Iodine deficiency in cattle: compensatory changes in thyroidal selenoenzymes

The trace elements selenium and iodine are both essential for normal thyroid hormone metabolism. To investigate the relationships between these functions, heifers were maintained on iodine-deficient or iodine-sufficient diets from mid pregnancy to term. In these heifers and their offspring the interrelationship between iodine and selenium was apparent with the preferential 10- to 12-fold induction of the selenoenzyme, thyroidal type I, selenium-containing iodothyronine deiodinase activity by iodine deficiency. This was accompanied by two- to four-fold increases in cytosolic glutathione peroxidase activity, probably reflecting increased oxidative activity and metabolism in the thyroid gland in response to iodine deficiency. The above selenoenzyme activities were not affected in liver, kidney, pituitary and brain by iodine deficiency. The results are consistent with a critical role for selenium in both the normal function of cattle thyroid and key enzymes to compensate for the effects of iodine deficiency.

Selenoprotein expression and brain development in preweanling selenium- and iodine-deficient rats

Selenium deficiency causes further impairment of thyroid hormone metabolism in iodine-deficient rats and therefore could have a role in the aetiology of both myxoedematous and neurological cretinism in humans. Thyroidal type I iodothyronine deiodinase (ID-I), cytosolic glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase activities were increased in iodine-deficient adult rats and their offspring at 11 days of age. Thyroidal ID-I activity was unchanged and thyroidal cytosolic glutathione peroxidase activity was decreased by more than 75% by combined selenium and iodine deficiency in 11-day-old rats, indicating that, while the thyroid retained an ability to produce 3,3',5-triiodothyronine (T3), the gland was probably more susceptible to peroxidative damage caused by increased hydrogen peroxide concentrations driven by increased thyrotrophin. Thyroidal atrophy, common in myxoedematous cretinism, did not occur in iodine- or selenium and iodine-deficient rat pups. Iodine deficiency increased brain type II iodothyronine deiodinase activity 1.5-fold in 4-day-old rats and 3-fold in 11-day-old rats, regardless of selenium status. Thus rats were able to activate compensatory mechanisms in brain that would maintain T3 concentrations in selenium and iodine deficiencies. Surprisingly, however, selenium deficiency had a greater effect than iodine deficiency on markers of brain development in rat pups. Expression of the brain-derived neurotrophic factor (BDNF) mRNA was decreased in selenium deficiency in 4- and 11-day-old pups and in combined selenium and iodine deficiency in 4-day-old pups. Iodine deficiency caused an increase in BDNF expression in 11-day-old pups but had no effect on 4-day-old pups. Myelin basic protein mRNA expression in brain was decreased by combined selenium and iodine deficiency in 11-day-old rats.

Plant cell expansion: scaling the wall

The regulation of plant cell size and shape is poorly understood at the molecular level. Recently, two loci required for normal cell expansion in Arabidopsis were cloned. They both encode enzymes involved in the construction of the cell wall. These studies are the first promising examples of the use of Arabidopsis molecular genetics for the study of wall synthesis and assembly during plant cell elongation.

Selenium and iodine deficiencies: effects on brain and brown adipose tissue selenoenzyme activity and expression

Adequate dietary iodine supplies and thyroid hormones are needed for the development of the central nervous system (CNS) and brown adipose tissue (BAT) function. Decreases in plasma thyroxine (T4) concentrations may increase the requirement for the selenoenzymes types I and II iodothyronine deiodinase (ID-I and ID-II) in the brain and ID-II in BAT to protect against any fall in intracellular 3,3',5 tri-iodothyronine (T3) concentrations in these organs. We have therefore investigated selenoenzyme activity and expression and some developmental markers in brain and BAT of second generation selenium- and iodine-deficient rats. Despite substantial alterations in plasma thyroid hormone concentrations and thyroidal and hepatic selenoprotein expression in selenium and iodine deficiencies, ID-I, cytosolic glutathione peroxidase (cGSHPx) and phospholipid hydroperoxide glutathione peroxidase (phGSHPx) activities and expression remained relatively constant in most brain regions studied. Additionally, brain and pituitary ID-II activities were increased in iodine deficiency regardless of selenium status. This can help maintain tissue T3 concentrations in hypothyroidism. Consistent with this, no significant effects of iodine or selenium deficiency on the development of the brain were observed, as assessed by the activities of marker enzymes. In contrast, BAT from selenium- and iodine deficient rats had impaired thyroid hormone metabolism and less uncoupling protein than in tissue from selenium- and iodine-supplemented animals. Thus, the effects of selenium and iodine deficiency on the brain are limited due to the activation of the compensatory mechanisms but these mechanisms are less effective in BAT.

