Plasma Thyroxine in the Sow During Pregnancy and Lactation and During Resumption of Ovarian Activity After Weaning

Total thyroxine in plasma was studied during pregnancy, lactation and during the post weaning period. The ovarian activity was monitored by progesterone determinations, and oestrous symptoms were recorded. In the two sows studied during pregnancy there was a distinct decrease in total thyroxine values in the last month of pregnancy, reaching a minimum about the time of farrowing. Total thyroxine values stayed low during lactation, but from about the time of weaning and during the following two weeks the concentrations increased rapidly. There was no difference in the thyroxine pattern in sows resuming ovarian activity within normal time (10 days) after weaning (72 sows) compared with sows with delayed resumption of ovarian activity (19 sows). The thyroxine level after weaning did not differ between sows with “silent 11631” and sows with overt oestrus. Primiparous and pluriparous sows had also similar thyroxine values after weaning. Sows weaned in January—June had a little higher thyroxine concentrations after weaning than sows weaned in July—December. There was a significant negative correlation between number of suckling piglets and thyroxine concentrations before weaning. Free thyroxine index was calculated in some selected samples. The results suggested that the changes observed in total thyroxine reflect changes in the free thyroxine concentrations.     

Mechanism of inhibition of malate dehydrogenase by thyroxine derivatives and reactivation by antibodies: homogeneous enzyme immunoassay for thryroxine.

Pig heart mitochondrial malate dehydrogenase (L-malate:NAD+ oxidoreductase, EC 1.1.1.37) is about 90% inhibited upon labelling an average of two amino groups per subunit with an active ester of thyroxine. Inhibition is probably associated primarily with thyroxine binding to one specific group which is normally unreactive but becomes activated upon noncovalent binding of thyroxine derivatives to the enzyme. Enzyme inhibition is due to a decrease in the rate of association of NAD. Antibodies to thyroxine induce a slow conformational change with partial reversal of inhibition of more heavily labelled conjugates. The antibody-induced activation is not cooperative and does not require bivalent association of the antibody. Activation can be blocked by the presence of free thyroxine and is the basis for a clinically useful assay for serum thyroxine.     

Effect on wool growth of thyroxine replacement in thyroidectomized merino rams

The effect on wool growth of thyroidectomy with subsequent thyroxine replacement at subnormal and supranormal levels has been investigated in Merino rams fed a restricted basal diet. Thyroidectomy without thyroxine replacement caused a greater than 60% reduction in wool growth. Provision of 30% of normal plasma thyroxine concentrations was sufficient to return wool growth to normal. Similarly, complete replacement of plasma thyroxine levels gave normal wool growth while increasing thyroxine concentrations to three times normal increased wool growth to levels slightly above normal. These results provide a possible explanation for the inability of other workers to correlate seasonal thyroxine patterns and annual wool growth variations.     

Total and free thyroid hormone concentrations in patients receiving maintenance replacement treatment with thyroxine.

Total and free serum concentrations of thyroxine and triiodothyronine were measured in 122 subjects with hypothyroidism who were clinically well while receiving conventional replacement treatment with thyroxine. In a third of patients concentrations of total and free thyroxine were raised, often considerably; nevertheless concentrations of total and free triiodothyronine were usually normal. Though significant correlations were obtained between total triiodothyronine concentrations and total thyroxine concentrations (p less than 0.001) and between the triiodothyronine concentrations and free thyroxine concentrations (p less than 0.001) the slope of the line of the regression equation describing these correlations was small, hence large increases in both total and free thyroxine concentrations were accompanied by only modest increases in total and free triiodothyronine concentrations. The presence of total or free thyroxine concentrations above normal in patients taking thyroxine therefore are not necessarily of clinical consequence. In the assessment of adequacy of replacement treatment with thyroxine the most logical combination of in vitro thyroid function test results may be a normal thyrotrophin concentration and normal free triiodothyronine concentration.     

Evidence for a secretion of thyroxine by isolated hog thyroid cells.

