Re-expression of thyroxine-binding globulin in post-weaning rats during protein or energy malnutrition

A review of species differences in the control of, and response to, chemical-induced thyroid hormone perturbations leading to thyroid cancer

This review summarises the current state of knowledge regarding the physiology and control of production of thyroid hormones, the effects of chemicals in perturbing their synthesis and release that result in thyroid cancer. It does not consider the potential neurodevelopmental consequences of low thyroid hormones. There are a number of known molecular initiating events (MIEs) that affect thyroid hormone synthesis in mammals and many chemicals are able to activate multiple MIEs simultaneously. AOP analysis of chemical-induced thyroid cancer in rodents has defined the key events that predispose to the development of rodent cancer and many of these will operate in humans under appropriate conditions, if they were exposed to high enough concentrations of the affecting chemicals. There are conditions however that, at the very least, would indicate significant quantitative differences in the sensitivity of humans to these effects, with rodents being considerably more sensitive to thyroid effects by virtue of differences in the biology, transport and control of thyroid hormones in these species as opposed to humans where turnover is appreciably lower and where serum transport of T4/T3 is different to that operating in rodents. There is heated debate around claimed qualitative differences between the rodent and human thyroid physiology, and significant reservations, both scientific and regulatory, still exist in terms of the potential neurodevelopmental consequences of low thyroid hormone levels at critical windows of time. In contrast, the situation for the chemical induction of thyroid cancer, through effects on thyroid hormone production and release, is less ambiguous with both theoretical, and actual data, showing clear dose-related thresholds for the key events predisposing to chemically induced thyroid cancer in rodents. In addition, qualitative differences in transport, and quantitative differences in half life, catabolism and turnover of thyroid hormones, exist that would not operate under normal situations in humans.

Effects of Dietary Protein on Thyroid Axis Activity

Thyroid hormones (TH) are essential for the normal development and function of every vertebrate. The hypothalamic-pituitary-thyroid (HPT) axis is regulated to maintain euthyroid status. One of the most influential environmental factors that determines HPT axis activity is nutrition. Both food availability and substrate diversity affect thyroid hormone economy. The present paper aims to summarize literature data concerning the influence of the amount and the type of protein on thyroid axis activity. This review sheds light on the contribution of a low-protein diet or insufficient intake of essential amino acids to TH abnormalities. We believe that the knowledge of these dependencies could improve the results of nutritional interventions in thyroid axis disorders and enhance the efficiency of animal breeding.

In Vivo Acute Exposure to Polychlorinated Biphenyls: Effects on Free and Total Thyroxine in Rats

Hypothyroxinemia in rats has been well documented as a result of exposure to polychlorinated biphenyls (PCBs). Hypothetical mechanisms include induction of hepatic catabolic enzymes and cellular hormone transporters, and/or interference with plasma transport proteins. We hypothesized that if thyroxine displacement from transport proteins by PCBs occurs in vivo, it would result in increased free thyroxine (FT4). This study investigates the effects of a single oral dose of 2,2’,4,4’,5,5'-hexachlorobiphenyl (PCB 153 at 60 mg/kg) or 3,3’,4,4’,5,5'-hexachlorobiphenyl (PCB 169 at 1 mg/kg) on rats at 28 or 76 days of age. Total thyroxine (TT4) and FT4 were measured at 0.5, 1, 2, 4, 8, 24, or 48 hours post -dosing. Microsomal ethoxy- and pentoxy-resorufin-O-deethylase (EROD and PROD) activity and uridine diphosphoglucuronosyl transferase (UGT) activity were determined. No significant increase in TT4 or FT4 concentrations was seen at any time point. PCB 153 significantly decreased TT4 and FT4 in young and adult rats, with young rats showing a time-by-treatment interaction from 2 to 48 hours post -dosing in serum FT4. With PCB 169 exposure, young rats showed a decrease in FT4 only, whereas adult rats showed decreases in TT4 only. Hepatic EROD and PROD activities were both dramatically increased following PCB 169 and 153, respectively. Uridine diphosphoglucuronosyl transferase activity was increased only after PCB 169 exposure. These data demonstrate that neither PCB 153 nor PCB169 increased FT4, which supports the conclusion that these PCBs do not displace thyroxine from serum TTR, or if it does occur, there is no subsequent increase in serum FT4 in vivo.

