition. To examine this apparent paradox, we administered EMD 21388, a compound which inhibits the binding of T4 to transthyretin resulting in a rapid increase in circulating free T4 levels, to rats pretreated with radiolabeled T4 and T3. We observed increases in pituitary and liver T4 content of greater than 150%, without increases in the respective tissue T3 contents. The EMD 21388-treated rats also exhibited a 25% decrease in pituitary 5'D-II activity (103.8 +/- 15.8 fmol 125I released.mg protein-1.h-1, vs. control, 137.4 +/- 15.9, mean +/- SE), as did rats treated with sodium salicylate, another compound that inhibits T4-TTR binding (100.8 +/- 7.1). TSH levels significantly decreased 2 h after the administration of EMD 21388. These data demonstrate that despite a T4-mediated decrease in pituitary 5'D-II activity, an increase in T4 independently decreases TSH secretion.

Hepatic 5'‐deiodination in chickens from lines selected for high and low body weight and their Fl cross

1. Assay conditions for measuring hepatic 5'-deiodinase (5'D) activity at initial velocity, using reverse T3 as substrate, have been validated for adult chicken liver. 2. The characteristics of hepatic 5'D activity in adult chickens from lines selected for high (HW) and low (LW) juvenile body weight are similar to those in mammals and in the other birds that have been investigated. 3. Chickens from the HW line have significantly higher specific activity of hepatic 5'D, and thus potentially higher T3 production, than those from either the LW line or the F1 cross (HL) between the HW and LW lines.

In vivo effects of flavonoid EMD 21388 on thyroid hormone secretion and metabolism in rats

In vitro, the synthetic flavonoid EMD 21388 appears to be a potent inhibitor of thyroxine (T4) 5'-deiodinase and diminishes binding of T4 to transthyretin. In this study, in vivo effects of long-term administration of EMD 21388 on thyroid hormone production and metabolism were investigated. Intact male rats received EMD 21388 (20 mumol.kg body wt-1.rat-1.day-1) for 14 days. [125I]T4 and 3,5,3'-[131I]triiodotyronine (T3) were infused continuously and intravenously in a double-isotope protocol for the last 10 and 7 days, respectively. EMD 21388 decreased plasma thyroid hormone concentrations, but thyrotropin levels in plasma and pituitary did not change. Plasma clearance rates for T4 and T3 increased. Thyroidal T4 secretion was diminished, but T3 secretion was elevated. Extrathyroidal T3 production by 5'-deiodination was lower. T4 concentrations were markedly lower in all tissues investigated. Total tissue T3 was lower in brown adipose tissue, brain, cerebellum, and pituitary, tissues that express the type II 5'-deiodinase isozyme due to decreased local T3 production. Most tissues showed increased tissue/plasma ratios for T4 and T3. These results indicate that this flavonoid diminished T4 and increased T3 secretion by the thyroid, probably in analogy with other natural flavonoids, by interference with one or several steps between iodide uptake, organification, and hormone synthesis.

Significance of cation transport in control of energy metabolism and thermogenesis.

Significance of cation transport in control of energy metabolism and thermogenesis.

Possible role of adrenergic mechanism in starvation-induced reduction in circulating thyroxine and triiodothyronine in rats.

To elucidate the possible role of adrenergic mechanism in thyroid hormone metabolism during starvation, serum thyrotropin (TSH), thyroxine (T4), and 3,5,3'-triiodothyronine (T3) levels and conversion of T4 to T3 in perfused liver were investigated in fasting (60 h) and fed rats. Propranolol (0.5 mg/kg), yohimbine (0.3 mg/kg) or phentolamine (5.0 mg/kg) was subcutaneously injected to the rat every 12 h. Serum levels of TSH, T4, and T3 were significantly lower in fasting rats than in fed rats. Although propranolol, yohimbine, and phentolamine administration did not significantly alter circulating TSH, T4 and T3 levels in fed rats, phentolamine partly inhibited the starvation-induced reduction in circulating TSH, T4, and T3. Thyroxine uptake and T3 production in perfused liver were significantly lower in fasting rats than in fed rats. Phentolamine treatment did not alter the T4 uptake and T3 production in perfused liver of fasting rats. These results suggest that alpha-adrenergic mechanism may have some role in starvation-induced reduction in circulating T4 and T3, and that phentolamine partly inhibited this phenomenon probably through the inhibitory effect on reduction in circulating TSH during starvation.

