Maternal thyroid-blocking immunoglobulins in congenital hypothyroidism

The Clinical Value and Variation of Antithyroid Antibodies during Pregnancy

Antithyroid antibodies, which include thyroid-stimulating hormone receptor antibodies (TRAbs), thyroid peroxidase antibodies (TPOAbs), and thyroid globulin antibodies (TgAbs), are widely known for their tight association with thyroid autoimmune diseases. The variation in all three kinds of antibodies also showed different trends during and after pregnancy (Weetman, 2010). This article reviewed the the physiological changes, while focusing on the variation of thyroid antibodies concentration in women during and after pregnancy, and adverse consequences related to their elevation. Since abnormal elevations of these antithyroid antibodies may lead to adverse outcomes in both mothers and fetuses, special attention must be paid to the titer of the antibodies during pregnancy. The molecular mechanisms of the variations in those antibodies have yet to be explained. The frequency and timing of thyroid antibody measurement, as well as different reference levels, also remain to be elucidated.

Thyrotropin Receptor Blocking Antibodies

Autoantibodies (Ab) against the thyroid-stimulating hormone receptor (TSHR) are frequently found in autoimmune thyroid disease (AITD). Autoantibodies to the TSHR (anti-TSHR-Ab) may mimic or block the action of TSH or be functionally neutral. Measurement of anti-TSHR-Ab can be done either via competitive-binding immunoassays or with functional cell-based bioassays. Antibody-binding assays do not assess anti-TSHR-Ab functionality, but rather measure the concentration of total anti-TSHR binding activity. In contrast, functional cell-based bioassays indicate whether anti-TSHR-Ab have stimulatory or blocking activity. Historically bioassays for anti-TSHR-Ab were research tools and were used to study the pathophysiology of Graves' disease and Hashimoto's thyroiditis. In the past, bioassays for anti-TSHR-Abs were laborious and time-consuming and varied widely in performance from laboratory to laboratory. Recent advances in the development of cell-based assays, including the application of molecular engineering, have led to significant improvements that have enabled bioassays to be employed routinely in clinical laboratories. The prevalence and functional significance of TSHR blocking autoantibodies (TBAb) in autoimmune hypothyroidism has been less well investigated compared to TSHR stimulating Ab. There is an increasing body of data, however, that demonstrate the clinical utility and relevance of TBAb, and thus the importance of TBAb bioassays, in the diagnosis and management of patients with AITD. In the present review, we summarize the different methods used to measure TBAb, and discuss their prevalence and clinical relevance.

Prevalence and clinical relevance of thyroid stimulating hormone receptor-blocking antibodies in autoimmune thyroid disease

The prevalence and clinical relevance of thyroid stimulating hormone (TSH) receptor (TSHR) blocking antibodies (TBAb) in patients with autoimmune thyroid disease (AITD) was investigated. Serum TBAb were measured with a reporter gene bioassay using Chinese hamster ovary cells. Blocking activity was defined as percentage inhibition of luciferase expression relative to induction with bovine TSH alone (cut-off 40% inhibition). All samples were measured for TSHR stimulatory antibody (TSAb) and TSHR binding inhibiting immunoglobulins (TBII). A total of 1079 unselected, consecutive patients with AITD and 302 healthy controls were included. All unselected controls were negative for TBAb and TSAb. In contrast, the prevalence of TBAb-positive patients with Hashimoto's thyroiditis and Graves' disease was 67 of 722 (9·3%) and 15 of 357 (4·2%). Of the 82 TBAb-positive patients, thirty-nine (48%), 33 (40%) and 10 (12%) were hypothyroid, euthyroid and hyperthyroid, respectively. Ten patients were both TBAb- and TSAb-positive (four hypothyroid, two euthyroid and four hyperthyroid). Thyroid-associated orbitopathy was present in four of 82 (4·9%) TBAb-positive patients, with dual TSHR antibody positivity being observed in three. TBAb correlated positively with TBII (r = 0·67, P < 0·001) and negatively with TSAb (r = -0·86, P < 0·05). The percentage of TBII-positive patients was higher the higher the level of inhibition in the TBAb assay. Of the TBAb-positive samples with  > 70% inhibition, 87% were TBII-positive. Functional TSHR antibodies impact thyroid status. TBAb determination is helpful in the evaluation and management of patients with AITD. The TBAb assay is a relevant and important tool to identify potentially reversible hypothyroidism.

Analytical Performance and Validation of a Bioassay for Thyroid-Blocking Antibodies

BACKGROUND AND OBJECTIVE

A cell-based bioassay for the measurement of thyroid blocking autoantibodies (TBAb) has been recently reported. The analytical performance and validation of this bioassay is assessed and described.