Effects of leucine and phenylalanine supplementation during intermittent periods of food restriction and refeeding in adult rats

Although many studies have shown that amino acid ingestion acutely stimulates protein anabolism, only few studies have investigated whether long-term supplementation promotes changes in body composition. We therefore tested the hypothesis that l-leucine (LEU) and l-phenylalanine (PHE) supplementation might have a positive impact on the body composition of rats submitted to intermittent periods of food restriction and refeeding (weight cycling or WC). The WC protocol comprised three cycles, each consisting of 1 week of 50% food restriction followed by 2 weeks of ad libitum ingestion. The groups submitted to WC ingested the control diet (WC-CON) or the diet supplemented with LEU+PHE (WC-AA). A pair-fed group receiving the control diet (PF-CON) was used as a reference for the effects of WC. Although food intake was the same in all groups, higher body weight and energy efficiency were observed in the WC-AA group compared to the PF-CON and WC-CON groups although not significantly in relation to the latter. These results were the consequence of a significant increase of lean body mass and body protein content in the WC-AA group compared to the PF-CON and WC-CON groups. The WC-CON and WC-AA groups presented 36.1% and 18.9% more body fat, respectively, than the PF-CON group but this difference was not significant. Neither fasting insulin nor glucose concentration nor postprandial insulin secretion was significantly affected by the supplemented diet. In conclusion, supplementation with LEU+PHE improved the body composition profile of rats submitted to WC, mainly by increasing lean body mass and body protein content.

Food restriction induced thyroid changes and their reversal after refeeding in female rats and their pups

In the present study, two groups of pregnant female rats were submitted to food restriction (24 h fast versus 24 h diet intake) from the 14th day of pregnancy until either the 14th day (group B) or the 4th day after parturition (group C). All pups and their mothers were sacrificed on day 14 after delivery. The body weight of the 14-day-old pups (group B) was 46% less than the controls (group A). Free thyroxine and free triiodothyronine levels in the plasma were reduced by 44 and 16% in pups and by 20 and 36% in their mothers, respectively. These reductions were correlated with a decrease in thyroid iodine content of the pups (-50%) and their mothers (-24%). Radioiodine uptake (131I) by the thyroid gland of pups was significantly increased by 27%. Plasma TSH levels were decreased by 38% in pups and by 44% in dams. Morphological changes in thyroid glands were observed in energy restricted dams and in their pups. Some of follicles in pups were empty. Moroever in dams, we noted the presence of peripheral resorbed vacuoles, sign of thyroid hyperactivity. After a refeeding (group C) period of ten days, total recovery occurred in plasma thyroid hormone levels (FT4 and FT3) and in thyroid iodine contents of pups in spite of a partial recovery of body weights and plasma TSH levels. In dams, a partial recovery occurred in plasma thyroid hormone levels in spite of total recovery in thyroid iodine contents, while plasma TSH levels exceeded control values. A significant amelioration in thyroid histological aspects was observed in pups and their dams.