Response of hepatic thyroxine 5′‐deiodinase of rainbow trout, Oncorhynchus mykiss, to chronic ingestion of 3,5,3′‐triiodo‐L‐thyronine

AbstractRainbow trout held at 6.5°C were fed food supplemented with 12 ppm T3 (3,5,3′‐triiodo‐L‐thyronine). By 2 weeks, plasma T3 had increased from 3.7 ng/ml (controls) to 12 ng/ml and stayed at that level for 12 weeks with minor change in plasma L‐thyroxine (T4). The hepatic microsomal 5′‐deiodinase (5′‐D) activity (Vmax fell to 25% of control values by 2 weeks and by 12 weeks was undetectable. Trout were also fed the T3‐supplemented food for 6 weeks and then reverted to control food (0 ppm T3) for 6 weeks. After 2 weeks reversion, plasma T3 was 48% lower and 5′‐D Vmax 78% lower than for control fish fed 0 ppm T3 throughout; after 6 weeks reversion, plasma T3 had recovered, but 5′‐D had regained only 58% of control values. These data confirm that exogenous T3 suppresses endogenous T3 production and suggest extrathyroidal autoregulation of T3 production. Furthermore, even 6 weeks after T3 treatment had ceased, hepatic 5′‐D was subnormal, indicating development of T3 dependency.

Thyroid hormones during development of a marsupial, the tammar wallaby, Macropus eugenii

The levels of thyroid hormones in the plasma and the activities of 5'-deiodinase activity in liver and kidney were determined in the tammar wallaby, Macropus eugenii, from early pouch life to adulthood. The total concentration of plasma thyroxine (T4) was below 15 nmol/l before day 75 of pouch life, rose to about 75 nmol/l at day 160, and then decreased to about 12 nmol/l in the adult. The total concentration of plasma tri-iodothyronine (T3) was below 0.4 nmol/l before day 120, increased to 3 nmol/l by about day 220 and then decreased to 1.0 nmol/l in adults. Concentrations of free T4 and free T3 followed a similar pattern but peaked at 45 and 160 pmol/l respectively. Concentrations of reverse T3 (rT3) were extremely variable, ranging from 0 to 1 nmol/l at day 100, and from 0 to greater than 2 nmol/l at day 180. After about day 230, rT3 levels fell rapidly and were below 0.3 nmol/l in adults. Liver and kidney 5'-deiodinase activities, which were undetectable before day 80, reached adult levels by day 220. Half-maximal activity of both these enzymes occurred at about day 205, mid-way between the peaks of T4 and T3. These findings suggest that the systems supporting synthesis and release of hormones from the thyroid gland are probably mature by about day 160 of pouch life in the tammar, while peripheral deiodinase activity, which is a major factor in the production of T3 in the plasma, matures by about day 220. These events thus precede the development of physiological independence of the young tammar from its mother.

Growth Hormone Induced Stimulation of the T4to T3Conversion in Fed and Fasting Dwarf Goats

Fed and food deprived (7 days) adult dwarf goats were injected intravenously with 75 micrograms/kg b. w. of ovine growth hormone (o-GH). Blood samples were collected from the jugular vein -2, -1 and 0 hr prior to and 30, 60, 90 and 120 min after injection and assayed for thyroxine (T4) and triiodothyronine (T3) concentrations. The 5'-monodeiodination (5'-D) activity was consequently determined in liver and kidney samples following slaughtering. Fasting alone increased plasma concentrations of T3 and T4, whereas injection of o-GH raised T3 additionally and more profoundly in food deprived animals compared to fed ones. A small increase in plasma T4 was also observed following o-GH injection, but only in starved goats. At the same time the hepatic, but not the kidney, 5'-D activity was stimulated in food deprived and GH injected animals. It is concluded that during prolonged fasting an increased peripheral T4 to T3 conversion is occurring contrary to the known decrease in T3 production during short periods of food deprivation. This increased conversion may be under the control of GH.

Thyroxine, 3,5,3'-Triiodothyronine, and 3,3',5-Triiodothyronine Concentrations in Several Tissues of the Rat: Effects of Amiodarone and Desethylamiodarone on Thyroid Hormone Metabolism

