Selenium Deficiency Inhibits the Conversion of Thyroidal Thyroxine (T4) to Triiodothyronine (T3) in Chicken Thyroids

Selenium (Se) influences the metabolism of thyroid hormones in mammals. However, the role of Se deficiency in the regulation of thyroid hormones in chickens is not well known. In the present study, we examined the levels of thyroidal triiodothyronine (T₃), thyroidal thyroxine (T₄), free triiodothyronine, free thyroxine (FT₄), and thyroid-stimulating hormone in the serum and the mRNA expression levels of 25 selenoproteins in chicken thyroids. Then, principal component analysis (PCA) was performed to analyze the relationships between the selenoproteins. The results indicated that Se deficiency influenced the conversion of T₄ to T₃ and induced the accumulation of T₄ and FT₄. In addition, the mRNA expression levels of the selenoproteins were generally decreased by Se deficiency. The PCA showed that eight selenoproteins (deiodinase 1 (Dio1), Dio2, Dio3, thioredoxin reductase 2 (Txnrd2), selenoprotein i (Seli), selenoprotein u (Selu), glutathione peroxidase 1 (Gpx1), and Gpx2) have similar trends, which indicated that they may play similar roles in the metabolism of thyroid hormones. The results showed that Se deficiency inhibited the conversion of T₄ to T₃ and decreased the levels of the crucial metabolic enzymes of the thyroid hormones, Dio1, Dio2, and Dio3, in chickens. In addition, the decreased selenoproteins (Dio1, Dio2, Dio3, Txnrd2, Seli, Selu, Gpx1, and Gpx2) induced by Se deficiency may indirectly limit the conversion of T₄ to T₃ in chicken thyroids. The information presented in this study is helpful to understand the role of Se in the thyroid function of chickens.     

Sex-specific phenotypes of hyperthyroidism and hypothyroidism in mice

Background

Thyroid dysfunction is more common in the female population, however, the impact of sex on disease characteristics has rarely been addressed. Using a murine model, we asked whether sex has an influence on phenotypes, thyroid hormone status, and thyroid hormone tissue response in hyper- and hypothyroidism.

Methods

Hypo- and hyperthyroidism were induced in 5-month-old female and male wildtype C57BL/6N mice, by LoI/MMI/ClO₄ ^? or T₄ i.p. treatment over 7 weeks, and control animals underwent sham treatment (N?=?8 animals/sex/treatment). Animals were investigated for impact of sex on body weight, food and water intake, body temperature, heart rate, behaviour (locomotor activity, motor coordination, and strength), liver function, serum thyroid hormone status, and cellular TH effects on gene expression in brown adipose tissue, heart, and liver.

Results

Male and female mice showed significant differences in behavioural, functional, metabolic, biochemical, and molecular traits of hyper- and hypothyroidism. Hyperthyroidism resulted in increased locomotor activity in female mice but decreased muscle strength and motor coordination preferably in male animals. Hypothyroidism led to increased water intake in male but not female mice and significantly higher serum cholesterol in male mice. Natural sex differences in body temperature, body weight gain, food and water intake were preserved under hyperthyroid conditions. In contrast, natural sex differences in heart rate disappeared with TH excess and deprivation. The variations of hyper- or hypothyroid traits of male and female mice were not explained by classical T₃/T₄ serum state. TH serum concentrations were significantly increased in female mice under hyperthyroidism, but no sex differences were found under eu- or hypothyroid conditions. Interestingly, analysis of expression of TH target genes and TH transporters revealed little sex dependency in heart, while sex differences in target genes were present in liver and brown adipose tissue in line with altered functional and metabolic traits of hyper- and hypothyroidism.

Conclusions

These data demonstrate that the phenotypes of hypo- and hyperthyroidism differ between male and female mice and indicate that sex is an important modifier of phenotypic manifestations.