Effects of lysine deficiencies on plasma levels of thyroid hormones, insulin-like growth factors I and II, liver and body weights, and feed intake in growing chickens

Adequate (1.10%) and deficient (0.88, 0.66, and 0.53%) levels of Lys were fed to broiler chicks from 9 to 23 d of age. Groups fed the control diet (1.10% Lys) were also pair-fed daily with each deficient group. Compared with the free-fed control, graded decreases in feed intake occurred as the deficiency worsened, and these were significantly different with 0.66 and 0.53% Lys. Growth decreased significantly with each deficient level of Lys compared with the free-fed control and was always significantly lower than in the pair-fed control groups in each set. Plasma triiodothyronine (T3) was elevated in chicks fed 0.88 and 0.66% lysine but not with 0.53% when compared with the full-fed control treatment. However, in deficient chicks receiving 0.66 and 0.53% Lys, T3 levels were significantly higher compared with their pair-fed controls. Plasma T4 was not significantly different between any treatments. Liver weights decreased significantly at each level of Lys deficiency, but most of the differences disappeared when expressed relative to body weight. Plasma insulin-like growth factor (IGF)-I decreased significantly with the most severe Lys deficiency. However, it decreased to a similar degree in the pair-fed controls, showing that this effect was primarily due to the lower feed intake. Plasma IGF-II levels did not differ between any treatments. No correlations were evident between thyroid hormones and IGF-I or IGF-II values. We concluded that the primary effect of Lys deficiency was an elevation in plasma T3 levels without accompanying changes in plasma T4. No effect of the Lys deficiency per se on plasma IGF-I and IGF-II and liver weights relative to body weights was found.

Role of thyroid hormones in human and laboratory animal reproductive health

The highly conserved nature of the thyroid gland and the thyroid system among mammalian species suggests it is critical to species survival. Studies show the thyroid system plays a critical role in the development of several organ systems, including the reproductive tract. Despite its highly conserved nature, the thyroid system can have widely different effects on reproduction and reproductive tract development in different species. The present review focuses on assessing the role of thyroid hormones in human reproduction and reproductive tract development and comparing it to the role of thyroid hormones in laboratory animal reproduction and reproductive tract development. The review also assesses the effects of thyroid dysfunction on reproductive tract development and function in humans and laboratory animals. Consideration of such information is important in designing, conducting, and interpreting studies to assess the potential effects of thyroid toxicants on reproduction and development.

Effects of methionine deficiencies on plasma levels of thyroid hormones, insulin-like growth factors-I and -II, liver and body weights, and feed intake in growing chickens

We showed previously that Met deficiency at 0.25% of the diet causes elevations in plasma triiodothyronine (T3) in broilers. In the present study, plasma levels of thyroid hormones as well as insulin-like growth factors (IGF)-I and -II were measured in chicks fed 3 deficient levels of total Met. Control (0.5%) and Met-deficient diets (0.4, 0.3, and 0.2%) were fed to male broilers from 8 to 22 d of age. Additional groups of control chicks were pair-fed with the Met-deficient ones. Chicks receiving 0.4% Met increased feed intake by 10% with no significant change in body weight. The more severe Met deficiencies of 0.3 and 0.2% caused graded reductions in feed intake and weight gain. However, corresponding pair-fed control chicks were significantly heavier. These changes suggest more marked alterations in metabolic processes with 0.3 and 0.2% Met than with 0.4% Met. Liver weights were heavier in chicks fed 0.3 and 0.2% Met but not 0.4%. Plasma T3 was higher in all deficient chicks compared with the free-fed control, which was significant only with 0.3% Met. However, with 0.3 and 0.2% Met, plasma T3 was significantly elevated compared to pair-fed controls. Plasma thyroxine (T4) was lower in all deficient groups, which was significant only with 0.2% Met, whereas no significant differences occurred between deficient chicks and their pair-fed controls. Plasma IGF-I levels were not significantly different, but they were consistently lower in deficient chicks and deserve further study. Plasma IGF-II was significantly less in chicks fed 0.2% Met compared to pair-fed controls suggesting that Met deficiency interferes with IGF-II metabolism. We concluded that a deficit of dietary Met altered plasma T3 and IGF-II levels, but the effect was dependent on the degree of deficiency.

