Plasma Thyroid Hormone Concentrations Research Articles

The Effect of Oral Consumption of Perchlorate, Alone and in Combination with Ethanol, on Plasma Thyroid Hormone and Brain Catecholamine Concentrations in the Rat

Literature has reported a controversy concerning the effects of the environmental pollutant perchlorate on pertinent physiological systems. However, no research to date has evaluated the effect of concomitant consumption of perchlorate and an additional environmental contaminant on physiological systems. The present preliminary investigation served to assess the effects of oral consumption of perchlorate, alone and in combination with ethanol, on thyroid hormone and brain catecholamine concentrations in female rats of gestational age. Forty, female Myers' high ethanol-preferring rats were randomly assigned to 1 of 7 groups that received: (1) deionized water, both bottles (2) deionized water and 10% ethanol (v/v), two separate bottles (3) 300 microg/l perchlorate solution in deionized water, both bottles (4) 300 microg/l perchlorate in deionized water and in 10% ethanol (v/v), two separate bottles (5) 3000 microg/l perchlorate solution in deionized water, both bottles (6) 3000 microg/l perchlorate in deionized water and in 10% ethanol (v/v), two separate bottles (7) 0.01% propylthiouracil solution in deionized water, both bottles. At cessation of the treatment period, plasma triiodothyronine (T3) and thyroxine (T4) levels were measured by radioimmunoassay and brain area concentrations of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), and norepinephrine were measured by high performance liquid chromatography. Perchlorate consumption, alone and/or in combination with ethanol consumption, failed to produce significant alterations from control values for triiodothyronine, thyroxine, dopamine, DOPAC, or norepinephrine. The data suggest that the no-observed effect level of perchlorate consumption on thyroid hormone and brain catecholamine concentrations is above the 3000 microg/l concentration in the adult female rat.

Euthyroid Sick Syndrome in Dogs with Idiopathic Epilepsy before Treatment with Anticonvulsant Drugs

Euthyroid sick syndrome is a common finding in dogs and is attributable to nonthyroidal illness or treatment with any of a variety of drugs such as phenobarbital. In dogs with epilepsy, treatment with anticonvulsant drugs can lead to subnormal plasma thyroid hormone concentrations despite normal thyroid function. One‐hundred thirteen dogs with seizure activity were retrospectively evaluated to determine the influence of idiopathic epilepsy (IE) on thyroid hormone concentrations. Blood samples were taken from 60 dogs with IE before initiation of anticonvulsant therapy. Control groups consisted of 34 dogs with IE and receiving anticonvulsants and 19 dogs with secondary epilepsy. Thyroid concentrations consistent with euthyroid sick syndrome were diagnosed in 38% of dogs with untreated IE without clinical signs of hypothyroidism or concomitant diseases. There was a significant correlation (r= 0.363, P= .01) between seizure frequency and plasma thyroid hormone concentrations: the longer the interval between 2 seizure events, the higher the serum total thyroxine concentration. There was no correlation between the degree of alteration of thyroid hormone concentrations and the time span between the most recent seizure event and blood collection, the type of the most recent seizure event, the duration of the complete seizure history, or the predominant seizure type. These results suggest that IE can be a reason for euthyroid sick syndrome in dogs. The effect of phenobarbital on plasma thyroid hormone concentrations must be investigated in future studies, as it might be less pronounced than expected.

