Disposal rates of thyroxine and triiodothyronine in iodine-deficient rats

Sustained Pituitary T3 Production Explains the T4-mediated TSH Feedback Mechanism

The regulation of thyroid activity and thyroid hormone (TH) secretion is based on feedback mechanisms that involve the anterior pituitary TSH and medial basal hypothalamus TSH-releasing hormone. Plasma T3 levels can be "sensed" directly by the anterior pituitary and medial basal hypothalamus; plasma T4 levels require local conversion of T4 to T3, which is mediated by the type 2 deiodinase (D2). To study D2-mediated T4 to T3 conversion and T3 production in the anterior pituitary gland, we used mouse pituitary explants incubated with 125I-T4 for 48 hours to measure T3 production at different concentrations of free T4. The results were compared with cultures of D1- or D2-expressing cells, as well as freshly isolated mouse tissue. These studies revealed a unique regulation of the D2 pathway in the anterior pituitary gland, distinct from that observed in nonpituitary tissues. In the anterior pituitary, increasing T4 levels reduced D2 activity slightly but caused a direct increase in T3 production. However, the same changes in T4 levels decreased T3 production in human HSkM cells and murine C2C12 cells (both skeletal muscle) and mouse bone marrow tissue, which reached zero at 50 pM free T4. In contrast, the increase in T4 levels caused the pig kidney LLC-PK1 cells and kidney fragments to proportionally increase T3 production. These findings have important implications for both physiology and clinical practice because they clarify the mechanism by which fluctuations in plasma T4 levels are transduced in the anterior pituitary gland to mediate the TSH feedback mechanism.

Effect of maternal iodine supplementation on thyroid function and birth outcome in goiter endemic areas

OBJECTIVE

The study was undertaken to examine the clinical and endocrine parameters of thyroid in a total of 460 pregnant women belonging to non-goiter areas (group 1; n = 156) and endemic areas without (group 2; n = 154) and with iodine supplementation (group 3; n = 150), and their respective newborns.

METHODS

Women of group 3 with visible goiter were administered two capsules of iodized oil orally each containing 200 mg of iodine, from weeks 6--8 of pregnancy. Blood samples were obtained from all groups during each trimester, at parturition (umbilical cord blood) and after delivery. Serum triiodothyronine (T3), thyroxine (T4) and thyroid stimulating hormone (TSH) levels were measured by specific enzyme immunoassays.

RESULTS

In group 2, serum T4 concentrations were low while T3 and TSH levels were high which showed hypothyroidism in the women of endemic areas. Goiter size decreased in most of the subjects who received a single dose of iodized oil and resulted in increase in serum concentrations of thyroid hormones; whereas, TSH levels decreased. Iodine supplementation also resulted in raised T4 and low TSH levels in the cord blood of neonates. During the course of study, two abortions, three still births and one cretin were reported in group 2; none was reported in group 3; and one still birth was reported in group 1.

CONCLUSIONS

The oral administration of a single dose of iodized oil is capable of correcting iodine deficiency both clinically and endocrinologically in mothers and neonates. Iodine supplementation has the potential to positively impact the birth weight of newborns.

The effects of iodine deficiency on thyroid hormone deiodination

Iodine deficiency induces multiple intrathyroidal autoregulatory changes leading to an increased triiodothyronine (T(3)) production and secretion, at the expense of thyroxine (T(4)). It is characterized by low serum T(4), normal or slightly elevated T(3), and as a consequence of the latter, normal thyrotropin (TSH). Tissues are also hypothyroxinemic, but their T(3) concentrations are mostly normal and ensure clinical euthyroidism, except for those that depend to a high degree on local generation from T(4) by extrathyroidal mechanisms involving the iodothyronine deiodinases isoenzymes. Thus, unless iodine deficiency is so severe and chronic that intrathyroidal and extrathyroidal mechanisms are no longer sufficient to maintain a normal T(3) in most tissues, individuals are clinically and biochemically euthyroid, but some tissues may be selectively hypothyroid (i.e., the brain). In adults both the intrathyroidal and the extrathyroidal mechanisms reacting to the iodine deficiency are fully operative even when the latter is mild. They contribute jointly to the maintenance of elevated or normal T(3) in those tissues deriving most of it from the plasma, until iodine deficiency becomes very severe. Those depending to a large extent from local generation from T(4), mostly by an interplay between type 2 iodothyronine deiodinase (D2) and type 3 (D3), may already be T(3)-deficient (and hypothyroid) with mild iodine deficiency. Therefore, thyroid status of the iodine-deficient individual not only depends on the degree of iodine shortage, but is mostly tissue-specific, and is difficult to define for the individual as a whole: elevated, normal, and low concentrations of T(3) are found simultaneously in different tissues of the same animal, even with severe deficiencies. Most effects of iodine deficiency are reversed in the adults with an adequate iodine prophylaxis, but the absence of T(4) during early fetal life leads to irreversible brain damage (neurologic cretinism). Thyroid hormones of maternal origin are available to the embryo early in development and continue contributing to fetal thyroid hormone status, even after onset of fetal thyroid secretion. In the case of congenital hypothyroidism and normal maternal T(4), the transfer of the latter, together with increased D2 activity, protects the fetal brain from T(3) deficiency, even when it may be insufficient to maintain euthyroidism in other fetal tissues. Practically all of the T(3) found in the fetal brain is derived locally from T(4), and not from circulating T(3). In the case of severe iodine deficiency, both the embryo and the mother are T(4)-deficient; therefore, the fetal brain is exposed to T(3)-deficiency, both before and after onset of fetal thyroid function. This leads to irreversible alterations and damage to the central nervous system (i.e. abnormal corticogenesis). Moreover, because intrathyroidal autoregulatory mechanisms are not yet operative in the fetus, both T(4) and T(3) continue to be very low until birth, and the fetus is not only hypothyroxinemic, similar to its mother, but also clinically and biochemically hypothyroid.

Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases

The goal of this review is to place the exciting advances that have occurred in our understanding of the molecular biology of the types 1, 2, and 3 (D1, D2, and D3, respectively) iodothyronine deiodinases into a biochemical and physiological context. We review new data regarding the mechanism of selenoprotein synthesis, the molecular and cellular biological properties of the individual deiodinases, including gene structure, mRNA and protein characteristics, tissue distribution, subcellular localization and topology, enzymatic properties, structure-activity relationships, and regulation of synthesis, inactivation, and degradation. These provide the background for a discussion of their role in thyroid physiology in humans and other vertebrates, including evidence that D2 plays a significant role in human plasma T(3) production. We discuss the pathological role of D3 overexpression causing "consumptive hypothyroidism" as well as our current understanding of the pathophysiology of iodothyronine deiodination during illness and amiodarone therapy. Finally, we review the new insights from analysis of mice with targeted disruption of the Dio2 gene and overexpression of D2 in the myocardium.

Regional physiological adaptation of the central nervous system deiodinases to iodine deficiency

The goal of the present investigation was to analyze the types 2 (D2) and 3 (D3) iodothyronine deiodinases in various structures within the central nervous system (CNS) in response to iodine deficiency. After 5-6 wk of low-iodine diet (LID) or LID + 2 microg potassium iodide/ml (LID + KI; control), rats' brains were processed for in situ hybridization histochemistry for D2 and D3 mRNA or dissected, frozen in liquid nitrogen, and processed for D2 and D3 activities. LID did not affect weight gain or serum triiodothyronine, but plasma thyroxine (T4) was undetectable. In the LID + KI animals, D3 activities were highest in the cerebral cortex (CO) and hippocampus (HI), followed by the olfactory bulb and was lowest in cerebellum (CE). Iodine deficiency decreased D3 mRNA expression in all CNS regions, and these changes were accompanied by three- to eightfold decreases in D3 activity. In control animals, D2 activity in the medial basal hypothalamus (MBH) was similar to that in pituitary gland. Of the CNS D2-expressing regions analyzed, the two most responsive to iodine deficiency were the CO and HI, in which an approximately 20-fold increase in D2 activity occurred. Other regions, i.e., CE, lateral hypothalamus, MBH, and pituitary gland, showed smaller increases. The distribution of and changes in D2 mRNA were similar to those of D2 activity. Our results indicate that decreases in the expression of D3 and increases in D2 are an integral peripheral component of the physiological response of the CNS to iodine deficiency.

Iodothyronine deiodinase activities in fetal rat tissues at several levels of iodine deficiency: a role for the skin in 3,5,3'-triiodothyronine economy?

Iodothyronine deiodinases, types I, II, and III (D1, D2, and D3) activities were measured in tissues of fetal rats, at 18 and 21 days of gestation, at several levels of iodine deficiency (ID): mild ID diet (MID) and moderately severe ID, MID + 0.005% perchlorate (MID+P). D2 was present in fetal skin, increased between days 18 and 21, and also in MID and MID+P. In skin, D3 increased during ID at day 18, whereas there was a decrease at day 21. Skin T4 decreased in MID and MID+P, showing an inverse relationship with D2. Skin T3 decreased at day 18 in MID and MID+P but increased at day 21, probably because of the increased D2 and decreased D3, maintaining T3 concentrations. No effect of ID was observed on hepatic D1. D2 increased in brain and brown adipose tissue at day 21 in MID+P. No changes were found in maternal placental D2 and D3, but D2 and D3 increased in the fetal placenta at day 18 in MID+P. A higher level of D2 is present in fetal skin than in the brain. As the activity is increased, in even mild ID (and already at 18 days) it can be concluded that skin D2 is likely to be of considerable physiological importance, at least for fetal thyroid hormone economy, by contributing to the intracellular T3 content of the skin and, possibly, to the plasma T3.

