The quantity of thyroid hormone in human milk is too low to influence plasma thyroid hormone levels in the very preterm infant

BACKGROUND

Thyroid hormone is crucial for brain development during foetal and neonatal life. In very preterm infants, transient low levels of plasma T4 and T3 are commonly found, a phenomenon referred to as transient hypothyroxinaemia of prematurity. We investigated whether breast milk is a substantial resource of thyroid hormone for very preterm neonates and can alleviate transient hypothyroxinaemia. Both the influence of breast feeding on plasma thyroid hormone levels and the thyroid hormone concentration in preterm human milk were studied.

METHODS

Two groups were formed from the placebo group of a randomized thyroxine supplementation trial in infants born at < 30 weeks' gestational age on the basis of the mean breast milk intake during the third, fourth and fifth weeks of life. One group received more than 50% breast milk (mean breast milk intake 84%, n = 32) and the other group less than 25% breast milk (mean breast milk intake 3.3%, n = 25). Plasma thyroid hormone concentrations were compared between the two groups. Breast milk was collected from mothers of infants participating in the same trial and the thyroxine concentration in breast milk was measured with RIA after extraction.

RESULTS

No significant differences were found between both groups in plasma concentrations of T4, free T4, T3, TSH, rT3 and thyroxine-binding globulin (TBG), which were measured once a week. Thyroxine concentration in breast milk ranged between 0.17 microg/l and 1.83 microg/l (mean 0.83, SD 0.3 microg/l) resulting in a maximum T4 supply of 0.3 microg/kg via ingested breast milk. In formula milk, the T4 concentration was equally low. Protease treatment did not influence the measured T4 concentrations.

CONCLUSIONS

No differences in plasma thyroid hormone between breast milk-fed and formula-fed infants were found. The amount of T4 present in human milk and formula milk is too low to alter the hypothyroxinaemic state of preterm infants.

Iodine species and the endocrine system: thyroid hormone levels in adult Danio rerio and developing Xenopus laevis

Recently a new approach for the analysis of iodinated organic species in human serum has been developed using liquid chromatography-inductively coupled plasma-mass spectrometry (LC-ICP-MS). This method enables quantification of iodide, T4 and T3, as well as reverse T3 (rT3) and the synthetic precursors of TH, monoiodotyrosine (MIT), and diiodotyrosine (DIT) in a single injection. In this work, the LC-ICP-MS approach was used to analyze whole-body homogenates of adult male and female zebrafish (Danio rerio) and tadpoles of the African clawed frog (Xenopus laevis) at two different developmental stages (NF58 and 61) according to Nieuwkoop and Faber. The data demonstrate that the LC-ICP-MS method was successful at measuring I-, MIT, DIT, T4, T3, and rT3 in these two species. Furthermore, the method also detected five additional iodinated compounds which are currently unidentified.

The effects of PCB exposure and fish consumption on endogenous hormones

Previous studies have suggested that exposure to polychlorinated biphenyls (PCBs) may alter thyroid function, but data on effects of PCB exposure on other endogenous hormones has been lacking. The current study is ancillary to a larger investigation of the effects of Great Lakes fish consumption on PCBs and reproductive function. In the current study we examine associations of PCBs, 1,1-bis (4-chlorophenyl)-2,2-dichloroethene (DDE), and fish consumption with thyroid and steroid hormones in 178 men and PCBs, DDE, and fish consumption with thyroid hormones in 51 women from the original study. Serum PCB level and consumption of Great Lakes fish are associated with significantly lower levels of thyroxine (T(4)) and free thyroxine index (FTI) in women and with significantly lower levels of T(4) in men. Fish consumption, but not PCB level, is significantly and inversely associated with triiodothyronine (T(3)) in men. Results for thyroid-stimulating hormone (TSH) are inconsistent. Among men, there are significant inverse associations of both PCB and fish consumption with sex hormone-binding globulin (SHBG)-bound testosterone, but no association with SHBG or free testosterone. There are no significant overall associations of PCB, DDE, or fish consumption with estrone sulfate, follicle-stimulating hormone, luteinizing hormone, or dehydroepiandrosterone sulfate. The results of this study are consistent with previous studies showing effects of fish consumption and PCB exposure on thyroid hormones and suggest that PCBs may also decrease steroid binding to SHBG. Elucidation of specific mechanisms must await future investigations.