The effects of amiodarone, an iodinated antiarrhythmic drug, on thyroid hormone metabolism are known. It is not known whether the main metabolite, desethylamiodarone, is responsible. We investigated the influence of both compounds on the intracellular rT3, T4, and T3 concentrations in tissues of the rat, the source of T3 (plasma-derived vs. produced locally from T4) and T3 and T4 production by the thyroid. Special attention is paid to the heart. We found that both amiodarone and desethylamiodarone cause a decrease in intracellular T3 in all tissues (P less than 0.001), in most tissues an increase in T4 and a greater increase in the rT3 concentration. Both compounds inhibit both deiodination (P less than 0.0001) and T3 production by the thyroid (P less than 0.0001); T4 production was enhanced (P less than 0.05). In the heart a hypothyroid-like state, caused by decreased plasma-derived T3 (P less than 0.0001), was found. But a pool of T3 produced locally from T4 was present (21% of the total T3, P less than 0.01), which has never been demonstrated under normal conditions. This pool might play a role in the mechanism of action of the drugs. Differences between the drugs were organ-specific, but the effects of desethylamiodarone were as strong as or stronger than those of amiodarone. We conclude that desethylamiodarone was responsible for the changes, although the possibility of a common metabolite, generated later, has not been excluded.

Influence of dietary lipid and carbohydrate levels and chronic 3,5,3′-triiodo-l-thyronine treatment on thyroid function in immature rainbow trout, Oncorhynchus mykiss

The influence of varying dietary levels of nonprotein energy sources (lipid, L; carbohydrate, C) and 3,5,3'-triiodo-L-thyronine (T3) on thyroid function in immature rainbow trout was studied. Three diets of equivalent available energy content and identical nutrient composition, except for dissimilar concentrations of L and C (diet 1, L = 7%, C = 28.3%; diet 2, L = 13%, C = 14.9%; diet 3, L = 19%, C = 1.5%), were each supplemented with 0, 4, 8, or 12 ppm T3 and fed to satiation to trout at 6.5 +/- 0.5 degrees on a 12-hr photoperiod for 12 weeks. Dietary L and C concentrations did not influence plasma total L-thyroxine (T4) or T3 levels, indices of free T4 or free T3 levels, hepatic T4 5'-monodeiodinase (5'D) activity, capacity or affinity of hepatic nuclear T3 receptors, or thyroid follicle epithelial cell height. T3 treatment elevated total and free T3 levels and decreased 5'D activity (Vmax) in approximate proportion to T3 dose, and without effect on plasma total or free T4 levels or T3 receptor properties. However, thyroid follicle epithelial cell height was depressed at 8 or 12 ppm dietary T3. In trout reverted for 20 days to a T3-free diet from a T3 (12 ppm) diet, plasma total T3 levels fell to 30% of those of control trout (0 ppm T3 throughout). It was concluded that, under our experimental conditions, (i) trout thyroid function was refractory to dietary concentrations of L and C, (ii) the primary response to T3 supplementation was suppressed hepatic 5'D level and T3 production, which was sustained for at least 20 days after T3 treatment ceased, and (iii) despite causing a probable indirect decrease in thyroidal secretion, T3 did not modify the set point of the hypothalamo-hypophyseal-thyroid axis based on plasma total or free T4 levels.

Seasonal and estradiol-17β-stimulated changes in thyroid function of adult Geotria australis, a southern hemisphere lamprey

Measurable in vitro hepatic monodeiodinase activity of the southern hemisphere lamprey, Geotria australis, was present only during the first 5 of the 16 month upstream spawning migration of this species. The production of T3 from T4 in vitro was pH-sensitive, and exhibited typical Michaelis-Menton kinetics. No consistent differences in the serum T4 concentrations were found in animals sampled at different times during the period of their residence in fresh water. However, serum T3 concentrations underwent a progressive decline during this period. Estradiol-17β (E2), administered as a suspension in hydrogenated coconut oil, induced a lowering of serum T4 concentrations and a rise in serum T3:T4 ratios, but had no measureable effect on liver size and serum concentrations of total calcium and protein. In the males, E2 induced the production of a small amount of a serum protein assumed to be vitellogenin, but there was no conspicuous increase in the amount of the same protein in females. This response to E2-challenge parallels more closely that of cyprinids than that of salmonids.

Calorie restriction and iopanoic acid effects on thyroid hormone metabolism

We measured the effects of iopanoic acid on thyroid hormone metabolism in obese men during severe calorie restriction to study the nutrition regulation of thyroid hormone metabolism. Eight morbidly obese men received a weight-maintenance diet followed by 6 wk of 600 kcal/d. During underfeeding, patients received iopanoic acid or placebo for 2-wk periods in a double-blind crossover fashion. Underfeeding alone (UF) produced a 28.3% decline in the serum triiodothyronine (T3) concentration, and iopanoic acid plus underfeeding (IOP) produced a 49.5% decline in T3 concentration from baseline. Serum reverse T3 concentrations increased 289% during IOP compared with UF alone (p less than 0.001). Serum TSH concentration was unchanged by underfeeding but increased twofold during IOP treatment. Thyroid hormone kinetics demonstrated a decrease in T3 production during IOP compared with UF. These findings suggest that calorie restriction regulates T3 production by modulating only type I 5'-deiodinase activity.