     

Association of First-Trimester Thyroid Function Test Values with Thyroperoxidase Antibody Status,Smoking, and Multivitamin Use

ObjectiveTo determine first-trimester thyroid function values and associations with thyroperoxidase antibody (TPO-Ab) status, smoking, emesis, and iodine-containing multivitamin use.MethodsWe collected information by interview, questionnaire, and blood draw at the initial obstetric visit in 668 pregnant women without known thyroid disease. We compared thyroid-stimulating hormone (TSH), total thyroxine (T₄), and free T₄ index (FT₄I) values by TPO-Ab status. Multiple regression was used to identify characteristics associated with thyroid function values.ResultsThe following median (range containing 95% of the data points) thyroid function test values were obtained in 585 TPO-Ab–negative women: TSH, 1.1 mIU/L (0.04-3.6); FT₄I, 2.1 (1.5-2.9); and T₄, 9.9 μg/dL (7.0-14.0). The following median (range containing 95% of the data points) thyroid function test values were obtained in 83 TPO-Ab–positive women: TSH, 1.8 mIU/L (0.3-6.4) (P < .001); FT₄I, 2.0 (1.4-2.7) (P = .06); and T₄, 9.3 μg/dL (6.8-13.0) (P = .03) (P values denote statistically significant differences between TPO-Ab–positive and negative participants). Among TPO-Ab–negative participants, TSH level was not associated with use of iodine-containing multivitamins, smoking, or race. TSH increased 0.03 mIU/L for every year of maternal age (P = .03) and decreased by 0.3 mIU/L for every increase in parity (P < .001). T₄ decreased 0.04 μg/dL for every year of maternal age (P = .04). Mean FT₄I was 2.05 in smokers and 2.20 in nonsmokers (P < .01). There were no relationships between T₄ or FT₄I and parity, race, or iodine-containing multivitamin use.ConclusionTPO-Ab status of pregnant women should be considered when constructing trimester-specific reference ranges because elevated serum TPO-Ab levels are associated with higher TSH and lower T₄ values. (Endocr Pract. 2008;14:33-39)     

Use of the Free Thyroxine Index to Refine the Lower Limit of a Free Thyroxine Immunoassay for Detection of Secondary Hypothyroidism

ObjectiveTo determine the utility of measuring free T₄ index (FT₄I) in patients with low free T₄ (FT₄) levels using immunoassay and normal thyroid-stimulating hormone for the evaluation of secondary hypothyroidism.MethodsWe performed a retrospective medical chart review of patients seen at a single institution as outpatients who had a simultaneously normal thyroid-stimulating hormone level, low FT₄ level, and any FT₄I measured between June 2014 and October 2016. Demographic, laboratory, and imaging data were collected. Using FT₄I as the reference for diagnosis of hypothyroidism, the sensitivity and specificity of the FT₄ immunoassay’s lower-limit thresholds were determined. Within each threshold group, available brain imaging and biochemical evaluation were categorized according to the presence or absence of pituitary disease.ResultsA total of 155 sets of result pairs (FT₄ and FT₄I) performed on 118 subjects were analyzed. The lower limit of a normal FT₄ level by immunoassay at this institution was 0.93 ng/dL, though all pairs with FT₄ ≥0.89 ng/dL had a normal FT₄I. All pairs with FT₄ ≤0.67 ng/dL had a low FT₄I. No pituitary macroadenomas were identified in any subject, though the rates of pituitary imaging in this patient sample were low.ConclusionPatients with a borderline low FT₄ level by immunoassay often have normal FT₄I. In such patients at our center, significant structural and biochemical pituitary pathology was uncommon.     

Conversion and binding of tetraiodothyronine in developing rat brain

In this study we have examined whether rat brain nuclear thyroid hormone receptors bind T₄ or metabolites of T₄ and whether there is a developmental change in brain T₄ metabolism and binding. Developing animals were injected with trace [¹²⁵I]3,5-tetraiodothyronine ([¹²⁵I]T₄) and after sacrifice brain nuclear and cytoplasmic fractions were examined to determine whether their radioactivity was represented by the injected [¹²⁵I]T₄ or any of its metabolites. Of the radiothyronines specifically bound to the nucleus, 90% was found to be triiodothyronine ([¹²⁵I] T₃) and 10% was [¹²⁵I]T₄. Of the cytoplasmic, protamine sulfate-precipitable fraction, 40% was [¹²⁵I]T₄ and 60% [¹²⁵I]T₃. Inasmuch as the percentage of [¹²⁵I] T₃ found in plasma during the same postinjection interval was similar to that present as contaminant of the injected material, it was concluded that brain [¹²⁵I] T₃ derives from local monodeiodination of T₄ to T₃. The main developmental change observed was a marked decline in the total cytoplasmic and nuclear [¹²⁵I] T₄ uptake. However, with development, the T₃/T₄ ratio remained constant in the nuclear fraction while it decreased in the cytoplasmic fraction. It is concluded that although T₃, deriving from monodeiodianation of T₄, is the main form of thyroid hormone that regulates brain development by its binding to brain nuclear receptors, the fact that T₄ is the most available from during the critical period makes it, indirectly, very important to brain development. Further, the decline observed with development in T₄ uptake and monodeiodination to T₃, may contribute to the concomitantly declining role of thyroid hormones on brain tissue.     