Transthyretin as a thyroid hormone carrier: function revisited

Thyroid hormones are essential for normal mammalian development and for normal metabolism. Thyroxine (T4) is the principal product synthesized by the thyroid follicles, and triiodothyronine (T3), the biologically active hormone, derives mainly from tissue T4 deiodination. More than 99% of the circulating hormone is bound to plasma proteins, mainly to thyroxine-binding globulin, transthyretin and albumin in man, and to transthyretin and albumin in rodents. The role of plasma proteins in the transport of hormones to target tissues has, for a long time, been controversial. The liver and the choroid plexus are the major sites of transthyretin synthesis, tissues from which transthyretin is secreted into the blood and the cerebrospinal fluid, respectively. Transthyretin has been proposed to mediate thyroid hormone transfer into the tissues, particularly into the brain across the choroid-plexus-cerebrospinal fluid barrier. Studies in a transthyretin-null mice strain have shown conclusively that transthyretin is not indespensable for thyroid hormones' entry into the brain and other tissues, nor for the maintenance of an euthyroid status. An euthyroid status is also observed in man totally deprived of thyroxine-binding globulin and in rats without albumin. Taken together, these results exclude dependence of thyroid hormone homeostasis on any major plasma carrier per se. This evidence agrees with the free hormone hypothesis which states that the biologically significant fraction, that is taken up by the tissues, is the free circulating hormone.

Endocrine markers of semistarvation in healthy lean men in a multistressor environment

We tested the hypothesis that key endocrine responses to semistarvation would be attenuated by changing only the food intake in a multistressor environment that also included sustained workload, inadequate sleep, and thermal strain. Serum hormones were compared within and between two groups of healthy young male volunteers participating in the 8-wk US Army Ranger course, with four repeated cycles of restricted energy intakes and refeeding: group 1 (n = 49) and group 2 (n = 48); energy deficits averaged 1,200 and 1,000 kcal/day, respectively. After 8 wk, most of group 1 achieved a minimum body fat, serum 3,5,3'-triiodothyronine (T(3)) was below normal (78 +/- 20 ng/dl), testosterone (T) approached castrate levels (4.5 +/- 3.9 nmol/l), insulin-like growth factor I (IGF-I) declined by one-half (75 +/- 25 microg/l), and cholesterol rose from 158 +/- 31 to 217 +/- 39 mg/dl. Bioavailable T(3) and T were further reduced by increases in their specific binding proteins in response to declining insulin. Refeeding, even with continuation of the other stressors, produced prompt recovery of T(3), T, and IGF-I. Higher energy intakes in group 2 attenuated the subclinical hypothyroidism and hypercholesterolemia, whereas consistent luteinizing hormone suppression indicated centrally mediated threshold effects on gonadal hormone suppression. We conclude that low T, T(3), and IGF-I remained reliable markers of acute energy deficits in the presence of other stressors; elevated cholesterol and cortisol provided information about chronic status, corresponding to diminishing body fat stores.

Low levels of plasma proteins: malnutrition or inflammation?

Levels of several plasma proteins, including albumin, transferrin, and transthyretin (prealbumin), have been proposed as markers for protein energy malnutrition. However, many other factors, especially inflammatory disease and drug or hormone therapy, affect levels of these proteins. These factors probably account for the majority of low levels of transthyretin. Levels of albumin and other proteins may be helpful in determining increased risk of morbidity and mortality, but better markers are needed for diagnosis of protein energy malnutrition per se.

Serum thyroid hormone and thyroid gland weight measurements in protein-energy malnutrition

Protein-energy malnutrition (PEM) is a problem which concerns about half the world's children. We investigated the effects of malnutrition on thyroid gland weight and thyroid hormone levels. 22 children suffering from malnutrition (14 children suffering from marasmus and 8 children suffering from kwashiorkor) and 7 healthy controls were studied. Malnutrition was confirmed clinically and according to the Wellcome classification definition of malnutrition. Serum thyroid hormone concentrations were measured by radioimmunoassay and the weights of the thyroid gland were evaluated scintigraphically. In the groups with marasmus and kwashiorkor the mean TT4, TT3 and FT3 levels were significantly lower, and TSH levels were significantly higher, compared to controls. FT4 was not influenced by PEM. The mean thyroid gland weights of the groups with marasmus and kwashiorkor were higher than that of the control group. We found no significant differences in all these parameters between groups with marasmus and kwashiorkor. In each of the three groups, the most marked positive correlation was between thyroid gland weight and ratio of thyroid gland weight to body surface area.