METHODS

Chinese hamster ovary cells expressing a chimeric thyrotropin receptor were treated with bovine (b) TSH and different concentrations of an immunoglobulin G (IgG) monoclonal human TBAb (K1-70). TBAb was measured as a function of luciferase activity relative to bTSH alone and expressed as percent inhibition. Results obtained in the chimeric cell line were compared with those of a wild-type cell line. Analytical performance studies were subsequently performed with the chimeric cell line only.

RESULTS

Immunodepletion of K1-70 IgG by using a protein G-Sepharose column showed that positive percent inhibition in the TBAb bioassay was detectable from K1-70 IgG only. The limit of blank was determined to be 12.2%. The limit of detection was 14% inhibition, equivalent to 0.4 ng/mL K1-70, while the limit of quantitation was 22% (coefficient of variation [CV] 12%) equivalent to 0.625 ng/mL K1-70. The dynamic range was between 14 ± 3.7 (mean % inhibition ± standard deviation) and 101 ± 2.6, equivalent to 0.4-10 ng/mL K1-70. The linear range was between 22 ± 2.6 and 93 ± 0.6 inhibition, equivalent to 0.625-5 ng/mL K1-70. The upper limit of the 99th percent reference range was 34% inhibition. In two laboratories, CV values for the intra- and inter-assay precisions for K1-70 ranged from 2% to 12% and from 1.7% to 14.5%, respectively. For patient sera, the CV values for the intra- and inter-assay precisions ranged from 3% to 9% and from 3% to 11%, respectively. No interference was found when follicle-stimulating hormone, luteinizing hormone, and human chorionic gonadotrophin were tested in the TBAb bioassay. The median of % inhibition values in 40 TBAb positive sera from patients with autoimmune thyroid disease were 93.5 (range 25-103) and 92 (range 64-107) for the wild type and chimeric cell lines, respectively. Further, all 40 samples of patients with various non-thyroidal autoimmune diseases were TBAb negative.

CONCLUSIONS

This TBAb bioassay exhibits excellent analytical performance and high level of reproducibility.

Severe unsuspected maternal hypothyroidism discovered after the diagnosis of thyrotropin receptor-blocking antibody-induced congenital hypothyroidism in the neonate: failure to recognize and implications to the fetus

BACKGROUND

Whereas most adequately treated children with congenital hypothyroidism (CH) do well neurodevelopmentally, when both the maternal and fetal thyroid glands are compromised, significant cognitive delay can occur despite early and aggressive postnatal therapy. Maternal thyrotropin-stimulating hormone receptor (TSHR)-blocking antibodies (Abs) can be transmitted to the fetus and cause combined maternal-fetal hypothyroidism. Current guidelines recommend their measurement only if mothers have known autoimmune thyroid disease, there is a history of a previously affected sibling, or when transient CH is suspected.

RESULTS

We report 3 infants in whom the diagnosis of maternal hypothyroidism was not known and was identified only after CH was diagnosed in their babies. One of these infants had developmental delay despite rapid normalization of thyroid function postnatally. All 3 mothers had potent TSHR Abs in serum, but thyroid peroxidase Abs and thyroglobulin Abs were detectable in only 2 of them.

CONCLUSIONS

TSHR-blocking Ab-induced CH should be suspected in any baby with CH irrespective of the known family history, especially if the hypothyroidism is severe and a eutopic thyroid gland is demonstrated on imaging. Measurement of TSHR Abs is necessary to establish the diagnosis; the presence of other thyroid Abs is insufficiently sensitive and may miss some cases.

Thyroid autoantibodies in cord blood sera from children and adolescents with autoimmune thyroiditis

Autoimmune thyroid disease is common among women of childbearing age. Thyroid autoantibodies are predominantly of the immunoglobulin G (IgG)-type and pass the placental barrier from mother to child. Recent studies have suggested a pathogenetic role for transplacentally transmitted autoantibodies in the development of autoimmune disease. The aim of the present study was to investigate if children and adolescents with autoimmune thyroiditis (AIT) have been exposed to thyroid autoantibodies already in utero. Cord blood sera taken at delivery from 34 newborns who had developed AIT during childhood and adolescence, and sera from 31 of their mothers, were analyzed for the presence of autoantibodies against thyroid peroxidase (TPOAb) and thyroglobulin (TgAb), and compared to 233 randomly selected control children and their mothers. The prevalence of TPOAb in cord blood sera was elevated among the children and adolescents with AIT compared to controls (38% versus 14%; odds ratio [OR] 4.12, p < 0.001). An increased prevalence of TPOAb was also found among their mothers (29% versus 15%; OR 2.17, p < 0.048). No significant difference in the prevalence of TgAb was found either between children with AIT and the control children (18% versus 9%; OR 2.16, p < 0.15), or between their mothers and the control mothers (23% versus 12%; OR 2.17, p < 0.16). Most of the TPOAb-positive children had TPOAb-positive mothers, indicating the maternal origin of their TPOAb. In conclusion, a large proportion of children who later developed AIT had already been exposed to transplacentally transmitted TPOAb in utero. Whether these autoantibodies have any pathogenetic role in the development of AIT in offspring or if they simply mirror the inheritance of AIT, remains to be investigated.