Changes in thyroid and vitamin A status in mink fed polyhalogenated-aromatic-hydrocarbon-contaminated carp from the Saginaw River, Michigan, USA

A study was conducted to determine whether changes in thyroid and vitamin A dynamics were induced in ranch mink exposed to environmentally relevant concentrations of polyhalogenated aromatic hydrocarbons. Adult female mink were fed diets that contained 0% (control), 10%, 20%, or 30% wild carp ( Cyprinus carpio) collected from the Saginaw River, Michigan, USA. Total polychlorinated biphenyls concentrations were 0.03, 0.83, 1.05, and 1.69 mg/kg feed, respectively; the 2,3,7,8-tetrachlorodibenzo- p-diozin toxic equivalents were 3.4, 27.9, 47.6, and 73.2 ng/kg, respectively. Diets were fed 3 weeks prior to breeding and throughout gestation and lactation. When the kits were weaned at 6 weeks of age, they were continued on their respective diets until 27 weeks of age. Plasma thyroid hormone concentrations, thyroid gland activity and structure, and vitamin A dynamics were assessed in young mink at 6 and 27 weeks of age. Plasma total T 4 and free T 4 in 6-week-old female and male kits fed the 10% carp diet were significantly higher than those of controls, while kits fed the 20% and 30% carp diet had nonsignificant decreases relative to the control mink. Plasma total T 3 concentrations in 27-week-old juvenile males fed the 30% carp diet were significantly lower than those in individuals fed the 10% carp diet. No overt thyroid toxicity was apparent as thyroid weight, activity, and structure in kits and juveniles of both sexes were similar among diet groups. Plasma retinol and total ester concentrations in both kits and juveniles were reduced in mink fed the 30% carp diet relative to controls. The ratio of retinol to retinyl palmitate in livers of juveniles fed the 30% carp diet was two times higher than that in control mink. Significant reductions in kidney retinol and fatty acyl retinyl esters were observed in kits and juveniles fed the 30% carp diet relative to control values.

Plasma total T3 and T4 concentrations in goats at different physiological stages, as affected by the energy intake

The effects of different concentrate levels in the diet on plasma 3-3′-5-triiodothyronine (T3) and thyroxine (T4) concentrations in does of different physiological conditions were investigated. Blood samples were taken starting in autumn from 20 goats fed hay ad libitum, during the dry period (30 days and 3 days prior to the synchronized oestrus; D1 and D2, respectively), pregnancy (week 13, week 17 and week 21 of pregnancy; P1, P2 and P3, respectively) and lactation (5 weeks and 13 weeks following parturition; L1 and L2, respectively). The control group (C, n = 10) was fed either 0.2 (in periods D1, D2 and P1) or 0.4 kg maize grain(head per day) (in P2, P3, L1 and L2). The high energy diet group (H, n = 10) was fed either 0.4 (in D1, D2 and P1) or 0.7 kg maize grain(head day) (in P2, P3, L1 and L2). Plasma total T3 concentrations were affected only by the sampling period, being significantly higher in the P1 period (0.96 ± 0.05 ng/ml) than in the dry period (0.72 ± 0.04 ng/ml) and in P3 (0.70 ± 0.02 ng/ml). In the C goats the plasma T4 concentration recorded in the P1 (65.6 ± 2.6 ng/ml) and L1 (62.8 ± 5.2 ng/ml) periods was significantly higher than that in P3 (40.2 ± 3 ng/ml). The variation in plasma T4 concentration in the H does was not significantly different throughout the different physiological stages. The decrease in plasma T3 and T4 concentrations in the second half of pregnancy in the H goats was delayed by 1 month. In addition, during the last 2 months of pregnancy, the higher concentrate diet induced higher plasma T3 (0.92 ± 0.06 ng/ml) and T4 (67.2 ± 6.6 ng/ml) concentrations when compared to the C (0.76 ± 0.04 and 45.1 ± 3.2 ng/ml for T3 and T4, respectively) group. The effect of energy intake on plasma thyroid hormone secretion during late pregnancy could be explained by a negative energy balance that usually occurs during this period. During lactation the higher energy diet had no significant effect on milk yield nor on the plasma thyroid hormone concentrations. The concentrate level of the C diet seemed to be sufficient to meet the energy requirements for milk production in these goats, as indicated also by the lack of body weight and body condition score (BCS) changes during lactation. The results of the present study highlight the relationships between thyroid activity and energy balance in the goat species. Circulating thyroid hormones represent a relevant metabolic index of the animals’ nutritional state.