Genetics of heat tolerance and thyroid function in Athens-Canadian randombred chickens

Summary Five experiments were conducted to assess the genetic variation in thyroid function (T₃, T₄), body weight and heat stress survival time in chickens. Thyroxine (T₄) levels were found to be elevated in response to 4 and 8 g bovine thyroid stimulating hormone (TSH) in experiment I. In experiment II, 4 g of TSH was injected into chickens from 30 sire families of the Athens-Canadian Randombred population. The heritability of T₄ levels after TSH injection was high. In experiment III, families identified as having innate high or low T₄ levels after TSH injection and a group of control birds were subjected to a heat Stressor of 50 °C for up to 240 min at six weeks of age and heat stress survival time was studied. The groups did not differ from each other in heat stress survival time. Experiment IV was similar to experiment I except triiodothyronine (T₃) was also measured after TSH injection. Both T₄ and T₃ levels after TSH injection were moderately heritable. In experiment V birds were reared to six weeks of age and heritability calculated for body weight, T₄, T₃, and heat stress survival time. Heritabilities were high for body weight, moderate for T₄ and heat stress survival time, and low for T₃. Phenotypic correlations were significant and negative for heat stress survival time with body weight and T₄, and for body weight with T₃ after TSH. Significant positive correlations were found for T₄ with T₃ after TSH and also T₄ and body weight. Analysis of genetic correlations suggested that none of the traits studied would be an adequate selection parameter for achieving heat tolerance without reducing body weight.Supported by State and Hatch funds allocated to the Georgia Agricultural Experiment Stations of the University of Georgia     

Further studies on the regulation of the Harderian glands of golden hamsters by the thyroid gland

Summary Long-term increased or decreased circulating levels of thyroid hormones significantly modify porphyrin concentrations and morphology in the Harderian glands of male and female hamsters. Administration of T₃ reduced porphyrin concentrations in females; this treatment or decreasing thyroid hormone levels with KClO₄ suppressed the post-castration rise of porphyrins in males. Hypophysectomy led to increased porphyrins in the Harderian glands of males; this rise was suppressed in hypophysectomized males by T₃ or T₄. In females, hypophysectomy reduced porphyrins which were further reduced by daily administration of T₃ or T₄. These modifications in the normal females were identical in castrated males. Mitotic activity in the Harderian glands of females was stimulated by KClO₄ and by hypophysectomy with or without exogenous T₃. In males, castration increased mitotic activity which was suppressed by T₃ and exacerbated by KClO₄. Increased mitotic activity seemingly follows loss of tissue mass. The data show that thyroid hormones act directly on the Harderian glands rather than indirectly through modification of TSH synthesis/release. Female type glands in males are a consequence of loss of gonadal androgens by castration, or by suppression or loss of thyroid hormones by hypophysectomy or by treatment with KClO₄. However, male type glands in females are the result of androgen treatment, and/or increased levels of thyroid hormones via reduced ambient temperatures or of photic input. We conclude that regulation of the Harderian gland appears to be different in the two sexes.Abbreviations T ₃ Triiodothyronine - T ₄ Thyroxine - TSH Thyroid Stimulating Hormone - KClO ₄ Potassium Perchlorate - h hours - ml milliliter - mg milligram - g gram - male - female - castrated male - AP hypophysectomized - CON Control - ALA delta aminole-vulenic acid - HG Harderian Gland     

Evaluation of zinc in the regulation of serum T3 and T4 levels and hepatic functions in carbontetrachloride-intoxicated rats

Circulating tri-iodothyronine (T₃) and thyroxine (T₄) concentrations were determined after 6 wk of zinc treatment to carbontetrachloride (CCl₄) intoxicated male albino rats. Concentrations of T₃ were observed to be significantly depressed following CCl₄ treatment alone. On the contrary, no significant change was noticed in the concentrations of T₄ when compared to controls. However, zinc administration to hepatotoxic animals resulted in restoring the T₃ activity to within normal limits, thus indicating the indirect effects of zinc on the regulation of thyroid hormone concentrations. The activities of all the serum and hepatic marker enzymes were found to be significantly elevated following CCl₄ treatment. However, following zinc supplementation to these intoxicated animals, the levels of the marker enzymes decreased significantly when compared to the CCl₄-treated animals. A similar trend was seen in the case of lipid peroxidation following zinc treatment.     