Autoimmune thyroid disease in pregnant women and their offspring

OBJECTIVE

To present an overview of autoimmune thyroid disease (AITD) that can occur in pregnancy.

METHODS

The major thyroid antibodies that can traverse the maternal-fetal circulation and affect the fetus are summarized, those women at risk of having affected fetuses are identified, and the diagnosis, course, and treatment of AITD in maternal and neonatal patients are discussed.

SUMMARY

AITD, including Graves' disease and autoimmune thyroiditis, is common in women of childbearing age. Rarely, the fetus can be affected because of transplacental passage of maternal IgG. Of the thyroid autoantibodies found in AITD, only those directed against the thyroid-stimulating hormone (TSH) receptor have been shown to cause fetal thyroid dysfunction. Both transient neonatal hyperthyroidism and hypothyroidism have been described, as has delayed onset of neonatal hyperthyroidism due to the coexistence of stimulating and blocking TSH receptor antibodies. In general, affected infants are those born to mothers with the most potent antibody activity, and the duration of the neonatal thyroid dysfunction is dependent on the antibody titer and the rate of metabolic clearance from the infant's circulation. If fetal hyperthyroidism is suspected, maternal TSH receptor antibodies should be measured during the third trimester of pregnancy. For neonatal hypothyroidism, this measurement in the mother or baby soon after birth will suffice. Screening for the presence of TSH receptor antibodies by radioreceptor assay is the most cost-effective approach. If results are positive, bioassay should be done to determine the nature of the antibody activity. Women at risk of having babies with neonatal hyperthyroidism include those with a history of previous affected infants, with difficult to control thyrotoxicosis, or with a history of Graves' disease and development of hypothyroidism either spontaneously or as a result of thyroid gland ablation. Transient neonatal hypothyroidism due to TSH receptor-blocking antibodies should be suspected in any infant with hypothyroidism born to a mother with AITD (particularly those with previously affected offspring).

CONCLUSION

Treatment of maternal hyperthyroidism must consider both maternal and fetal thyroid status. In general, the lowest dose of antithyroid medication sufficient to produce maternal euthyroidism or slight hyperthyroidism is used. In pregnant women with hypothyroidism, doses of L-thyroxine should be sufficient to normalize maternal thyroid function without regard to the fetus. Identification and treatment of affected infants soon after birth will ensure a normal outcome. Whether inadequately treated maternal hypothyroidism is associated with a permanent intellectual deficit in the offspring is currently unknown.

Epitope heterogeneity of thyrotropin receptor-blocking antibodies in Graves' patients as detected with wild-type versus chimeric thyrotropin receptors