The effects of PCB exposure and fish consumption on endogenous hormones

Previous studies have suggested that exposure to polychlorinated biphenyls (PCBs) may alter thyroid function, but data on effects of PCB exposure on other endogenous hormones has been lacking. The current study is ancillary to a larger investigation of the effects of Great Lakes fish consumption on PCBs and reproductive function. In the current study we examine associations of PCBs, 1,1-bis (4-chlorophenyl)-2,2-dichloroethene (DDE), and fish consumption with thyroid and steroid hormones in 178 men and PCBs, DDE, and fish consumption with thyroid hormones in 51 women from the original study. Serum PCB level and consumption of Great Lakes fish are associated with significantly lower levels of thyroxine (T(4)) and free thyroxine index (FTI) in women and with significantly lower levels of T(4) in men. Fish consumption, but not PCB level, is significantly and inversely associated with triiodothyronine (T(3)) in men. Results for thyroid-stimulating hormone (TSH) are inconsistent. Among men, there are significant inverse associations of both PCB and fish consumption with sex hormone-binding globulin (SHBG)-bound testosterone, but no association with SHBG or free testosterone. There are no significant overall associations of PCB, DDE, or fish consumption with estrone sulfate, follicle-stimulating hormone, luteinizing hormone, or dehydroepiandrosterone sulfate. The results of this study are consistent with previous studies showing effects of fish consumption and PCB exposure on thyroid hormones and suggest that PCBs may also decrease steroid binding to SHBG. Elucidation of specific mechanisms must await future investigations.

High prevalence of psychiatric morbidity in a medical intensive care unit

This study seeks to determine the prevalence of psychiatric morbidity within a medical intensive care unit, examine its correlation with the various physiological parameters and delineate any clinical predictors for psychiatric morbidity. Seventy-seven patients who gave informed consent were administered the General Health Questionnaire (GHQ), Acute Physiological And Chronic Health Evaluation II (APACHE II) and thyroid function tests were performed. A high prevalence of psychiatric morbidity was found (36.4%). However, no statistically significant association was found between psychiatric morbidity and gender, age, APACHE II scores and thyroid function indices. Nevertheless, it is hoped that the index of suspicion for psychiatric morbidity can be raised in order to optimise the clinical management of patients within this setting.

Vitamin and thyroid status in arctic grayling (Thymallus arcticus) exposed to doses of 3,3',4,4'-tetrachlorobiphenyl that induce the phase I enzyme system

Induction of phase I biotransformation enzymes is recognized as a hallmark response in fish exposed to coplanar PCBs. Depletions of vitamins A and E and disrupted thyroid hormone and glandular structure secondary to this induction have not yet been examined in an arctic fish species. Arctic grayling were exposed to a single oral dose of 0 (control), 10, 100 or 1000 ng 3,3',4,4'-tetrachlorobiphenyl (TCB) g(-1) bodyweight, a contaminant found in most arctic fish. After 30 and 90 days of exposure, TCB concentrations in tissues, hepatic phase I activity (as ethoxyresorufin-O-deethylase (EROD)), plasma and tissue vitamin A and E concentrations, plasma thyroid hormone levels and thyroid glandular structure were examined. Total plasma osmolality, as an indicator of overall fish health was also monitored. TCB recovery in tissues was low and extremely variable, making comparisons between intended dose groups inappropriate. Therefore, correlation analysis between actual recovered TCB concentrations and biochemical responses was employed. Hepatic EROD activity correlated strongly with liver TCB concentrations. Liver concentrations of vitamin A were altered as a function of TCB concentrations and EROD activity, but plasma vitamin A status was not affected. Vitamin E was depleted by TCB accumulation in blood and EROD induction in liver of males only at 90 days postexposure. Thyroid hormones status and glandular structure were not affected by the short duration TCB exposures used in this experiment. TCB concentrations were correlated with an elevation in plasma osmolality. Results from this experiment indicate that the vitamin status and osmoregulation of arctic grayling exposed to TCB can be compromised. Further studies of field populations exposed to this type of contaminant are warranted.