Pig Epidermis: A Cell Kinetic Study

The basal cell density (BCD), labelling index (LI), duration of DNA synthesis (T_s) and cell cycle time (T_c) have been calculated for the epidermis of pigs in the age range 4–27 months. the BCD declined progressively from 143.4 ± 6.5 cells/mm at 4 months to 128.8 ± 8.3 cells/mm at 15 months, whereafter the values showed little change. There was a small decrease in LI with increasing age, from 7.9 ± 1.5% at 4 months to 5.9 ± 1.0% at 27 months. However, the change to housing animals outdoors as compared with indoors had a greater effect on the LI (～10%). Severe weathering in the skin of animals housed outdoors resulted in a very high LI (～20%). Neither T_s or T_c varied significantly with age. T_s was within the range 8.8–9.2 hr and T_c 127–161 hr. In animals housed outdoors T_c was reduced relative to animals housed indoors. the BCD and T_s were not affected by housing conditions. the kinetic parameters investigated in the pig were similar to those reported for man.     

Effects of summer- and winter-like acclimation on the thermoregulatory behavior of fed and fasted desert hamsters, Phodopus roborovskii

1.

Thermoregulatory behavior of fed and fasted desert hamsters (Phodopus roborovskii) acclimated to summer- [16 light (L):8 dark (D), ambient temperature (T_a)=26.5 °C] and winter-like (8L:16D, T_a=10 °C) conditions was studied. Body temperature (T_b), selected temperature and activity were measured in hamsters placed in a thermal gradient system for 48 h.     

Harderian glands of golden hamsters: morphological and biochemical responses to thyroid hormones

Summary Manipulation of circulating levels of thyroid hormones modifies Harderian gland structure and porphyrin concentrations in male and female golden hamsters. Specifically, thyroxine (T₄) and triiodothyronine (T₃) induce the morphological conversion of the Harderian glands of females to approximate those of the male. Further, porphyrin concentrations are markedly decreased by this treatment. This effect occurs in ovariectomized animals as well, indicating that the gonads are not involved. Suppression of thyroid function by potassium perchlorate (KClO₄) drastically reduces Harderian gland weight in both males and females. However, KClO₄ decreases porphyrin levels in the Harderian glands of females and increases it in the male. Concurrently, KClO₄ also induces a morphological conversion of the Harderian glands of males to the female type. This effect is evident in photoperiods of either 14:10 (h) or 8:16 (h).     

Chronic exposure to short photoperiod inhibits free thyroxine index and plasma levels of TSH, T4, triiodothyronine (T3) and cholesterol in female Syrian hamsters

1. Chronic exposure of female Syrian hamsters (Mesocricetus auratus) for 9 weeks to a short photoperiod (10L:14D) depressed the pituitary-thyroid axis as indicated by a drop in circulating titers of thyroid stimulating hormone (TSH), thyroxine (T4), triiodothyronine (T3) and the free thyroxine index (FT4I) compared to animals maintained under long photoperiodic conditions (14L:10D). 2. Short day treatment also reduced plasma cholesterol levels. 3. Neither plasma triglycerides, glucose nor growth hormone (GH) levels differed between hamsters exposed to short or long daily photoperiods.     

The relationships among endurance performance measures as estimated from VO<Subscript>2PEAK</Subscript>, ventilatory threshold,and electromyographic fatigue threshold: a relationship design

Background

The use of surface electromyography has been accepted as a valid, non-invasive measure of neuromuscular fatigue. In particular, the electromyographic fatigue threshold test (EMG_FT) is a reliable submaximal tool to identify the onset of fatigue. This study examined the metabolic relationship between VO_2PEAK, ventilatory threshold (VT), and the EMG_FT, as well as compared the power output at VO_2PEAK, VT, and EMG_FT.

Methods

Thirty-eight college-aged males (mean ± SD = 22.5 ± 3.5 yrs) performed an incremental test to exhaustion on an electronically-braked cycle ergometer for the determination of VO_2PEAK and VT. Each subject also performed a discontinuous incremental cycle ergometer test to determine their EMG_FT value, determined from bipolar surface electrodes placed on the longitudinal axis of the vastus lateralis of the right thigh. Subjects completed a total of four, 2-minute work bouts (ranging from 75–325 W). Adequate rest was given between bouts to allow for subjects' heart rate to drop within 10 beats of their resting heart rate. The EMG amplitude was averaged over 10-second intervals and plotted over the 2-minute work bout. The resulting slopes from each successive work bout were used to calculate EMG_FT.