Isolated thyroid cells prepared from hog thyroid glands by tryptic dispersion were incubated with 131I- for 1--6 h. Free [131I]thyroxine was identified in the incubation medium by three chromatographic methods. Neither [131I]iodotyrosines nor [131I]triiodothyronine were detected. The [131I]thyroxine released in the medium by 100 mul of cells (packed cell volume) after a 6-h incubation period amounted to 1.16% (S.E. = +/- 0.39) of the total radioactivity. The medium [131I]thyroxine represented 15--25% of the total [131I]thyroxine synthesized during the 6 h of incubation. Thyrotropin, 1--60 munits/ml, increased the medium [131I]thyroxine content 2-4 fold. Dibutyryl cyclic AMP mimicked the effect of thyrotropin. The amount of medium [131]thyroxine was strictly related to the amount of incubated cells but was independent of the volume of the incubation medium. When prelabeled cells were incubated in the presence of methimazole the increase in medium [131I]thyroxine was quantitatively related to a decrease in the intracellular [131I]thyroxine. Addition of dinitrotyrosine, an inhibitor of the deiodinase activity, induced the release of iodotyrosines in the incubation medium. That the incubation supernatant of isolated thyroid cells did contain free thyroxine but not iodotyrosines suggests that the normal mechanisms of proteolysis of thyroglobulin and deiodination of iodotyrosines inside the cells are preserved. From these data, it was concluded that the thyroxine release by isolated cells represents a real secretion.     

Influence of thyroxine and luteinizing hormone on some enzymes concerned with lipogenesis in adipose tissue, testis and adrenal gland

The activities of hexokinase, citrate-cleavage enzyme, ;malic enzyme' and NADP-linked isocitrate dehydrogenase have been measured in the adipose tissue, testes and adrenals of normal rats, hypophysectomized rats and hypophysectomized rats treated with either thyroxine or thyroxine plus luteinizing hormone. Hypophysectomy reduced the activity of all four enzymes in all three tissues. Thyroxine alone restored the activity of all four enzymes in adipose tissue towards normal but failed to do so in either testes or adrenals. Thyroxine and luteinizing hormone restored the citrate-cleavage enzyme activity of testes and increased the activity of hexokinase from the low value after hypophysectomy. Neither ;malic enzyme' nor isocitrate dehydrogenase was increased by thyroxine or thyroxine and luteinizing hormone in testes. The differential stimulation of enzyme activity by thyroxine in the different tissues suggests thyroxine as having a special significance in adipose-tissue lipogenesis.     

Sister chromatid exchange and micronuclei in human peripheral blood lymphocytes treated with thyroxine in vitro

Thyroid hormones enhance the metabolic rate and the aerobic metabolism favoring oxidative stress, which is accompanied by induction of damage to cellular macromolecules including the DNA. The aim of the present study was to investigate the ability of thyroxine to induce sister chromatid exchange and micronuclei, and to modulate cell-cycle kinetics in cultured human lymphocytes. Eight experimental concentrations of thyroxine were used, ranging from 2 x 10(-9) to 0.5 x 10(-4)M. Treatment with thyroxine increased the frequency of SCE per cell at the higher concentrations (1.5 x 10(-6), 0.5 x 10(-5), 1.5 x 10(-5) and 0.5 x 10(-4)M). On the other hand, there were no significant aneugenic and/or clastogenic effects observed in the cytokinesis-block micronucleus assay. The results show that thyroxine acted as a relatively weak clastogen compared with the positive control N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). In addition to the genotoxic effects, two high concentrations of thyroxine decreased the mitotic index and caused cell-cycle delay. In conclusion, thyroxine exhibited weak clastogenic effects only at high concentrations. Therefore, effects in humans might appear in cases of acute thyroxine overdose.     

An investigation of the prolactin-thyroxine synergism in newt limb regeneration

The use of hormone replacement to support limb regeneration in hypophysectomized newts has been the subject of many investigations. Growth hormone, as well as prolactin (PL) in combination with exogenously supplied thyroxine, have all been shown to he effective. However, the bovine growth hormone used to support limb regeneration was contaminated by prolactin and thyroidstimulating hormone (TSH). The present investigation evaluates the significance of (1) prolactin contamination and (2) endogenous thyroxine synthesis resulting from TSH contamination on limb regeneration in hypophysectomized newts. The effect of supplying exogenous thyroxine was also evaluated. Our studies showed that when hypophysectomized newts were injected with contamination levels of PL and TSH, regeneration occurred, suggesting that the newt's thyroid synthesized sufficient thyroxine to support a prolactin-thyroxine synergism. The endogenous thyroxine was synthesized by thyroid glands that were indistinguishable from those of saline-injected, hypophysectomized controls.     

Reduced serum free thyroxine concentration in postmenopausal women receiving oestrogen treatment.

Thyroid hormone state was assessed in a group of postmenopausal women who had received long term treatment with oestrogen. Serum concentrations of total thyroxine, triiodothyronine, and thyroxine binding globulin were raised compared with those in a control group given placebo; serum concentrations of thyroid stimulating hormone did not differ between the groups. Oestrogen treatment resulted in a significant decrease in the serum free thyroxine concentration and in the ratio of thyroxine to thyroxine binding globulin, which supports the view that oestrogen is the causative factor of the physiological reduction in free thyroid hormone during pregnancy.     