[Endocrinological disorders in association with alopecia areata-a 27 patients study]

Alopecia areata is a dermatological disease, characterized by the loss of hair, which affect men, women and children and can evaluate alone or in association with a variety of other disorders. Between these endocrinological diseases, especial thyroid disorders, have a high incidence. Twenty-seven patients with alopecia areata (12 women and 15 men) aged between 3 and 46 years were endocrinologically investigated. Eighteen of them (66.6%) had endocrinological disorders. Thyroid diseases were present in 10 cases (37%): 4 cases with endemic goiter, 2 cases with nodular goiter and 4 cases with hypothyroidism (1 case with autoimmune thyroiditis, 1 case with nodular goiter, 1 case with cystic goiter and 1 case with hypothyroidism post thyroidectomy for thyroidal lymphoma). Twelve cases (44.4%) were found with tetania. The incidence of thyroid diseases in alopecia areata is higher then in general population (2%), as well as the incidence of tetania. These evidences suggest that it is necessary to make a screening of endocrinological disorders in patients with alopecia areata.

Effects of feeding high dosages of vitamin E to laying hens on thyroid hormone concentrations of hatching chicks

Long-term experimental feeding of 20,000 ppm alpha-tocopheryl acetate to laying hens caused a significant (P < 0.05) decrease in hatching rates as compared to the control group, which was fed a diet containing 19 ppm alpha-tocopherol. When the thyroid hormones in the developing chicks were checked on incubation days 16, 19, 21, and 22, the following results were ascertained: During the latter part of incubation, increases in plasma concentrations of thyroxine and triiodothyronine were observed. No significant differences in hormone concentrations (P > 0.05) between the control and the treatment group were observed during incubation days 16, 19, and 22. However, on the day of hatching (day 21 of incubation) significantly lower (P < 0.05) triiodothyronine concentrations in chick embryos of piped eggs were found in the treatment group. Moreover, thyroxine concentrations in non-piped eggs and in hatched chicks were found to be significantly higher as compared to the control group. Given these results, one concludes that extremely high dosages of vitamin E may affect thyroid hormone concentrations of hatching chicks, and therefore, the chicks might be inhibited in pipping the egg shell. Hypothetically, the hepatic enzyme 5'-monodeiodinase is involved in the mechanism of inhibition.

Are thyroid hormones or hCG responsible for hyperemesis gravidarum? A matched paired study in pregnant Chinese women

OBJECTIVE

To determine the relative importance of thyroid hormones and human chorionic gonadotropin in the etiology of hyperemesis gravidarum.

DESIGN

A prospective study comparing the hormonal status in women with hyperemesis gravidarum (5-18 weeks), with healthy pregnant controls, matched for gestational age. Sensitive thyroid stimulating hormone, free thyroxine, free triiodothyronine and total beta human chorionic gonadotropin were measured by immunoassays. The hormone results for hyperemesis gravidarum group (n=58) were compared with pregnant control women (n=58) using the Mann Whitney test. Stepwise logistic regression analysis was performed to determine which variables were significantly associated with hyperemesis gravidarum and to estimate the probability of each woman having hyperemesis gravidarum. The strength of the resulting association was tested by generating a receiver operating characteristic curve for predicting hyperemesis gravidarum using these probabilities.

RESULTS

Maternal age and all hormones were significantly different between the hyperemetic and control groups. However, logistic regression analysis demonstrated that only maternal age, free thyroxine and thyroid stimulating hormone were significant independent variables. The area beneath the receiver operating characteristic curve for prediction of hyperemesis gravidarum was 0.84.

CONCLUSION

Human chorionic gonadotropin is not independently involved in the etiology of hyperemesis gravidarum but may be indirectly involved by its ability to stimulate the thyroid. Differences in maternal age and thyroid function are highly discriminatory with regard to hyperemesis gravidarum.

Physiological neuroendocrinology of peptides, steroids and other hormones in cerebrospinal fluid

Cerebrospinal fluid acts as a conduit in neuroendocrine regulation. Valid assessment of normal cerebrospinal fluid levels of peptides, steroids and other hormones requires clarification of reference concentrations in control patients and normal volunteers. Awareness of factors which may alter neuronal activity and, in turn, the relative composition of cerebrospinal fluid constituents is essential to the accurate sampling and hormonal analysis of cerebrospinal fluid.