Selenium and iodine deficiencies and selenoprotein function

This paper reviews some recent findings on the interactions between selenium deficiency and iodine deficiency. Both micronutrients can control the levels of selenoprotein mRNAs, particularly in the thyroid and brain. When selenium and iodine supplies are limiting the compensatory mechanisms work to minimise adverse effects on thyroid hormone metabolism and thus neurological development. The mechanisms for regulation of selenoproteins in selenium and iodine deficiency are however very tissue-specific. For example, unlike the brain and thyroid, brown adipose tissue is unable to retain selenoproteins in selenium and iodine deficiency and is therefore at greater risk from injurious effects of the deficiencies.

A technique for recording and analysis of postural changes associated with thermal comfort

The recording of posture has a long history in the study of ergonomics, but has generally been concerned with the assessment of mechanical strain on the human body. This paper extends the concern to the thermal implications of posture. A change in posture can change the effective body surface area available for heat exchange with the environment and therefore the metabolic rate per unit body surface area. This effect is systematised into a method of postural coding which reflects the extent of changes in effective body surface area available for heat exchange. The paper reports a brief assessment of the extent of the effect of posture in normal office work and the extent to which posture is temperature-related.

Selenium status of a group of Scottish adults

OBJECTIVE

To examine dietary selenium intake and indices of selenium status (plasma and red blood cell selenium and glutathione peroxidase activities) in apparently healthy Scottish individuals.

DESIGN AND SUBJECTS

One hundred subjects, aged between 40 and 60 y, completed a seven day weighed food intake and provided blood samples for selenium status analysis.

SETTING

Inverurie, Aberdeenshire, Scotland.

RESULTS

Average reported selenium intake was low (43 micrograms/d). A significant number of subjects had reported intakes below the RNI. Low levels of plasma selenium were also found but no subject had values below 40 micrograms/1. Red blood cell selenium was within the reference range established for a healthy UK population. Smoking status had no consistent effect on selenium status.

CONCLUSIONS

The results of the present study suggest that selenium status of certain Scottish individuals may be compromised and that further studies are warranted.

SPONSORSHIP

BASF, Germany; The Tobacco Products Research Trust, UK; Scottish Office Agriculture Environment and Fisheries Department.

Design, synthesis, and characterization of a cationic peptide that binds to nucleic acids and permeabilizes bilayers

We have designed a cationic amphipathic peptide, KALA (WEAKLAKALAKALAKHLAKALAKALKACEA), that binds to DNA, destabilizes membranes, and mediates DNA transfection. KALA undergoes a pH-dependent random coil to amphipathic alpha-helical conformational change as the pH is increased from 5.0 to 7.5. One face displays hydrophobic leucine residues, and the opposite face displays hydrophilic lysine residues. KALA-mediated release of entrapped aqueous contents from neutral and negatively charged liposomes increases with increasing helical content. KALA binds to oligonucleotides or plasmid DNA and retards their migration in gel electrophoresis. It displaces 50% of ethidium bromide from DNA at a charge ratio (+/-) of 0.9/1. In cultured cells, KALA assists oligonucleotide nuclear delivery when complexes are prepared at a 10/1 (+/-) charge ratio. KALA/DNA (10/1)(+/-) complexes mediate transfection of a variety of cell lines. The KALA sequence provides a starting point for a family of peptides that incorporate other functions to improve DNA delivery systems.

Effect of cholesterol and charge on pore formation in bilayer vesicles by a pH-sensitive peptide