The stable transfectants of wild-type (W25) and mutant thyrotropin-receptor (TSH-R) allow detection of the bioactivities of TSH-R antibodies in Graves' patients. A mutant Chinese hamster ovary (CHO) cell line (Mc1+2) transfected with a chimeric construct, where residues 8 to 165 of the TSH-R are replaced with residues 10 to 166 of the lutropin/choriogonadotropin (LH/CGR) receptor, lacks the cyclic adenosine monophosphate (cAMP) response to most thyrotropin stimulating antibodies (TSAb), yet retains the response to TSH and acquires the response to LH/CG. We compared Mc1+2 cells with wild-type W25 cells for their ability to detect TSAb as well as thyrotropin-blocking antibodies (TBAb) in Graves' sera. Eighteen normal and 39 Graves' sera were tested for TSAb and TBAb levels by in vitro bioassays using W25 and Mc1+2 cells. In addition, these sera were also tested for thyrotropin-binding inhibitory activity (TBII) by a radioreceptor assay. Eighteen (47%) Graves' sera had TBAb activity measured with W25 cells but not with Mc1+2 cells. These TBAbs were, therefore, a population of antibodies with functional epitopes on the N-terminus of the extracellular domain. This TBAb activity by W25 cells exhibited a high degree of correlation with TBII levels by a radioreceptor assay (r = 0.70, p = 0.001). Ten (25.6%) Graves' sera had positive TBAb activity in both W25 and Mc1+2 cells; moreover, their activity in both assays was similar (r = 0.83, p < 0.001). The TBAb activity in these sera, however, did not correlate with TBII activity. Eleven (28%) Graves' sera had no TBAb activity. Overall, thyroid-stimulating antibodies were detected in 87% and 28% of the 39 Graves' sera by W25 and Mc1 +2 cells, respectively. Thus, using the 2 cell lines, at least 2 distinct populations of TBAbs were detected. One is detected in a similar fashion by both W25 and Mc1+2 cell lines and likely interacts with the epitopes residing in the unaltered C-terminus of the TSH-R. The other is reactive in W25 cells only, indicating the loss of TBAb epitope in the chimeric receptor located in the N-terminus of the TSH-R. Furthermore, our results indicate that the TBAb binding epitope in 8-165 residues of the native TSH-R is highly associated with TBII activity in Graves' disease. These results indicate that patients with Graves' disease harbor TBAbs with epitope heterogeneity and favor the notion that there are different sites and mechanisms by which TBAbs act in Graves' patients. It remains to be determined whether or not TBAb subtyping will have a useful predictive role in the management of patients with Graves' disease.

Characterization of monoclonal thyroid-stimulating and thyrotropin binding-inhibiting autoantibodies from a Hashimoto's patient whose children had intrauterine and neonatal thyroid disease

A multiplicity of TSH receptor autoantibodies (TSHRAbs) have been characterized after subcloning heterohybridomas produced from the lymphocytes of a patient who has Hashimoto's thyroiditis and had three children with intrauterine or neonatal hyperthyroidism. Twelve clones produced stimulating TSHRAbs that increased cAMP levels and iodide uptake in rat FRTL-5 thyroid cells and increased cAMP levels in Chinese hamster ovary (CHO) cells transfected with the human TSHR; like 95% of Graves' stimulating TSHRAbs, all 12 have their functional epitope on the N-terminus of the TSHR extracellular domain, requiring residues 90-165 for activity. All 12 bind to human thyroid membranes in the absence, but not the presence, of TSH, but are only weak inhibitors of TSH binding in assays measuring TSH binding-inhibiting Igs (TBIIs). In contrast, 8 different clones produced TSHRAbs that did not increase cAMP levels, but, instead, exhibited significant TBII activity. Four inhibited the ability of TSH or a stimulating TSHRAb to increase cAMP levels and had their functional epitope on the C-terminal portion of the TSHR external domain, residues 261-370, mimicking the properties of blocking TSHRAbs that cause hypothyroidism in patients with idiopathic myxedema. The 4 other TBIIs inhibited the ability of TSH, but not that of a stimulating TSHRAb, to increase cAMP levels, like TBIIs in Graves' patients. The functional epitope for 3 of these Graves'-like TBIIs was residues 90-165; the functional epitope for the fourth was residues 24-89. The fourth also increased arachidonic acid release and inositol phosphate levels in FRTL-5 thyroid cells and exhibited conversion activity, i.e. the ability to increase cAMP levels in the presence of an anti-human IgG. Thus, this TBII exhibited signal transduction activity, unlike the other 3 Graves'-like TBIIs. The patient, therefore, has stimulating TSHRAbs and 3 different types of TBIIs, each with different functional properties and different epitopes on the TSHR.