Euthyroid Sick Syndrome in Dogs with Idiopathic Epilepsy before Treatment with Anticonvulsant Drugs

Euthyroid sick syndrome is a common finding in dogs and is attributable to nonthyroidal illness or treatment with any of a variety of drugs such as phenobarbital. In dogs with epilepsy, treatment with anticonvulsant drugs can lead to subnormal plasma thyroid hormone concentrations despite normal thyroid function. One-hundred thirteen dogs with seizure activity were retrospectively evaluated to determine the influence of idiopathic epilepsy (IE) on thyroid hormone concentrations. Blood samples were taken from 60 dogs with IE before initiation of anticonvulsant therapy. Control groups consisted of 34 dogs with IE and receiving anticonvulsants and 19 dogs with secondary epilepsy. Thyroid concentrations consistent with euthyroid sick syndrome were diagnosed in 38% of dogs with untreated IE without clinical signs of hypothyroidism or concomitant diseases. There was a significant correlation (r = 0.363, P = .01) between seizure frequency and plasma thyroid hormone concentrations: the longer the interval between 2 seizure events, the higher the serum total thyroxine concentration. There was no correlation between the degree of alteration of thyroid hormone concentrations and the time span between the most recent seizure event and blood collection, the type of the most recent seizure event, the duration of the complete seizure history, or the predominant seizure type. These results suggest that IE can be a reason for euthyroid sick syndrome in dogs. The effect of phenobarbital on plasma thyroid hormone concentrations must be investigated in future studies, as it might be less pronounced than expected.

Ontogeny and Effects of Thyroid Hormone on β1-Adrenergic Receptor mRNA Expression in Ovine Fetal Kidney Cortex

Previous studies indicate that thyroidectomy (TX) decreases renin gene expression in ovine fetal renal cortex in late gestation. Fetal ovine renin-containing renocortical cells become increasingly responsive to beta-adrenergic stimulation as gestation proceeds. Increases in plasma thyroid hormone concentrations parallel this change, suggesting that there is a positive developmental relationship between the two. To examine this hypothesis, we determined the ontogeny of beta1-adrenergic receptor (beta1R) mRNA expression, and the effect of thyroid hormone on in vivo and in vitro expression in fetal sheep. Renocortical tissue was obtained from naive, TX, and sham-operated fetuses to determine beta1R mRNA levels. Renin-containing renocortical cells from TX or sham fetuses were treated with isoproterenol (Iso) or forskolin (FSK) for analysis of cellular cyclic adenosine monophosphate (cAMP) levels. Renocortical cells from naive fetuses were treated with triiodothyronine (T3) to assess cellular beta1R mRNA levels. Fetal plasma thyroxine (T4) level was determined. Renocortical beta1R mRNA expression increased significantly between 100 and 140 days' gestational age (dGA), while TX attenuated this increase (P <.01). Renocortical cellular cAMP levels were higher in sham compared to TX fetuses following incubation with Iso or FSK (P <.05). Cells incubated with T3 exhibited significantly increased beta1R mRNA expression (P <.05). The data suggest that thyroid hormone may be involved in modulating ovine fetal renocortical beta1R gene expression during development. We speculate that the increased beta1R mRNA expression in renal cortical cells as development progresses may mediate the increases in renin gene response to beta-adrenergic stimulation in late gestation.