Results

Power outputs and VO₂ values from each subject's incremental test to exhaustion were regressed. The linear equations were used to compute the VO₂ value that corresponded to each fatigue threshold. Two separate one-way repeated measure ANOVAs indicated significant differences (p < 0.05) among metabolic parameters and power outputs. However, the mean metabolic values for VT (1.90 ± 0.50 l·min^-1) and EMG_FTVO₂(1.84 ± 0.53 l·min^-1) were not significantly different (p > 0.05) and were highly correlated (r = 0.750). Furthermore, the mean workload at VT was 130.7 ± 37.8 W compared with 134.1 ± 43.5 W at EMG_FT (p > 0.05) with a strong correlation between the two variables (r = 0.766).

Conclusion

Metabolic measurements, as well as the power outputs at VT and EMG_FT, were strongly correlated. The significant relationship between VT and EMG_FT suggests that both procedures may reflect similar physiological factors associated with the onset of fatigue. As a result of these findings, the EMG_FT test may provide an attractive alternative to estimating VT.

     

Interaction of harderian glands,illumination, and temperature on thyroid hormones in golden hamsters

Summary In this study, Harderianectomy (Hdx) has been shown to differentially modify circulating levels of the thyroid hormones, T₄ and T₃, in male and female golden hamsters exposed to low photic intensities or to moderately low temperatures.Specifically, low photic intensities depress circulating levels of T₄ in both control and Harderianectomized (Hdx) male and female hamsters. In addition, T₃ is decreased in both control and Hdx males but not in females, as a consequence of reduced levels of illumination. Moderately low temperatures (10°C) depress T₄ in both control and Hdx males but not in females, while T₃ is increased in both control male and female hamsters, and in Hdx males, but not Hdx females. The data suggest that the Harderian gland of males enhances the sensitivity of the TSH-thyroid axis to photic intensity and to lower temperatures, and further, that there is a considerable sexual difference in the role the Harderian glands may play in response to these two environmental factors.     

The relationship between heat-stress and photobleaching in green and blue-green algae

Two characteristic temperatures were identified from measurements of the temperature dependence of O₂ evolution by Chlorella vulgaris and Anacystis nidulans: T₁, the threshold temperature for inhibition of O₂ evolution under saturating light conditions, and T₂, the upper temperature limit for O₂ evolution. Measurement of delayed light emission from photosystem II (PSII) showed that it passed through a maximum at T₁ and was virtually eliminated on heating the samples to T₂. Related changes were observed in low-temperature (77K) fluoresence emission spectra. Heat-stress had little effect on the absorption properties of the cells at temperatures below T₁ but incubation at higher temperatures, particularly under high-light conditions, resulted in extensive absorption losses. An analysis of these measurements suggests that this increased susceptibility to photobleaching is triggered by an inhibition of the flow of reducing equivalents from PSII that normally serves to protect the light-harvesting apparatus of the cells from photo-oxidation. Adaptation to higher growth temperatures resulted in increases in the values of T₁ and T₂ for Anacystis nidulans but not for Chlorella vulgaris.Abbreviations PSI photosystem I - PSII photosystem II - Chl a chlorophyll a - Chl b chlorophyll b - DCMU 3-(3 4 dichlorophenyl)-11-dimethylurea - PC plastocyanin - APC allophycocyanin CIW-DPB Publication No. 887.     

Thyroid function and thyrotropin levels in rabbits immunized to produce antibodies against thyroid hormones