Effect of iodine deficiency and cold exposure on thyroxine 5'-deiodinase activity in various rat tissues

We measured thyroxine 5'-deiodinase I (T(4)5'D-I) activity in thyroid, liver, and kidney and thyroxine 5'-deiodinase II (T(4)5'D-II) activity in brown adipose tissue (BAT) in rats on a low-iodine diet (LID) to test the possibility that increased deiodinase activity in these tissues might contribute to the maintenance of ther serum 3,5,3'-triiodothyronine (T3) level. Control rats received LID plus KI. Experiments were also performed with LID and LID plus KI rats exposed to cold. T(4)5'D-I activity was greatly increased in the thyroids of LID rats but not in liver or kidney. We consider it likely that increased thyroxine (T4)-to-T3 conversion in the greatly enlarged thyroids of LID rats contributed to the maintenance of serum T3. T(4)5'D-II activity in BAT was markedly increased in LID rats and was further greatly increased on cold exposure. However, we were unable to demonstrate an increase in uncoupling protein mRNA levels in BAT in response to cold in LID rats. We attribute this to the very low serum T4 level, which limits substrate availability. This factor also makes it unlikely that BAT contributes to maintenance of serum T3 in LID rats.

Triiodothyronine and thyroxine in the serum and thyroid glands of iodine-deficient rats

Triiodothyronine (T(3)) and thyroxine (T(4)) were measured by immunoassay in the serum and thyroid hydrolysates of control (group A), mildly iodine-deficient (group B), and severely iodine-deficient rats (group C). These results were correlated with changes in thyroidal weight, (131)I uptake and (127)I content as well as with the distribution of (131)I in Pronase digests of the thyroid. There was a progressive increase in thyroid weight and (131)I uptake at 24 h with decrease in iodine intake. The (127)I content of the thyroids of the group B animals was 44% and that of the group C animals 2% of that in group A. The mean labeled monoiodotyrosine/diiodotyrosine (MIT/DIT) and T(3)/T(4) ratios in group A were 0.42+/-0.07 (SD) and 0.12+/-0.01, 0.59+/-0.06 and 0.11+/-0.03 in group B, and 2.0+/-0.3 and 1.8+/-0.9 in the group thyroid digests.Mean serum T(4) concentration in the control rats was 4.2+/-0.6 (SD) mug T(4)/100 ml, 4.5+/-0.3 mug/100 ml in group B animals, and undectectable (<0.5 mu(4)/100 ml) in group C animals. There was no effect of iodine deficiency on serum T(3) concentrations, which were 44+/-9 (Mean+/-SD) ng/100 ml in A animals, 48+/-6 ng/100 ml n B animals, and 43+/-6 ng/100 ml in the C group. Thyroidal digest T(3) and T(4) concentrations were 39 and 400 ng/mg in group A animals and were reduced to 5 and 1% of this, respectively, in group C. The molar ratio of T(3)/T(4) in the thyroid digests of the groups A and B animals was identical to the ratio of labeled T(3)/T(4) and was slightly less (1.0+/-0.9) than the labeled T(3)/T(4) ratio in the group C animals. The mean ratio of labeled T(4) to labeled T(3) in the serum of the severely iodine-deficient animals 24 h after isotope injection was 11+/-1 (SEM). With previously published values, it was possible to correlate the ratio of labeled T(4)/T(3) in the thyroid digest with the labeled T(4)/T(3) ratio in the serum of each iodine-deficient animal. This analysis suggested that the labeled thyroid hormones in the severely iodine-deficient rat were secreted in the ratio in which they are present in the gland. Kinetic analysis of total iodothyronine turnover indicated that two-thirds of the T(3) utilized per day by the iodine-sufficient rat arises from T(4). If the T(4)-T(3) conversion ratio remains the same in iodine deficiency, then the analysis suggests that about 90% of the T(3) arises directly from the thyroid. Therefore, it would appear that absolute T(3) secretion by the thyroid increases severalfold during iodine deficiency. The fact that serum T(3) remains constant and T(4) decreases to extremely low levels, combined with previous observations that iodine-deficient animals appear to be euthyroid, is compatible with the hypothesis that T(4) in the normal rat serves primarily as a precursor of T(3).