Plasma total T3 and T4 concentrations in bucks as affected by photoperiod

In order to determine whether total thyroxine (T4) and 3-3′-5-triiodothyronine (T3) plasma concentrations are affected by photoperiodic changes in bucks, these hormones were assayed using RIA in samples collected weekly for a year period in Alpine and Saanen male goats exposed to natural photoperiod (46°N latitude, control group), or to artificial photoperiod alternating every 2 months (4M group) or 1 month (2M) with long days (16L:8D) and short days (8L:16D). In the control group, the thyroid hormones showed marked seasonal variation. Maximal monthly peaks were recorded at the beginning of spring (April) for plasma T4 (35.7 ± 1.8 ng/ml) and in late winter to early spring (March) for plasma T3 (0.91 ± 0.11 ng/ml)—with the lowest concentrations being recorded in October (autumn) for plasma T4 (22.8 ± 1.0 ng/ml) and in September (late summer to early autumn) for plasma T3 (0.42 ± 0.01 ng/ml). In the 4M group, T4 plasma concentrations varied with daylength ( p < 0.05), recording an overall mean concentration significantly ( p < 0.01) higher during the short days (37.3 ± 7.5 ng/ml), than during long days (32.2 ± 7.3 ng/ml) with a significant effect of sampling time within each photoperiod ( p < 0.0001). In both light-treated groups, T3 varied with photoperiod ( p < 0.0001) and time ( p < 0.0001), increasing during long days and decreasing during short days. T3 plasma concentrations (derived from peripheral monodeiodination) followed the photoperiodic changes, while it was hypothesized that the stimulatory effect of daylength on T4 (thyroid gland secretory activity) was characterized by a longer latency period and could be seen in the 4M group during the following photoperiodic treatment. It is concluded that photoperiod affects thyroid hormone plasma concentrations in male goats, since different lighting regimes resulted in different profiles of both plasma T3 and T4. Bearing in mind the pivotal role of thyroid hormones in stimulating the metabolic activity of the whole body, it is possible that the light-induced increase in circulating hormones could sustain and improve animal production. The suitability of supplementary light treatments needs to be considered.

Thyroid hormone modulates renin and ANG II receptor expression in fetal sheep

Fetal renin-angiotensin system (RAS) activity is developmentally regulated, increasing in late gestation toward term. At the same time, fetal hemodynamic parameters change, with blood pressure increasing and heart rate decreasing. During this period, fetal plasma thyroid hormone concentrations also increase significantly. In this study we utilized the technique of thyroidectomy (TX), which removes thyroid hormone from the circulation, to investigate the importance of thyroid hormone on the developmental changes in the RAS (in plasma, kidney, heart, and lung) and hemodynamic regulation in fetal sheep. TX was performed at 120 days of gestational age (dGA), and control fetuses were sham operated. Immediately before necropsy ( approximately 137 dGA), fetuses were infused with isoproterenol and the hemodynamic responses were noted. TX significantly decreased plasma thyroid hormone concentrations and renal renin mRNA and renal active renin levels but did not change fetal plasma active renin levels. TX decreased both angiotensin II receptor subtype 1 (AT1) mRNA and protein levels in kidney and lung but not in the left ventricle. TX also was associated with increased ANG II receptor subtype 2 (AT2) mRNA and protein at the 44-kDa band in kidney, whereas AT2 protein was decreased at the 78-kDa level in kidney and lung tissue only. TX fetuses had significantly lower basal mean arterial blood pressures (MAP) and heart rates than controls. Isoproterenol infusion decreased MAP in TX fetuses. These findings support the hypothesis that thyroid hormone is important in modulating maturation of RAS and cardiovascular function in the late-gestation fetal sheep.

Effects of dietary exposure of 4-nonylphenol on growth and smoltification of juvenile coho salmon ( Oncorhynchus kisutch)

There is considerable concern that endocrine disrupting substances such as 4-nonylphenol (4-NP) in the freshwater environment may have adverse effects on the growth, survival, and osmoregulatory ability of salmonids during and after their transfer to sea water. This study was conducted to examine the effects of dietary exposure of coho salmon ( Oncorhynchus kisutch) to 4-NP during the parr-smolt transformation phase of their life cycle. Under laboratory conditions, juvenile fish were fed by hand twice daily to satiation diets dosed with one of several concentrations of 4-NP (doses varied between 0 (control) and 2000 mg/kg) for 4 weeks, then immediately transferred to sea water. Growth was observed for two successive 6-week periods following sea water transfer when all groups were fed the control diet (no supplemental 4-NP) only. In addition to 4-NP measurement in fish tissues, thyroid hormone concentrations in blood plasma were followed and related to diet treatment and sampling time. Dietary treatment of 4-NP did not influence the growth and smoltification of coho salmon, a result that conflicts to some extent with other reports in which deleterious effects of water-borne 4-NP on the smoltification process of salmonids were linked to disruption of the endocrine system. Appreciable concentrations of 4-NP were present in the livers, gall bladders and tissues after the 4-week exposure of coho salmon to the highest dietary dose of 4-NP, but 4-NP appeared to be effectively eliminated from the fish by the biliary-fecal pathway after sea water transfer.