The effect of cholesterol on the bilayer partitioning of the peptide GALA (WEAALAEALAEALAEHLAEALAEALEALAA) and its assembly into a pore in large unilamellar vesicles composed of neutral and negatively charged phospholipids has been determined. GALA undergoes a conformational change from a random coil to an amphipathic alpha-helix when the pH is reduced from 7.0 to 5.0, inducing at low pH leakage of contents from vesicles. Leakage from neutral or negatively charged vesicles at pH 5.0 was similar and could be adequately explained by the mathematical model (Parente, R. A., S. Nir, and F. C. Szoka, Jr., 1990. Mechanism of leakage of phospholipid vesicle contents induced by the peptide GALA. Biochemistry. 29:8720-8728) which assumed that GALA becomes incorporated into the vesicle bilayer and irreversibly aggregates to form a pore consisting of 10 +/- 2 peptides. Increasing cholesterol content in the membranes resulted in a reduced efficiency of the peptide to induce leakage. Part of the cholesterol effect was due to reduced binding of the peptide to cholesterol-containing membranes. An additional effect of cholesterol was to increase reversibility of surface aggregation of the peptide in the membrane. Results could be explained and predicted with a model that retains the same pore size, i.e., 10 +/- 2 peptides, but includes reversible aggregation of the monomers to form the pore. Resonance energy transfer experiments using fluorescently labeled peptides confirmed that the degree of reversibility of surface aggregation of GALA was significantly larger in cholesterol-containing liposomes, thus reducing the efficiency of pore formation.

Selenoenzyme expression in thyroid and liver of second generation selenium- and iodine-deficient rats

The stimulation of thyroid hormone synthesis in iodine deficiency may increase the requirement for the selenoproteins which are involved in thyroid hormone synthesis in the thyroid gland. Selenoenzyme activity and expression were investigated in the thyroid and liver of second generation selenium-and/or iodine-deficient rats. Selenium deficiency caused substantial decreases in hepatic selenium-containing type I iodothyronine deiodinase (ID-I) and cytosolic glutathione peroxidase (cGSHPx) activities and mRNA abundances, but phospholipid hydroperoxide glutathione peroxidase (phGSHPx) activity was only 55% of selenium-supplemented control levels, despite the absence of change in its mRNA abundance. Selenoenzyme mRNA concentrations were maintained at control levels in thyroid glands from the selenium-deficient rat pups. Despite this, a differential effect was observed in selenoenzyme activities: ID-I activity was decreased to 61%, cGSHPx activity to 45% and phGSHPx to 29% of that in selenium-adequate controls. In iodine-deficient thyroid glands, mRNA levels were increased 2.2, 5.0 and 2.8 times for ID-I, cGSHPx and phGSHPx respectively. ID-I and cGSHPx enzyme activities were also increased but the activity of phGSHPx was decreased despite the high mRNA abundance. Thyroid selenoprotein mRNA levels were also increased in combined selenium and iodine deficiency but again there were differential effects on enzyme activities, with ID-I activity increased, cGSHPx unchanged and phGSHPx decreased. Thus, iodine deficiency may produce an oxidant stress on the thyroid gland, increasing the requirement for selenium to maintain selenoenzyme activity. When dietary supplies of selenium are limiting, thyroid selenoprotein mRNA levels are increased to compensate for overall lack of the micronutrient. Furthermore, there is a preferential supply of available selenium to ID-I and cGSHPx to allow maintenance of thyroid function.

Selenoprotein gene expression during selenium-repletion of selenium-deficient rats

Selenium repletion of selenium-deficient rats with 20 micrograms selenium / kg body weight as Na2SeO3 was used as a model to investigate the mechanisms that control the distribution of the trace element to specific selenoproteins in liver and thyroid. Cytosolic glutathione peroxidase (cGSHPx), phospholipid hydroperoxide glutathione peroxidase (PHGSHPx), and iodothyronine 5'-deiodinase (IDI) activities were all transiently increased in liver 16 to 32 h after ip injection with selenium. However, only cGSHPx and PHGSHPx activities increased in the thyroid where IDI activity was already increased by selenium deficiency. These responses were owing to synthesis of the seleoproteins on newly synthesised and/or existing mRNAs. The selenoprotein mRNAs in the thyroid gland were increased two- and threefold after the transitory increases in selenoprotein activity. In contrast, there were parallel changes in selenoprotein mRNAs and enzyme activities in the liver, with no prolonged rises in mRNA levels. The organ differences suggest that increased thryotrophin (TSH) concentrations, which are known to induce thyrodial IDI and mRNA, may control the mRNAs for all the thyroidal selenoproteins investigated and be a major mechanism for the preservation of thyroidal selenoproteins when selenium supplies are limited.