A thyroid hormone receptor that is required for the development of green cone photoreceptors

Color vision is facilitated by distinct populations of cone photoreceptors in the retina. In rodents, cones expressing different opsin photopigments are sensitive to middle (M, 'green') and short (S, 'blue') wavelengths, and are differentially distributed across the retina. The mechanisms that control which opsin is expressed in a particular cone are poorly understood, but previous in vitro studies implicated thyroid hormone in cone differentiation. Thyroid hormone receptor beta 2 (TR beta 2) is a ligand-activated transcription factor that is expressed in the outer nuclear layer of the embryonic retina. Here we delete Thrb (encoding Tr beta 2) in mice, causing the selective loss of M-cones and a concomitant increase in S-opsin immunoreactive cones. Moreover, the gradient of cone distribution is disturbed, with S-cones becoming widespread across the retina. The results indicate that cone photoreceptors throughout the retina have the potential to follow a default S-cone pathway and reveal an essential role for Tr beta 2 in the commitment to an M-cone identity. Our findings raise the possibility that Thrb mutations may be associated with human cone disorders.

Evaluation of thyroid function in dogs suffering from recurrent flank alopecia

Thyroid function was assessed in euthyroid dogs (n = 20), dogs suffering from canine recurrent flank alopecia (CRFA, n = 18), and hypothyroid dogs (n = 21). Blood samples obtained from all dogs in each group were assayed for total thyroxine (TT4), thyrotropin (TSH), and thyroglobulin autoantibody (TgAA) serum concentrations. Total T4 and TSH serum concentrations were significantly decreased and increased, respectively, in the hypothyroid group compared with the other 2 groups. No significant differences in TT4 and TSH serum values were found between the euthyroid and CRFA groups. Thyroglobulin autoantibodies were detected in 10, 11.1, and 61.9% of euthyroid dogs, dogs with CRFA, and hypothyroid dogs, respectively. In conclusion, dogs suffering from CRFA have a normal thyroid function, and the determination of TT4 and TSH serum concentrations allows differentiation of these dogs from dogs with hypothyroidism, in most cases. Occasionally, the 2 diseases can be concomitant.

[How reliable is the laboratory? Increased needs of patient-related quality assurance]

Recent developments in medical care and research involve the increased use of immunochemical assays for hormones, tumour markers, vitamins and drugs. External quality assurance programmes using pooled human sera usually fail to detect analytical interference due to substances (e.g. anti-immunoglobulin or anti-ligand antibodies) present in individual serum specimens. The article reports on experience gained during a three-year period when specimens from individual patients attending a thyroid unit were distributed to hospital laboratories in Sweden for analysis. Specimen selection criteria were based on contradictory findings at the initial clinical or laboratory evaluation. The programme has given rise to the formation of a network of the laboratories involved, under the co-ordination of EQUALIS (External quality assurance in laboratory medicine in Sweden).

Thyrotoxicosis: clinical and laboratory assessment

Thyroid hormones are triiodothyronine (T3) and thyroxine (T4). The hypophysial thyrotropic hormone, thyroid stimulating hormone (TSH) is their physiologic regulator. Thyrotoxicosis is characterized by clinical symptoms caused by high thyroid hormone concentrations. The commonest forms are: 1) toxic diffuse goiter (Basedow-Flajani-Graves disease), 2) toxic multinodular goiter, 3) toxic adenoma. Other less frequent forms are the iodide-induced, that during Hashimoto thyroiditis, that from inappropriate TSH secretion. The diagnosis is predominantly clinical and confirmed by hormone level determination associated in some cases to functional and morphofunctional tests (TRH test, scintigraphy, thyroid I uptake) and antithyroid antibody assay.

Clinical and laboratory assessment of subclinical thyroid disease

The availability of highly sensitive in vitro tests for the determination of the free fractions of thyroid hormones (FT3 and FT4) and TSH have allowed the identification of latent thyroid disorders (subclinical hyper- and hypothyroidism) where the clinical signs are minor or absent. The TSH determination is the most sensitive in vitro test to define the thyroid function. FT3 is the most suitable for the correct diagnosis of subclinical hyperthyroidism, while FT4 is diagnostic for subclinical hypothyroidism. In patients with acute or chronic non thyroid disease or in case of assumption of drugs which interfere with in vitro diagnostic tests, the interpretation may be difficult.