Thyroid hormone status in gray seal (Halichoerus grypus) pups from the baltic sea and the atlantic ocean in relation to organochlorine pollutants

Marine apex predators, such as the gray seal (Halichoerus grypus), are exposed to high concentrations of organochlorine (OC) compounds via biomagnification in the food web. These lipophilic pollutants are transferred from the female to her offspring during lactation, and there is particular concern for endocrine disruption in the offspring. Herein we examined associations between the blubber concentrations of polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT) and metabolites, chlordanes, hexachlorocyclohexanes (HCHs) and hexachlorobenzene (HCB), and plasma concentrations of thyroid hormones (thyroxine [T4] and tri-iodothyronine [T3]) in free-ranging, newly weaned gray seal pups from the Baltic Sea and the Norwegian waters of the Atlantic Ocean. Concentrations of total T3 (TT3) and free T3 (FT3) were significantly lower in the polluted seals from the Baltic Sea. Blubber concentrations of the sum of the analyzed PCB congeners (sigmaPCBs) and the sum of the DDT compounds (sigmaDDTs) were correlated negatively with plasma TT3 and FT3 concentrations. The results from this study indicate that OCs may affect levels of thyroid hormones in free-ranging gray seal pups, and that more focus should be put on T3 when addressing the possible effects of OCs on thyroid hormone function and status in nonrodent species.

Effects of perchlorate exposure on resting metabolism, peak metabolism, and thyroid function in the prairie vole (Microtus ochrogaster)

Perchlorate, the oxidizer component in most solid rocket propellant formulations, is known to inhibit the uptake of iodide into the thyroid gland, thereby reducing production of the thyroid hormones, triiodothyronine and thyroxine (T4). Thyroid hormones regulate metabolism in endothermic organisms and are responsible for maintenance of homeothermic body temperatures. Little is known about the effects of perchlorate on metabolic capacity. The objectives of the present study were to determine if subchronic (51 d; 0, 1, and 10 mg/kg/d) and chronic (180 d; 0.75 mg/kg/d) perchlorate exposure in adult male prairie voles (Microtus ochrogaster) would alter resting metabolic rates as a result of decreased circulating thyroid hormone concentrations and to determine if perchlorate exposure disrupts thermogenesis in mammals exposed to cold stress. Voles exposed to perchlorate for 51 or 180 d experienced no significant alterations in resting metabolic rates at any point during the exposure period. Additionally, the treatment had no effect on peak metabolic rates or plasma thyroid hormone concentrations. However, thyroid gland T4 concentrations were significantly lower in perchlorate-exposed voles than in controls, indicating that thyroid gland T4 content may be a more sensitive endpoint than other thyroid variables for assessing perchlorate exposure. Overall, the present study did not provide evidence for energetic alterations associated with perchlorate exposure at concentrations that are higher than those typically found in groundwater or surface water in the environment.