Tissue-specific regulation of selenoenzyme gene expression during selenium deficiency in rats

Regulation of synthesis of the selenoenzymes cytosolic glutathione peroxidase (GSH-Px), phospholipid hydroperoxide glutathione peroxidase (PHGSH-Px) and type-1 iodothyronine 5'-deiodinase (5'IDI) was investigated in liver, thyroid and heart of rats fed on diets containing 0.405, 0.104 (Se-adequate), 0.052, 0.024 or 0.003 mg of Se/kg. Severe Se deficiency (0.003 mg of Se/kg) caused almost total loss of GSH-Px activity and mRNA in liver and heart. 5'IDI activity decreased by 95% in liver and its mRNA by 50%; in the thyroid, activity increased by 15% and mRNA by 95%. PHGSH-Px activity was reduced by 75% in the liver and 60% in the heart but mRNA levels were unchanged; in the thyroid, PHGSH-Px activity was unaffected by Se depletion but its mRNA increased by 52%. Thus there is differential regulation of the three mRNAs and subsequent protein synthesis within and between organs, suggesting both that mechanisms exist to channel Se for synthesis of a particular enzyme and that there is tissue-specific regulation of selenoenzyme mRNAs. During Se depletion, the levels of selenoenzyme mRNA did not necessarily parallel the changes in enzyme activity, suggesting a distinct mechanism for regulating mRNA levels. Nuclear run-off assays with isolated liver nuclei showed severe Se deficiency to have no effect on transcription of the three genes, suggesting that there is post-transcriptional control of the three selenoenzymes, probably involving regulation of mRNA stability.

Making reaccreditation meaningful

Reaccreditation is a well-accepted fact for many doctors outside the United Kingdom and is likely to become a reality for British general practitioners. The author'sabbatical year in the United States of America studying reaccreditation and its relationship to continuing medical education has enabled a critical analysis of recent proposals in the UK to be carried out. The aim of reaccreditation must be understood by the profession and must be clearly stated. To be credible it will have to be mandatory and linked to continuing medical education. Current types of continuing medical education must be developed so that they are meaningful, influence doctors' behaviour and include research, audit, training, reading and medical writing. The profession must confront the need to penalize the small number of doctors who have an unacceptable standard of practice. The potential benefits of an appropriate form of reaccreditation may include improved quality of care and patient outcome, enhanced job satisfaction and reduced rates of burnout.

Effect of selenium depletion on thyroidal type-I iodothyronine deiodinase activity in isolated human thyrocytes and rat thyroid and liver

The effects of dietary selenium deficiency on hepatic and thyroidal type I iodothyronine deiodinase (ID-I) and selenium-dependent glutathione peroxidase (GPx) activities have been studied in weanling rats. In selenium-deficient animals hepatic ID-I activity was reduced to 11% of the activity found in the selenium-replete groups, whilst thyroidal ID-I activity increased by 42%. Hepatic and thyroidal GPx activities were also reduced by selenium deficiency to approximately 0.6 and 70%, respectively, of the values found in the selenium-replete animals. We have also studied the effects of thyrotropin (TSH), and selenium supply on the activity of IDI and GPx in human thyrocytes grown in primary culture. When thyrocytes were grown in selenium-deficient (< 1 nmol l-1 Se) medium in the absence of TSH, addition of sodium selenite up to 1000 nmol l-1 had little or no effect on ID-I activity. In the absence of added selenite, TSH addition produced a significant increase in ID-I activity and this stimulation was increased further when selenite was added at concentrations of 50-1000 nmol l-1 with an optimal effect on ID-I activity being observed at a 500 nmol l-1. Selenium content and GPx activity in human thyrocytes grown in selenium-free media (selenium content < 1 nmol l-1) were not significantly lower than the corresponding measurements made in cells grown in media containing selenium at a concentration of 5.4 nmol l-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization and postnatal development of 5'-deiodinase activity in goat perirenal fat

The capacity to generate the active thyroid hormone 3,3',5-triiodothyronine (T3) from thyroxine (T4) has been assessed in the major adipose tissue depot (perirenal) of newborn goats through measurements of iodothyronine 5'-deiodinase activity. High activity of the enzyme was found in the perirenal adipose tissue. Developmental studies demonstrated that there was a rise, then a fall, in iodothyronine 5'-deiodinase activity (per milligram of protein) over the first week of postnatal life, with a further decline between the second and third weeks. At 3 wk of age, the activity per gram of tissue was only 2% of that of the newborn; however, because of age-related increases in the lipid content of the perirenal adipose tissue, the activity per cell (per microgram of DNA) at 3 wk of age was approximately 10% of the activity in the newborn. Kinetic studies and inhibition characteristics with propylthiouracil, gold thioglucose, and T4 indicated that the iodothyronine 5'-deiodinase in goat perirenal adipose tissue is of the type I form; there was no evidence for the type II enzyme. Affinity labeling of the iodothyronine 5'-deiodinase with bromoacetyl-rT3 or bromoacetyl-T4 was consistent with this view. In contrast to goats, only the type II enzyme is evident in rats and mice. It is concluded that the perirenal (brown) adipose tissue is likely to be a significant source of T3 for other tissues in newborn goats and that there are major species variations in the form of iodothyronine 5'-deiodinase present in brown fat, with the goat exclusively exhibiting type I.