In utero methylmercury exposure differentially affects the activities of selenoenzymes in the fetal mouse brain

Pregnant ICR mice were subcutaneously injected with 0,5, or 3x3 mg Hg/kg of methylmercury (MeHg) on days 12,13, and 14(G12-14) of gestation and were sacrificed on G17. Activity of selenoenzymes, including glutathione peroxidase (GPx) and 5'- or 5-iodothyronine deiodinases (5'-DI, 5-DI), was determined in fetal brain and placenta. MeHg did not affect the concentration of Se in these tissues, while it significantly inhibited the activity of GPx in the fetal brain and placenta, but not in the maternal brain. Although the levels of thyroid hormones in the maternal and fetal plasma were not affected by MeHg, 5-DI decreased and 5'-DI increased in the fetal brain, as if they had responded to hypothyroidism. Because the level of T4 in the fetal plasma was not affected by MeHg, these changes in enzymatic activities may result in a harmful excess of T3 in the fetal brain. In addition, 5-DI activity was increased in the placenta of MeHg-treated mice. These effects of prenatal MeHg exposure on fetal and placental DIs differed from those of dietary-induced Se deficiency, where the activities of DIs were decreased or not affected. Further evaluation of the effect of MeHg on selenoenzymes, especially 5-DIs, is warranted.

Postnatal exposure to chlorinated dioxins and related chemicals on thyroid hormone status in Japanese breast-fed infants

Effects of postnatal exposure to polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and coplanar polychlorinated biphenyls (Co-PCBs) on thyroid hormone status were studied in the peripheral blood of 36 breast-fed Japanese infants. Estimated total intakes of these chemicals in toxic equivalent quantity (TEQ) converted into 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) from the breast milk significantly and negatively correlated with the levels of triiodothyronine (T3) and thyroxine (T4) in the blood of breast-fed babies. Therefore, exposure to background levels of the highly toxic organochlorine chemicals through the breast milk may cause some effects on thyroid hormone status in Japanese infants.

Alterations in the enzyme activity and protein contents of protein disulfide isomerase in rat tissues during fasting and refeeding

Protein disulfide isomerase (PDI) is an enzyme that participates in the formation of disulfide bonds. It is also known to be the subunits of some enzymes and the membrane-associated thyroid hormone-binding protein. In this study, we measured the quantitative distribution of PDI protein in rat tissues and examined the relationship between protein level and enzyme activity in PDI during fasting and refeeding. Western blotting with specific anti-PDI antiserum detected the PDI protein band of 55 kd. Among several tissues, liver contained the largest amount of PDI protein, followed by kidney and fat, in which one-third to one-fourth of the hepatic PDI protein existed. The PDI protein band was also detected in heart and muscle. Fasting for 3 days decreased PDI protein levels in rat liver by 40%; control levels were recovered after 3 days of refeeding. The same change was observed in kidney. PDI activity, measured by the scrambled ribonuclease method, did not show the parallel alteration to PDI protein level in liver and kidney. Isomerase activity decreased to 50% of control values during fasting, but did not recover by refeeding. Thyroidal status did not affect either PDI protein level or isomerase activity. These findings show that fasting and refeeding affect PDI protein and enzyme activity, and that PDI protein level does not always reflect PDI activity.

Neonatal screening for congenital hypothyroidism in Estonia

Screening for congenital hypothyroidism was carried out by measuring thyroid stimulating hormone (TSH) on dried blood spots (mean + 2SD cut off value 12 microU/ml) by fluoroimmunoassay using DELFIA kits. A total of 20,021 infants were screened, and seven cases with congenital hypothyroidism were detected, giving an incidence of congenital hypothyroidism of 1:2860 (female:male ratio 6:1). In four of seven infants with congenital hypothyroidism (57%) the mother also had thyroid disease, supporting the importance of genetic factors as a cause of congenital hypothyroidism. Transient hyperthyrotropinaemia occurred in 654 infants (recall rate 3.3%). There was a significant association of transient hyperthyrotropinaemia only with cardiac failure at birth or caesarean section (p < 0.01). Family studies showed no predisposition to thyroid diseases associated with a transient increase of TSH.

Recurrent phyllodes tumor in a man

The case of a 35-year-old man with a borderline-type cystosarcoma phyllodes is presented. Four years after the primary excision of the tumor, wide excision of a local recurrence and postoperative radiotherapy were performed. No repeated relapse was observed during a 5-year follow-up. Neither significant endocrine changes nor genetic alteration could be proven. However, a slightly increased SHBG concentration was detected, resulting in a decreased biologically available androgen level reduced testosterone/SHBG index. This phenomenon might be a consequence of the chronic liver disease of the patient due to his type II diabetes mellitus and alcohol abuse. In addition to the conventional histopathological examinations, immunohistochemical and electron-microscopic investigations were carried out on tissue sections, and the steroid receptors, EGF receptors and EGF-like activity of the tumor were also studied.