Identification of an intracellular pathway of thyroxine synthesis by dispersed thyroid cells

This study deals with the identification of the biochemical events involved in the metabolic sequence leading from the synthesis to the release of thyroxine in the dispersed thyroid cell system. (1) Using an experimental model allowing the differentiation between intracellular and extracellular sites of iodination, it is shown that thyroxine is synthesized inside the cells by an iodinating system sensitive to thyrotropin stimulation. (2) The secretion of thyroxine synthesized inside the cells is not mediated by an exocytotic-endocytotic phenomenon. Colchicine, vinblastine, fluoride, propanolol and chlorpromazine, at concentrations equal to or 10–100-times higher than those required to inhibit hormone release in follicular-organized thyroid tissue have no effect on thyrotropin-stimulated thyroxine secretion. (3) The secretion involves the intracellular proteolysis of hormone-containing iodoprotein(s) which, in addition to free thyroxine, generates free mono- and diiodotyrosines. Free thyroxine is released into the incubation medium and iodotyrosines are deiodinated under normal conditions and accumulate in the presence of an inhibitor of iodotyrosine deiodinase: 3,5-dinitrotyrosine. This proteolysis is inhibited by 5 mM chlorpromazine. These data indicate that the complete metabolic sequence leading from the uptake of iodide to the release of free thyroxine into the incubation medium can be described as an ‘intracellular metabolic sequence for thyroxine synthesis’.     

Effects of chronic hemodialysis on thyroid function in chronic renal failure

Thyroid function was studied in 54 patients undergoing chronic hemodialysis. Serum thyroxine, triiodothyronine and free thyroxine and the free thyroxine index were significantly lower than normal. The levels of both serum thyroxine and the free thyroxine index tended to fall progressively the longer the patients were on hemodialysis. These findings, in association with low serum TSH levels and normal increase in radioactive iodine uptake by the thyroid after TSH injection, suggest that a defect in pituitary secretion of TSH may be responsible. Although some patients experienced symptomatic improvement after treatment with L-thyroxine the efficacy of this form of treatment in patients on chronic hemodialysis has not yet been established.     

On the relation of dystonic movements to serum thyroxine levels

A patient with psychomotor retardation secondary to delayed treatment of cretinism developed abnormal dystonic movements in the absence of other signs of toxicity during levothyroxine replacement therapy at a serum thyroxine level of 16 μg./100 ml. The dystonic movements disappeared when the serum thyroxine level fell. The abnormal movements were considered to be related to high thyroxine levels in this patient with pre-existing central nervous system dysfunction.     

Neonatal screening for congenital hypothyroidism by measurement of plasma thyroxine and thyroid stimulating hormone concentrations.

Neonatal screening for congenital hypothyroidism was introduced in the City of Birmingham in 1980 by measuring concentrations of both thyroid stimulating hormone and thyroxine in plasma. Over two years 30 108 babies were tested. Thirty one babies were recalled because of thyroid stimulating hormone concentrations greater than 40 mU/l, of whom 12 were treated with replacement thyroxine. Six babies were found to have low thyroxine concentrations because of reduced thyroxine binding globulin and five raised thyroxine values because of increased thyroxine binding globulin. As a result of this study screening was continued with measurement of thyroid stimulating hormone only as the primary test for congenital hypothyroidism, the thyroxine value being measured only when the concentration of thyroid stimulating hormone exceeded 20 mU/l.     

Effects of developmental and growth history on metamorphosis in the gray treefrog, Hyla versicolor (Amphibia, Anura)

In ecological models, the timing of amphibian metamorphosis is dependent upon rate of larval growth, e.g., tadpoles that experience a decrease in growth rate can initiate metamorphosis early. Recent authors have suggested that this plasticity may be lost at some point during the larval period. We tested this hypothesis by exposing groups of tadpoles of the gray treefrog, Hyla versicolor, to different growth schedules. In endocrine models, metamorphosis is dependent on thyroxine levels and thyroxine is antagonized by prolactin (amphibian larval growth hormone), consistent with the idea that a rapidly growing tadpole can delay metamorphosis. Thus, we also manipulated the rate of development by supplementing or maintaining natural thyroxine levels for half of the tadpoles in each growth treatment. All tadpoles that received thyroxine supplements metamorphosed at the same time regardless of growth history. They also metamorphosed earlier than tadpoles not treated with thyroxine. Tadpoles not given thyroxine supplements metamorphosed at different times: those growing rapidly during day 15-34 metamorphosed earlier than tadpoles growing slowly. Growth rate before day 15 and after day 34 had no effect on metamorphic timing. The difference in larval period between these rapidly growing tadpoles and their sisters given thyroxine treatments was less than the same comparison for tadpoles that grew slowly during the same period. This apparent prolactin/thyroxine antagonism did not exist after day 34. These results are consistent with the hypothesis of a loss of plasticity in metamorphic timing.     