Presence of the brown fat-specific mitochondrial uncoupling protein and iodothyronine 5'-deiodinase activity in subcutaneous adipose tissue of neonatal lambs

Subcutaneous adipose tissue of neonatal lambs has been examined for the presence of markers diagnostic of thermogenic brown fat. Uncoupling protein, uncoupling protein mRNA, and iodothyronine 5'-deiodinase activity were each detected in subcutaneous adipose tissue, as well as in the major internal fat depot (perirenal), of newborn lambs. These brown fat markers were not present, however, in adipose tissue of adult sheep. It is concluded that subcutaneous fat in newborn lambs is functionally 'brown', and similar to the internal fat; subcutaneous and internal adipose tissues follow a similar developmental path--from 'brown' to 'white'.

The role of thyroidal type-I iodothyronine deiodinase in tri-iodothyronine production by human and sheep thyrocytes in primary culture

We have studied the origin of tri-iodothyronine (T3) secreted by human and sheep thyrocytes in primary culture and also the expression of type-I thyroidal iodothyronine deiodinase (ID-I) in the thyroid and liver of man and various other animals. Inhibitors of ID-I reduced T3 secretion from human but not sheep thyrocytes. In contrast, inhibitors of de-novo thyroid hormone synthesis reduced both thyroxine (T4) and T3 production in sheep thyrocytes, but had no effect on the T3 secreted by human thyrocytes. Human thyrocytes did not produce T4 under the culture conditions used, although some endogenous T4 was present in the cells following their isolation. Although thyrotrophin (TSH) stimulated T3 production in both human and sheep thyrocytes, iodine in the form of potassium iodide was only essential for T3 and T4 production by the sheep cells. Although 125I from Na125I was incorporated into T3 and T4 in TSH-stimulated sheep thyrocytes, no 125I incorporation into T3 or T4 was detected in TSH-stimulated human thyrocytes. Using activity measurements and affinity labelling, ID-I was present in the livers of all species studied, but ID-I could not be detected in thyroid tissue from cattle, pigs, sheep, goats, rabbits, deer or llamas. In contrast, thyroid tissue from man, mice, guinea-pigs and rats had significant ID-I activity and expressed an affinity-labelled protein with a molecular mass of approximately 28.1 kDa on SDS-PAGE. These data show that under the culture conditions used, sheep thyrocytes produced T3 by de-novo synthesis, whilst human thyrocytes produced T3 by deiodination of endogenous T4. We conclude that thyroidal ID-I shows marked species difference in its expression and that, in those species which express the enzyme (man, mice, guinea-pigs and rats, in this study), it appears that it may make an important contribution to thyroidal T3 production.

Effects of combined iodine and selenium deficiency on thyroid hormone metabolism in rats

This paper compares the effects of combined iodine and selenium deficiency, of single deficiencies of these trace elements, and of no deficiency on thyroid hormone metabolism in rats. In rats deficient in both trace elements, thyroidal triiodothyronine (T3), thyroidal thyroxin (T4), thyroidal total iodine, hepatic T4, and plasma T4 were significantly lower, and plasma thyroid-stimulating hormone (TSH) and thyroid weight were significantly higher than in rats deficient in iodine alone. Plasma and hepatic T3 concentrations were similar in the dietary groups. Hepatic type I iodothyronine deiodinase (ID-I) activity was inhibited by selenium deficiency irrespective of the iodine status. Type II deiodinase (ID-II) activity in the brain was significantly higher and in pituitary, significantly lower in combined deficiency than in iodine deficiency alone. These data show that selenium can play an important role in determining the severity of the hypothyroidism associated with iodine deficiency.