Interactions of Iron Deficiency and Exercise Training Relative to Tissue Norepinephrine Turnover, Triiodothyronine Production and Metabolic Rate in Rats

The interactions of iron deficiency and exercise training relative to resting metabolic rate (RMR), tissue norepinephrine (NE) turnover and triiodothyronine (T3) production were examined in male Sprague-Dawley rats. Animals were assigned to iron-deficient (ID) or control (CN) diets and to sedentary (SD) or treadmill-exercise (EX) groups for 6 or 12 wk. Iron-deficient animals (hemoglobin 7.2 +/- 0.2 g 100 mL-1) had a 17% higher RMR and had slower growth rates than CN animals. Exercise training affected growth but not RMR in iron deficiency. Oxygen consumption (Vo2) following pharmacologic injection of norepinephrine was similar in both iron-deficient and exercised groups despite a 35% lower maximal exercise Vo2 in trained iron-deficient rats. NE turnover was lower in heart (55%) and liver (80%) of iron-deficient animals relative to controls. Trained iron-deficient animals had lower NE turnover in interscapular brown adipose tissue (IBAT) than sedentary iron-deficient animals; however, training did not alter NE turnover in control animals. In vitro liver and IBAT T3 production was similar in all groups except for lower activities (40%) in 6-wk iron-deficient, exercised animals. The significant effect of exercise on the growth attenuation of iron-deficient, exercised animals is thus not explained by increased IBAT metabolic activity or a generalized sympathetic nervous system activation. Decreased T3 production during periods of rapid growth and lean body mass development, however, may be important in exercised, iron-deficient animals.

Effects of low ambient pH and aluminum on plasma kinetics of cortisol, T3, and T4 in rainbow trout (Oncorhynchus mykiss)

Plasma kinetics for cortisol, 3,5,3′-triiodo-L-thyronine (T3), and L-thyroxine (T4), and other blood parameters were examined in catheterized rainbow trout (Oncorhynchus mykiss) exposed for 7 days to a sublethal ambient pH of 4.7 (adjusted with H2SO4), an ambient pH of 4.7 plus Al (AlKSO4, 20 μmol/L), or an ambient pH of 7.7 (control). Sublethal pH alone increased the cortisol level and the plasma degradation rate but did not change the plasma clearance rate; it did not modify plasma T3 and T4 levels and their kinetics or hepatic microsomal 5′-monodeiodinase activity; it increased plasma glucose and decreased plasma Cl− and osmolality with no change in packed cell volume. Low pH in combination with Al increased the cortisol plasma level, plasma clearance rate and plasma degradation rate; it increased the T4 plasma degradation rate but decreased the T3 plasma clearance rate, T3 plasma appearance rate, and 5′-monodeiodinase activity by increasing Km and decreasing Vmax; it also increased plasma glucose and depressed plasma Cl− and osmolality but increased packed cell volume. We concluded that Al exacerbates the effects of low ambient pH to increase cortisol prodcntion and decrease T3 production. Depressed T3 production through 5′-monodeiodinase inhibition may contribute to the poor growth of Al-exposed fish.

Increasing Age Impairs the Thyroid Hormone Response to Overfeeding

Aging in both man and rodent is associated with increases in body weight and body fat. In young adult rats, mixed calorie overfeeding increases triiodothyronine production and decreases weight gain efficiency compared with chow diets. This study was undertaken to determine whether 4- and 14-month-old adult rats have similar responses in thyroxine (T4) and triiodothyronine (T3) metabolism during mixed calorie overfeeding. Four- and 14-month-old male Sprague-Dawley rats were each separated into two groups (n = 14): control (CHOW) versus chow + mixed calorie (CAFE) overfeeding for 28 days. The 4-month-old CAFE-fed rats ingested 662 +/- 54 extra kcal over 28 days and gained 147 +/- 7 vs 113 +/- 5 g (P less than 0.01) for their age-matched CHOW group. The 14-month-old CAFE group ingested 309 +/- 45 extra kcal and gained 58 +/- 6 g weight vs -12 +/- 5 g for their age-matched CHOW group (P less than 0.01). Serum T4 concentrations were unchanged during overfeeding or aging. The serum T3 concentration was increased 24% in the 4-month-old CAFE group compared with the age-matched CHOW group (P less than 0.05), but there was no difference in the serum T3 concentration between the 14-month-old CAFE and CHOW groups. The metabolic clearance and production rates of T3 and T4 were decreased in the 14-month-old vs 4-month-old groups (P less than 0.01). The T3 metabolic clearance rate was increased in the CAFE versus CHOW group in the 4-month-old groups (137.3 +/- 11.2 vs 103.0 +/- 10.1 ml/kg/hr, P less than 0.01) but unchanged in the 14-month-old CAFE and CHOW groups (47.6 +/- 6.6 vs 53.4 +/- 5.3 ml/kg/hr, not significant). Liver type I iodothyronine 5'-deiodinase activity increased during overfeeding in the young (63.6%, P less than 0.02) but not in the older rat group. T4 and T3 production rates were decreased in the older rats and did not increase during overfeeding as observed in the young adult rat.