Effects of exogenous thyroxine or prior exposure to raised water-flow on the downstream movement of hatchery-reared Atlantic salmon smolts

Administration of thyroxine to hatchery-reared Atlantic salmon smolts raised blood serum thyroxine levels 24 h later to exceed those in controls. Treatment also slowed downstream movement when smolts were released into a stream above a fish trap. The descent of smolts released immediately after being challenged with high-water flows for several hours was also slowed, and proportionately fewer challenged fish than controls were captured at the fish trap. These findings were consistent in each of two years. Only in one year, however, could flow-challenge be shown to raise endogenous thyroxine levels in blood serum. It is concluded that exogenous thyroxine treatment and flow-challenge before release have qualitatively similar effects on downstream movement but that this similarity is possibly not a consequence of the common effect of increased serum thyroxine raised either exogenously or from the thyroid system itself.     

The Evolution of Thyroidal Function in Fishes

Although the thyroid gland evolved from the gut, there is noevidence that thyroxine functions as part of the gastro-intestinalendocrine system nor does it have any major function analogousto the control of glucose by the pancreatic islets. The controlof the thyroid evolved from the pituitary control of the gonadsuggesting that an early role of thyroxine was in reproduction.This idea is supported by the presence of cycles of thyroidactivity associated with reproduction in both elasmobranchsand teleosts. In teleosts thyroxine is necessary for gonadalmaturation. The numerous other effects of thyroxine in teleostsmay have evolved from this maturational effect or have beenadded to it during the course of teleost evolution.     

Effect of thiourea on intracranial bone tumour formation in AkR mice.

The effect of administration of thiourea (5 g/kg in diet) alone or simultaneously with thyroxine (1 mg/l in drinking water) on the frequency of hyperplastic benign osteoma of the skull was studied in AkR mice. Animals treated with both thiourea and thyroxine were in hyperthyroidism: the thyroxine dose received was higher that that required to prevent thiourea-induced thyroid gland hypertrophy. A significant increase of the intracranial bone tumour (IBT) frequency was observed both in mice treated with thiourea alone and those which received thiourea and thyroxine simultaneously. Increase of IBT frequency was not due to the antithyroid effect of thiourea but seems due to a direct toxic action of thiourea on the pituitary.     

Thyroxine,methimazole, and thyroid microsomal autoantibody titres in hypothyroid Hashimoto's thyroiditis.

Ten hypothyroid patients with Hashimoto''s thyroiditis were treated with methimazole 30 mg in addition to thyroxine 0.15 mg daily. Another 10 hypothyroid patients with Hashimoto''s thyroiditis were given thyroxine 0.15 mg alone. After 22 weeks of treatment significant decreases in thyroid microsomal autoantibody titres were observed in both groups (p less than 0.01). There was no difference in the mean change in titre between the two groups. When the patients treated with methimazole were subsequently given thyroxine 0.15 mg alone for a further 22 weeks no additional change in titre was observed. The data suggest that thyroxine, by normalising serum thyroid stimulating hormone concentrations, may reduce the autoantigenic properties of the thyrocytes with a subsequent decrease in autoantibody titres.     

Temperature-responsive release of thyroxine and its environmental adaptation in Australians

The hormone thyroxine that regulates mammalian metabolism is carried and stored in the blood by thyroxine-binding globulin (TBG). We demonstrate here that the release of thyroxine from TBG occurs by a temperature-sensitive mechanism and show how this will provide a homoeostatic adjustment of the concentration of thyroxine to match metabolic needs, as with the hypothermia and torpor of small animals. In humans, a rise in temperature, as in infections, will trigger an accelerated release of thyroxine, resulting in a predictable 23% increase in the concentration of free thyroxine at 39°C. The in vivo relevance of this fever-response is affirmed in an environmental adaptation in aboriginal Australians. We show how two mutations incorporated in their TBG interact in a way that will halve the surge in thyroxine release, and hence the boost in metabolic rate that would otherwise occur as body temperatures exceed 37°C. The overall findings open insights into physiological changes that accompany variations in body temperature, as notably in fevers.