Selenium deficiency, thyroid hormone metabolism, and thyroid hormone deiodinases

Much research into the functions of selenium in the cell has concentrated on its role in selenium-containing glutathione peroxidases. However, selenium was recently shown to be an essential component of type I iodothyronine 5'-deiodinase in rats, which converts thyroxin to the more biologically active hormone 3,5,3'-triiodothyronine. Thus, selenium-deficient rats have low tissue deiodinase activities and abnormal thyroid hormone metabolism. The discovery of this function for selenium in thyroid hormone metabolism has important implications for the interpretation of the effects of selenium deficiency, especially in individuals with an adequate vitamin E status.

The role of selenium in thyroid hormone metabolism and effects of selenium deficiency on thyroid hormone and iodine metabolism

Selenium deficiency impairs thyroid hormone metabolism by inhibiting the synthesis and activity of the iodothyronine deiodinases, which convert thyroxine (T4) to the more metabolically active 3,3'-5 triiodothyronine (T3). Hepatic type I iodothyronine deiodinase, identified in partially purified cell fractions using affinity labeling with [125I]N-bromoacetyl reverse triiodothyronine, is also labeled with 75Se by in vivo treatment of rats with 75Se-Na2SeO3. Thus, the type I iodothyronine 5'-deiodinase is a selenoenzyme. In rats, concurrent selenium and iodine deficiency produces greater increases in thyroid weight and plasma thyrotrophin than iodine deficiency alone. These results indicate that a concurrent selenium deficiency could be a major determinant of the severity of iodine deficiency.

The role of selenium in thyroid hormone metabolism and effects of selenium deficiency on thyroid hormone and iodine metabolism

Selenium deficiency impairs thyroid hormone metabolism by inhibiting the synthesis and activity of the iodothyronine deiodinases, which convert thyroxine (T4) to the more metabolically active 3,3'-5 triiodothyronine (T3). Hepatic type I iodothyronine deiodinase, identified in partially purified cell fractions using affinity labeling with [125I]N-bromoacetyl reverse triiodothyronine, is also labeled with 75Se by in vivo treatment of rats with 75Se-Na2SeO3. Thus, the type I iodothyronine 5'-deiodinase is a selenoenzyme. In rats, concurrent selenium and iodine deficiency produces greater increases in thyroid weight and plasma thyrotrophin than iodine deficiency alone. These results indicate that a concurrent selenium deficiency could be a major determinant of the severity of iodine deficiency.

Effect of selenium deficiency on hepatic type I 5-iodothyronine deiodinase activity and hepatic thyroid hormone levels in the rat

Selenium deficiency in rats for a period of up to 6 weeks inhibited both the production of 3,3',5-tri-iodothyronine (T3) from thyroxine (T4) (5'-deiodination) and also the catabolism of T3 to 3,3'-di-iodothyronine (5-deiodination) in liver homogenates. The hepatic stores of T3 were decreased by only 8% in selenium deficiency, despite the T3 production rate from T4 being only 7% of the rate found in selenium-supplemented rats. Hepatic glutathione S-transferase (GST) activity was increased in both hypothyroidism and selenium deficiency, but apparently by different mechanisms, since mRNA expression for this family of enzymes was lowered by hypothyroidism and increased in selenium deficiency. It is concluded that, since both T3 production and catabolism are inhibited by selenium deficiency, there is little change in hepatic T3 stores, and therefore the changes in the activity of certain hepatic enzymes, such as GST, that are found in selenium deficiency are not the result of tissue hypothyroidism.

Inter-relationships between selenium and thyroid hormone metabolism in the rat and man

Labelling of rat kidney microsomes in vitro with [125I]-bromoacetyl T4 produced two bands on SDS/PAGE with Mr of 55 kDa and 27.5 kDa representing protein disulphide isomerase and type I iodothyronine deiodinase (ID-I) respectively. The amount of the 55 kDa band was unchanged by selenium (Se) deficiency but the 27.5 kDa protein was markedly decreased in kidney microsomal fraction obtained from Se-deficient rats. Concurrent Se and iodine deficiency produced a significant increase in thyroid weight, plasma thyrotrophin (TSH) and a decrease in thyroidal iodine when compared with either single Se or iodine deficiency. These results suggest that ID-I is a selenoprotein and that Se deficiency can exacerbate the hypothyroidism observed in iodine deficiency. In man, blood glutathione peroxidase and blood Se levels were decreased in hyperthyroidism due to Graves' disease whilst normal levels of these analytes were found in patients treated for Graves' disease. These results suggest that thyroid status can affect Se balance rather than Se deficiency predisposes to Graves' disease.