Monitoring perchlorate exposure and thyroid hormone status among raccoons inhabiting a perchlorate-contaminated site

Perchlorate is a water soluble anion that is readily accumulated in vegetation. It inhibits uptake of iodide into thyroid gland tissue, thereby reducing production of thyroid hormones. Potential raccoon food items including berries, fish, and vegetation collected at a contaminated site contained quantifiable concentrations of perchlorate as determined by ion chromatography. Therefore, we monitored resident raccoons for exposure to perchlorate by examining plasma perchlorate and thyroid hormone concentrations. Resulting analytical data failed to demonstrate perchlorate exposure among raccoons that likely consumed food items collected along perchlorate-contaminated water bodies. There were no correlations between triiodothyronine or thyroxine and thyroid stimulating hormone concentrations, but triiodothyronine concentrations in raccoon plasma were significantly higher in 2000 than in 2001 (p = 0.0081). These data suggest that natural attenuation and remedial efforts initiated in January of 2001 may have reduced perchlorate exposure among raccoons inhabiting this site from 2000 to 2001. Temporal, spatial, and analytical factors limited our ability to quantify exposure among raccoons, however, our data do not indicate that raccoons currently inhabiting this site are at risk for significant exposure to perchlorate and subsequent effects.

L-Tri-iodothyronine is a major determinant of resting energy expenditure in underweight patients with anorexia nervosa and during weight gain

We aimed to define the effect of L-3,5,3'-tri-iodothyronine (T(3)) on metabolic adaptation in underweight patients with anorexia nervosa (AN) as well as during weight gain. This involved clinical investigation of 28 underweight patients with AN, who were compared with 49 normal-weight controls. A subgroup of 17 patients was followed during weight gain. Resting energy expenditure was measured by indirect calorimetry. Body composition was measured by anthropometry as well as bioelectrical impedance analysis. Energy intake (EI) was assessed by a 3-day dietary record. Plasma concentrations of thyroid hormones (thyroxine (T(4)), T(3) and thyrotropin (TSH)) were analyzed by enzyme immunoassays. When compared with normal-weight women, underweight patients with AN had reduced fat mass (FM) (-71.3%), fat-free mass (FFM) (-13.1%), resting energy expenditure (REE) (-21.8%), T(3)- (-33.4%) and T(4)-concentrations (-19.8%) at unchanged TSH. REE remained reduced after adjustment for FFM (-24.6%). T(3) showed a close association with REE. This association remained after adjustment of REE for FFM. Treatment of underweight AN patients resulted in a mean weight gain of 8.3 kg. This was mainly explained by an increase in FM with small or no changes in FFM. REE and T(3) also increased (+9.3% and +33.3% respectively) at unchanged TSH and T(4). There was a highly significant association between weight gain-induced changes in T(3) and changes in adjusted REE (r = 0.78, P < 0.001, based on Pearson's correlation). An increase in plasma T(3) concentrations of 1.8 pmol/l could explain an increase in REE of 0.6 MJ/day (that is, a 32% increase in T(3) was associated with a 13% increase in REE). Our data provide evidence that the low T(3) concentrations add to metabolic adaptation in underweight patients with AN. During weight gain, increases in T(3) are associated with increases in REE, which is independent of FFM. Both results are evidence for a physiologic role of T(3) in modulation of energy expenditure in humans.

Erythrocyte zinc content in critically ill patients

Abnormalities in thyroid hormone metabolism are common in critically ill patients. However, it is not known if these patients are truly hypothyroid at tissue level. Erythrocyte zinc has been shown to be a tissue marker of thyroid hormone status. In this study we have measured the erythrocyte zinc in critically ill patients. In this observational study we measured the zinc content of young erythrocytes in blood samples from 33 healthy subjects, 26 hypothyroid patients, four hyperthyroid patients, and 44 patients in the intensive care unit--22 of these were admitted after a major surgical procedure (surgical group) and the other 22 patients had a variety of conditions (non-surgical group). Erythrocytes were separated according to age by centrifugation. Plasma thyroid hormone concentrations were abnormal in 70% of the critically ill group. Erythrocyte zinc was significantly lower in hyperthyroid patients and higher in hypothyroid patients. In the non-surgical patients, erythrocyte zinc of young cells (median 256 micromol/L of cells) was significantly higher than (p<0.01) the corresponding cells in the healthy controls (202 micromol/L of cells), whereas in the surgical group it was not different (197 micromol/L of cells). We conclude that in non-surgical critically ill patients, erythrocyte zinc content is higher, suggesting that these patients may be hypothyroid at tissue level.