Growth and plasma thyroid hormone concentrations of chicks fed diets deficient in essential amino acids

Consumption of low protein (10%) diets is known to produce elevations in plasma triiodothyronine (T3) in growing chickens. Therefore, we evaluated the effect of individual essential amino acid deficiencies on plasma thyroid hormone concentrations. For 13 to 15 d, chicks were fed either a control diet free-choice, one of six amino acid-deficient diets free-choice, or the control diet, pair-fed at the level consumed by chicks fed each of the deficient diets. The control diet was a 50/50 mixture of broiler starter and purified amino acid diets. The amino acids, fed at the indicated percentages of National Research Council recommendations, were: arginine, 60%; lysine, 60%; threonine, 60%; leucine, 75%; isoleucine, 75%; and methionine, 50%. Feed consumption and weight gain were significantly lower in all deficient groups than in the free-choice control group. In all cases except leucine, deficient chicks also gained less weight than their pair-fed controls. Plasma T3 levels in the groups deficient in arginine, lysine, isoleucine, or methionine were higher than in their respective pair-fed controls. However, only with the isoleucine deficiency did T3 levels exceed those of control chicks given free access to feed. Thyroxine levels were significantly lower than control levels only with the lysine deficiency. These results suggest that changes in circulating levels of thyroid hormones in a protein deficiency may be a consequence of selected amino acid deficits, because individual essential amino acids, when deficient in the diet, do not exert the same effect on circulating levels of thyroid hormones.

Thyroid function in post-weaning rats whose dams were fed a low-protein diet during suckling

This study was designed to evaluate the thyroid and pituitary hormone levels in post-weaning rats whose dams were fed a low-protein diet during suckling (21 days). The dams and pups were divided into 2 groups: a control group fed a diet containing 22% protein that supplies the necessary amount of protein for the rat and is the usual content of protein in most commercial rat chow, and a diet group fed with a low-protein (8%) diet in which the protein was substituted by an isocaloric amount of starch. After weaning all dams and pups received the 22% protein diet. Two hours before sacrifice of pups aged 21, 30 and 60 days, a tracer dose (0.6 microCi) of 125I was injected (i.p.) into each animal. Blood and thyroid glands of pups were collected for the determination of serum T4, T3 and TSH and radioiodine uptake. Low protein diet caused a slight decrease in radioiodine uptake at 21 days, and a significant decrease in T3 levels (128 +/- 14 vs 74 +/- 9 ng/dl, P < 0.05), while T4 levels did not change and TSH was increased slightly. At 30 days, T3 and TSH did not change while there was a significant increase in both T4 levels (4.8 +/- 0.3 vs 6.1 +/- 0.2 micrograms/dl, P < 0.05) and in radioiodine uptake levels (0.34 +/- 0.02 vs 0.50 +/- 0.03%/mg thyroid, P < 0.05). At 60 days serum T3, T4 and TSH levels were normal, but radioiodine uptake was still significantly increased (0.33 +/- 0.02 vs 0.41 +/- 0.03%/mg thyroid, P < 0.05). Thus, it seems that protein malnutrition of the dams during suckling causes hypothyroidism in the pups at 21 days that has a compensatory mechanism increasing thyroid function after refeeding with a 22% protein diet. The radioiodine uptake still remained altered at 60 days, when all the hormonal serum levels returned to the normal values, suggesting a permanent change in the thyroid function.