Effects of long-term temperature acclimation on thyroid hormone deiodinase function, plasma thyroid hormone levels, growth, and reproductive status of male Atlantic cod, Gadus morhua

The recent collapse of the Northwestern Atlantic cod fisheries has coincided with a cooling of water temperatures. During this time the condition factor of cod has been poor. The objective of the present study was to determine the effects of long-term temperature acclimation on growth reproduction and thyroid function in laboratory held Atlantic cod (Gadus morhua). One of the key parameters used to assess thyroid function is the peripheral metabolism of L-thyroxine (T4) by microsomal deiodinase enzymes. Deiodinase function has not been described for gadid fish. T4 outer-ring deiodinating activity (apparent K(m) 1-2 nM) was confined primarily to liver. Its properties resembled those for hepatic T4ORD activity of other teleosts and the mammalian type II deiodinase. The T4ORD activity of cod liver exceeded that of salmonids and could explain the high plasma T3 levels (10-18 ng/ml), which were 2-5 times greater than T4 levels. T4 and T3 inner-ring deiodination was confined mainly to brain. In order to determine the effects of long-term temperature acclimation on cod, somatic growth, reproduction, and thyroidal status were assessed monthly in 400-900-g satiation-fed male Atlantic cod captured in June from the St. Lawrence Estuary and then acclimated from August to the following June under a natural photoperiod at 2-4 degrees C (LT) or 6-10 degrees C (HT). Reproductive status was determined from the gonadosomatic index (GSI), plasma testosterone (T) and 11-ketotestosterone (11-KT) levels, and the appearance of milt; thyroidal status was determined from plasma T4 and 3,5,3'-triiodo-L-thyronine (T3) levels and hepatic T4ORD activity to produce biologically active T3. Testis maturation (high levels of 1 and 11-KT, and milt release) occurred in April and May and was uninfluenced by acclimation temperature. LT cod grew more slowly than HT cod. Differences in body weight were particularly evident from December to February. In conclusion, (i) cod possess outer- and inner-ring deiodinase activities, predominating respectively in liver and brain, and with properties resembling those of other teleosts, (ii) T4ORD activity of liver is unusually high and may account for the high plasma T3 levels in this species, (iii) T4ORD activity tends to increase during periods of increased somatic growth, and (iv) chronic acclimation of male cod to 2-4 degrees C, as opposed to 6-10 degrees C, decreases somatic growth but does alter circulating levels of thyroid hormones and androgens and it does not change the time of sexual maturation.

Divergent deiodination of thyroid hormones in the separated parts of the fetal and maternal placenta in pigs

Previous work from this laboratory has shown that the thyroid gland of the fetal pig begins to function at about day 46-47 (0.40-0.415 fraction of gestational age). Sera from fetuses contain lower thyroxine (T4), 3,3',5-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) concentrations than maternal sera, except for about 2 weeks before term. The fetal T4 metabolism is dominated by the 5'-monodeiodinating activity (5'-MD). In the present study we measured the iodothyronines content, and the outer (5'-MD) and inner (5-MD) monodeiodinases activity, in homogenates of the placenta. The pig placenta, which is of the epitheliochorial type, was separated into the fetal and the maternal part. The concentrations of T4, T3 and rT3 were lower, and the deiodinating activity of 5'-MD and 5-MD higher, in the fetal than in the maternal placenta. The fetal placenta not only deiodinated more actively T4 to T3 and T4 to rT3, but degraded T3 to 3,3'-diiodothyronine (3,3'-T2) more actively than rT3 to 3,3'-T2. Such divergent deiodinating activity of T4 to T3, T3 to 3,3'-T2 and rT3 to 3,3'-T2 might favor establishing a relatively high and constant rT3 concentrations in fetal and maternal placentas, and a lower T3 in the fetal placenta. The inner ring deiodinating activity (excluding a day before parturition) was always more active in the fetal placenta, while the outer ring deiodinations varied in this respect, depending on the gestation stage. These results support the hypothesis that in the fetal pig, enzymatic deiodination of thyroid hormones forms a barrier which reduces transplacental passage of the hormones and that the fetal part of the placenta is the primary factor in the mechanism regulating the hormonal transfer. In spite of the presence of the barrier, there is an adequate maternal supply of thyroid hormones to the fetus in early gestation, which suggests that the enzymatic mechanism is influenced in some way by the thyroid status of the fetus.