Postmetamorphic Xenopus laevis shows decreased plasma triiodothyronine concentrations and phosphorylase activity due to subacute phytosterol exposure

Postmetamorphic South African clawed frogs ( Xenopus laevis) were exposed to a phytosterol mixture (ca. 80% β-sitosterol and less sitostanol, campesterol, and campestanol) for 14 days at 30 μg l −1 in a flow-through system. The effects of phytosterols (PS) on the plasma thyroid hormone (T 3 and T 4), testosterone, leptin-immunoreactive peptide and tissue glycogen concentrations were determined. The following enzyme activities were also analyzed from the liver and muscle: glycogen phosphorylase and lipase, and from the liver only: glucose-6-phosphatase. The plasma T 3 concentration was lower in the PS-exposed female frogs. Both muscle lipase and glycogen phosphorylase activities were also lower in the PS-exposed animals. These results could indicate that the basal metabolic rate and locomotion activity of the frogs were decreased. The effects could not be attributed to the possible estrogenicity of the PS mixture. Further studies will be needed to evaluate the possible significance of these effects.

The effects of dietary vitamin E and selenium deficiencies on plasma thyroid and thymic hormone concentrations in the chicken

Beginning at hatching, male Cornell K strain single comb white leghorn chickens were fed a basal diet, with or without vitamin E (100 IU/kg) and/or selenium (Se, 0.2 ppm). After 3 weeks of treatment, animals fed either the Se-deficient or basal diet had significantly reduced plasma Se-dependent glutathione peroxidase activities when compared to those fed a vitamin E and Se-supplemented diet. Similarly, animals fed the vitamin E-deficient or basal diet had significantly reduced plasma α-tocopherol levels. The effect of these treatments on plasma concentrations of thyroid hormones (T 3/T 4), growth hormone (GH), and thymic hormone (thymulin) was determined using radioimmunoassay and ELISA. A deficiency in Se, but not in vitamin E, resulted in an increase in plasma T 4 concentrations while plasma T 3 concentrations were decreased. Plasma GH levels showed some fluctuation as a result of the dietary treatments but there was no significant correlation between plasma GH levels and any of the other variables. A significant decrease in plasma thymulin levels was observed in Se-deficient birds compared to those receiving adequate Se in the diet. A vitamin E deficiency had no measurable effect on plasma thymulin levels. From these studies, we conclude that plasma thymulin concentrations directly correlate with plasma T 3 concentrations which are negatively affected by a Se deficiency.

Regulation of pituitary thyrotropin gene expression during Xenopus metamorphosis: negative feedback is functional throughout metamorphosis.

Several hypotheses have been proposed to explain the increase and sustained expression of pituitary thyrotropin (TSH) in the presence of elevated plasma thyroid hormone (TH) concentrations at metamorphic climax in amphibians. It has been proposed that the negative feedback of TH on TSH is inoperative until metamorphic climax, and that it is established at this time by the upregulation of pituitary deiodinase type II (DII); DII converts thyroxine (T(4)) to 3,5,3'-triiodothyronine (T(3)). However, earlier investigators, using indirect measures of TSH, reported that TH negative feedback on TSH was functional in premetamorphic tadpoles. In an effort to understand pituitary TSH regulation during amphibian metamorphosis, we analyzed multiple pituitary genes known or hypothesized to be involved in TSH regulation in tadpoles of Xenopus laevis. Tadpole pituitary explant cultures were used to examine direct negative feedback on TSH mRNA expression. Negative feedback is operative in the early prometamorphic tadpole pituitary and both T(3) and T(4) can downregulate TSH mRNA expression throughout metamorphosis. The expression of both DII and TH receptor betaA mRNAs increased during development and peaked at climax; however, these increases coincided with similar increases in deiodinase type III, which inactivates TH. Moreover, corticotropin-releasing factor (CRF) receptors, CRF binding protein and thyrotropin-releasing hormone receptor type 2 mRNA expression also peaked at climax. Our data suggest that the regulation of TSH is more complex than the timing of DII expression, and likely involves a balance between stimulation of TSH synthesis and secretion by neuropeptides (e.g. CRF) of hypothalamic or pituitary origin, increased pituitary sensitivity to neuropeptides through upregulation of their receptors, and intrapituitary TH levels.