Various parameters of thyroid function were studied in 27 rabbits, out of which 10 were immunized to produce antibodies against triiodothyronine (T₃), 9 against thyroxine (T₄) and 8 were normals. Estimations of T₃, T₄, Free T₄ (FT₄) and thyrotropin (TSH) in blood, qualitative and quantitative analysis of iodoamino acids in serum, protein bound iodine-131 (PB¹³¹I), butanol extractable iodine-125 (BE¹²⁵I) and measurement of the disappearance rates of ¹²⁵I-labelled T₃ and T₄ from plasma were done. In addition, glandular changes were also studied by measurement of ¹³¹I uptake, thyroid scanning and chromatographic analysis of hydrolysate of soluble iodoproteins. In T₃ immunized animals, levels of T₃ in serum increased by 38 to 125 times, levels of TSH also showed a significant rise (7.4 ± 1.2 vs 28 ± 9 ng/mL). Chromatographic analysis of iodoamino acids in serum as well as in the hydrolysate of the thyroid gland demonstrated a selective increase in synthesis of T₃. Rate of disappearance of T₃ from blood showed a significant decline. Thyroid glands in the immunized rabbits showed signs of hypertrophy and hyperplasia. Identical studies done in rabbits immunized to produce antibodies against T₄ showed a similar pattern though of variable degree. Our studies indicate that the thyroid glands of the immunized rabbits undergo marked alterations resulting in selective increase in the synthesis and secretion of the particular thyroid hormone against which they were immunized. They do so under the influence of increased levels of TSH.     

Serum T4, T3 and reverse T3 in ethanol fed rats.

To investigate the influence of chronic ethanol consumption on circulating thyroid hormone levels, male and female rats were given 20% ethanol as the only drinking solution daily for 8 weeks. Blood ethanol levels ranged 30–45 mg/L. In male rats serum T₄ decreased from the initial mean ± SD value of 5.2±1.4 to3.0 ±0.7 μg/dl; T₃ decreased from initial value of 97±14 to 66±11 ng/dl and rT₃ decreased from initial value of 19±9 to 10±1 ng/dl after 8 weeks of ethanol ingestion. Under similar experimental conditions, female rats showed a significant decrease in serum T₄ and rT₃ levels; however, T₃ levels decreased slightly but not significantly as compared to initial values. The results indicate adverse effect of chronic ethanol intake on serum thyroid hormone levels in rats.     

Free and total thyroid hormones in humans at extreme altitude

Alterations in circulatory levels of total T₄ (TT₄), total T₃ (TT₃), free T₄ (FT₄), free T₃ (FT₃), thyrotropin (TSH) and T₃ uptake (T₃U) were studied in male and female sea-level residents (SLR) at sea level, in Armed forces personnel staying at high altitude (3750 m) for prolonged duration (acclimatized lowlanders, ALL) and in high-altitude natives (HAN). Identical studies were also performed on male ALL who trekked to an extreme altitude of 5080 m and stayed at an altitude of more than 6300 m for about 6 months. The total as well as free thyroid hormones were found to be significantly higher in ALL and HAN as compared to SLR values. Both male as well as female HAN had higher levels of thyroid hormones. The rise in hormone levels in different ALL ethnic groups drawn from amongst the southern and northern parts of the country was more or less identical. In both HAN and ALL a decline in FT₃ and FT₄ occurred when these subjects trekked at subzero temperatures to extreme altitude of 5080 m but the levels were found to be higher in ALL who stayed at 6300 m for a prolonged duration. Plasma TSH did not show any appreciable change at lower altitudes but was found to be decreased at extreme altitude. The increase in thyroid hormones at high altitude was not due to an increase in hormone binding proteins, since T₃U was found to be higher at high altitudes. A decline in TSH and hormone binding proteins and an increase in the free moiety of the hormones is indicative of a subtle degree of tissue hyperthyroidism which may be playing an important role in combating the extreme cold and hypoxic environment of high altitudes.     

Effect of chronic ethanol administration on the rat pineal N-acetyltransferase and thyroxine type II 5′-deiodinase activities

Chronic ethanol intake resulted in a significant decrease in the rate of rat ponderal growth and an impaired nyctohemeral profile of pineal N-acetyltransferase (NAT) activity. In ethanol-treated animals, the onset of the nocturnal NAT increase is delayed by 2 hours when compared to control animals. Moreover, pineal NAT nocturnal peak was reached at 4 h (2 hours later than controls), while pineal type II thyroxine 5-deiodinase (5-D) nyctohemeral profile was not modified by ethanol administration. The effect of ethanol administration (12 weeks) on 5-D activity in different tissues was also studied. Ethanol induced a 5-D activity increase in hypothesis and brain frontal cortex, when compared to control animals. No change in 5-D activity is observed in either pineal gland, Harderian gland, or brown adipose tissue. Since basal values of 5-D activity in hypophysis or brain frontal cortex are particularly dependent on serum thyroxine (T₄) concentration, the effect of chronic ethanol administration on thyroid hormone levels was studied. Serum T₄ levels in ethanol-treated animals were significantly decreased when compared to controls at any time point studied. However, no change in serum 3,3,5-triiodothyronine (T₃) levels were found.