Early changes in thyroid hormone metabolism in the heart, liver, and brown adipose tissue during the induction of low T3 syndrome in streptozotocin-diabetic rats

In order to elucidate the day-by-day development of low T3 syndrome, we made rats diabetic by an injection of streptozotocin. Untreated controls killed at day 0 and rats treated for 8 days with insulin after they had received streptozotocin served as controls. Sub-groups of animals were killed 1, 2, 3, 4 and 8 days after streptozotocin. In serum, heart and liver, T3 was depressed to less than 50% of controls at day 4, whereas the insulin-treated rats differed from controls only as to heart T3. Heart iodothyronine 5'-deiodinase activity was depressed to a minimum at day 3 and depression was not prevented by insulin. Liver iodothyronine 5'-deiodinase activity had not reached a minimum at day 8, and again, insulin treatment did not normalize this parameter. T3 contents and iodothyronine 5'-deiodinase activity in brown adipose tissue did not differ from values in controls at any point of time. Thus, in the rats with low T3 syndrome induced by streptozotocin-diabetes, a lowered iodothyronine 5'-deiodinase activity is not fully inhibited by insulin treatment, whereas the T3 content in the liver is re-established during an observation period of 8 days. A direct toxic effect of streptozotocin seems unlikely as an in vitro study showed no influence of streptozotocin on iodothyronine 5'-deiodinase activity in the liver. The study thus indicates that iodothyronine 5'-deiodinase activity in the heart and liver is depressed in experimental diabetes, despite near optimal regulation of blood glucose, and we suggest that lowered intracellular T3 production could, after some time, result in a hypothyroid state in different tissues.

Effects of oral erythrosine (2′,4′,5′,7′-tetraiodofluorescein) on the pituitary-thyroid axis in rats

Erythrosine (FD&C Red Dye No. 3) is a tetraiodinated derivative of fluorescein. Rats fed a 4% erythrosine diet for 30 months beginning in utero have an increased incidence of thyroid adenomas and adenocarcinomas. These tumors may be secondary to increased stimulation of the thyroid gland by TSH. This study was undertaken to determine if dietary erythrosine disrupts the pituitary-thyroid axis thereby altering serum thyroid hormone levels, TSH levels, or the pituitary's response to TRH. Rats were fed diets containing erythrosine (0.5, 1.0, 4.0%), sodium iodide (0.16%), or fluorescein (1.6%) for 3 weeks, after which TRH testing was performed in vivo. Erythrosine produced a dose-dependent increase in serum T4 levels. With the 4% erythrosine diet, serum T4 and T3 levels and the free-T4 index were significantly increased, whereas the free-T3 index was unchanged. Rats fed the 4.0% erythrosine diet had an exaggerated TSH response to TRH; 10 min after the TRH injection, serum TSH levels were 80% greater than TSH levels of control rats. Short-term administration of erythrosine to rats decreased hepatic T3 production by decreasing its conversion of T4 to T3, indicating that erythrosine decreases hepatic 5′-deiodinase activity. These data demonstrate that dietary ingestion of 4% erythrosine disrupts the pituitary-thyroid axis as evidenced by an increased TSH response to TRH. This effect is mediated by erythrosine or an iodinated metabolite, since ingestion of its fluorescein nucleus had no effect. Erythrosine's effects were not likely mediated by iodide, because serum T4 and T3 levels were elevated and iodide administration did not increase the TSH response to TRH. These data suggest that erythrosine increases the pituitary's TSH response to TRH by altering thyrotroph cell conversion of T4 to T3. Chronic erythrosine ingestion may promote thyroid tumor formation in rats via chronic stimulation of the thyroid by TSH.