Dietary fat and body weight set point.

The level of body weight corresponding to the threshold where energy balance is spontaneously stable is influenced by factors contributing to variations in fat balance. In this regard, a high-fat diet is associated with an increase in ad libitum energy intake and can lead to an increase of several kilograms of body weight over a year. On a long-term basis, this spontaneous weight gain is expected to plateau because of the ability of fat gain to promote an increase in fat oxidation up to a level allowing the recovery of energy and fat balance. Recent evidence suggests that lipid-soluble organochlorine pollutants may also make energy and fat balance regulation more vulnerable in obese individuals by accentuating the decrease in plasma thyroid hormone concentrations, skeletal muscle oxidative enzymes, and resting metabolic rate induced by weight loss. These observations support the idea that the body weight set point may fluctuate according to the actual impact of factors influencing fat balance.

Effects of age on plasma metabolites and hormones in finishing Belgian Blue double-muscled cull females

AbstractTwo groups of 12 Belgian Blue double-muscled cull females were used successively in a 2-year repeated experiment and divided into three groups according to age, allowing four animals per group each year. The aim of the trial was to relate, during the finishing period, the metabolic and endocrine parameters with age. Females were fattened with a diet based on maize silage and were blood sampled on several occasions. The average daily gain (ADG) decreased with the age of the cows. Plasma glucose and triglycerides decreased also with age while the youngest females showed lower concentrations of urea than those older. The composition of plasma non-esterified fatty acids also differed considerably between groups. The hormones that best related with ADG were IGF-1 and insulin. Plasma concentrations of thyroid hormones were lower in the oldest animals. No difference between groups was found for GH. Heifers presented lower concentrations of GH and IGF-1 than those reported elsewhere in fattening bulls of the same breed. It may be concluded that in Belgian Blue double-muscled females, glucose, IGF-1 and insulin are good indicators of the growth potential. Young adult cows presented intermediate characteristics of metabolic and endocrine status that were close either to younger or to older cows.

Plasma thyroid hormone pattern in king penguin chicks: a semi-altricial bird with an extended posthatching developmental period

Plasma concentrations of thyroid hormones (TH) were investigated during the extended posthatching developmental period (∼11 months) of a semi-altricial bird species, the king penguin ( Aptenodytes patagonicus). The first period of growth in summer was marked by a progressive rise in plasma T 4 concentration that paralleled rapid increases in body mass and in structural and down growth. By contrast, plasma T 3 concentration had already reached adult levels in newly hatched chicks and did not change thereafter. Circulating TH of king penguin chicks thus follow an original pattern when comparing to altricial and precocial species. During the austral winter, the long period of undernutrition of king penguin chicks was characterized by a decrease in circulating TH that can be related to a seasonal stop in growth and energy saving mechanisms. Plasma TH concentrations increased again during the second growth phase in spring, and they reached their highest levels at the end of the fledging period, slightly before juveniles initiated their first foraging trip at sea. As expected, plasma T 4 levels were elevated when chicks moulted, developing a true-adult type waterproof plumage. The data also suggest that T 4 plays a major role in skeletal development and pectoral muscle maturation in anticipation of marine life. Plasma T 3 was at its highest during the period when juveniles improved resistance to cold waters by going back and forth to the sea, suggesting a role for circulating T 3 in cold acclimatization occurring at that time.