The disorders induced by iodine deficiency

This paper reviews present knowledge on the etiology, pathophysiology, complications, prevention, and therapy of the disorders induced by iodine deficiency. The recommended dietary allowances of iodine are 100 micrograms/day for adults and adolescents, 60-100 micrograms/day for children aged 1 to 10 years, and 35-40 micrograms/day in infants aged less than 1 year. When the physiological requirements of iodine are not met in a given population, a series of functional and developmental abnormalities occur including thyroid function abnormalities and, when iodine deficiency is severe, endemic goiter and cretinism, endemic mental retardation, decreased fertility rate, increased perinatal death, and infant mortality. These complications, which constitute a hindrance to the development of the affected populations, are grouped under the general heading of iodine deficiency disorders (IDD). At least one billion people are at risk of IDD. Iodine deficiency, therefore, constitutes one of the most common preventable causes of mental deficiency in the world today. Most of the affected populations live in mountainous areas in preindustrialized countries, but 50 to 100 million people are still at risk in Europe. The most important target groups to the effects of iodine deficiency from a public health point of view are pregnant mothers, fetuses, neonates, and young infants because the main complication of IDD, i.e., brain damage resulting in irreversible mental retardation, is the consequence of thyroid failure occurring during pregnancy, fetal, and early postnatal life. The main cause of endemic goiter and cretinism is an insufficient dietary supply of iodine. The additional role of naturally occurring goitrogens has been documented in the case of certain foods (milk, cassava, millet, nuts) and bacterial and chemical water pollutants. The mechanism by which the thyroid gland adapts to an insufficient iodine supply is to increase the trapping of iodide as well as the subsequent steps of the intrathyroidal metabolism of iodine leading to preferential synthesis and secretion of triiodotyronine (T3). They are triggered and maintained by increased secretion of TSH, which is ultimately responsible for the development of goiter. The acceleration of the main steps of iodine kinetics and the degree of hyperstimulation by TSH are much more marked in the pediatric age groups, including neonates, than in adults, and the development of goiter appears as an unfavorable side effect in the process of adaptation to iodine deficiency during growth. The most serious complication of iodine deficiency is endemic cretinism, a syndrome characterized by irreversible mental retardation together with either a predominant neurological syndrome or predominant hypothyroidism, or a combination of both syndromes.(ABSTRACT TRUNCATED AT 400 WORDS)

Transient congenital hypothyroidism due to maternal thyrotrophin binding inhibiting immunoglobulin

Transient congenital hypothyroidism due to maternal thyrotrophin binding inhibitor immunoglobulin (TBII), a thyroid-stimulating hormone (TSH)-receptor blocking antibody, is described in three male siblings born to a mother with autoimmune thyroiditis. These cases are believed to be the first described in Australia. The first child was found to have a serum TSH of 565 mU/L and had a negative thyroid scan when presented for neonatal screening. He was treated with thyroxine but became thyrotoxic at 3 months of age when he was on a dosage of 85 micrograms/m2 of body surface area. He was euthyroid 6 months after discontinuation of therapy. Nine years later a second hypothyroid sibling was born, with a serum TSH of 709 mU/L on day 4. Both mother and child were demonstrated to be strongly positive for TBII. Again this child was able to cease therapy by the age of 9 months. A third sibling, also TBII positive, was born 12 months after the second. His TSH was 90 mU/L and his serum thyroxine (T4) was 169 nmol/L. On this occasion, thyroid stimulation-blocking antibody was found to be present in the serum of both mother and child. Thyroxine therapy was ceased at 1 month. The family present a picture of varying degrees of transient neonatal hypothyroidism due to the transplacental passage of a maternal receptor blocking antibody. The condition is self-limiting, resolving when the immunoglobulin is cleared from the infant's circulation.

Thyrotropin receptor antibodies: advances and importance of detection techniques in thyroid diseases

The study of autoimmune thyroid disorders (AITD) has greatly contributed to our knowledge of autoimmunity. Graves' disease and Hashimoto's thyroiditis represent two ends of the range of autoimmune responses seen in AITD. Autoantibodies reactive to cytoplasmic antigens are associated with cell damage, and thyrotropin (TSH)-receptor antibodies (TRAb) influence the function and growth of the gland and play a major role in pathogenesis. The heterogeneous nature of TRAb is well accepted. Besides their long-known thyroid stimulating activity, TRAb can act as blocking antibodies or growth-promoting antibodies and, thus, cause hypothyroidism (primary myxedema) or endemic and sporadic goiters, respectively. Advanced methodologies for detection of these antibodies with the TSH-receptor assay and thyroid cell bioassay allow various activities to be measured. Current data using these assays confirm the presence of heterogeneity of functional activities of TRAb(s) in vivo. The activity of predominating antibody may relate to clinical presentation. This indicates a need for paired determinations of both TSH-binding inhibitory immunoglobulin (TBII) and thyroid-stimulating immunoglobulin (TSI) for accurate clinical correlations. Cloning the TSH-receptor gene has clarified its structure and function. The future identification of its epitopes will further delineate the clinical role of these antibodies and may allow development of new diagnostic and therapeutic approaches.

Thyrotrophin-blocking antibodies in congenital hypothyroidism

The role of transplacental transfer of maternal thyrotrophin (TSH)-blocking antibodies causing congenital hypothyroidism in Southern Chinese children was examined in this study. Twenty-two mothers of 24 patients with congenital hypothyroidism were studied 3-5 years after delivery. None of them had thyroid dysfunction at delivery or at the time of study. None had antithyroglobulin or antimicrosomal antibody. Only one mother was found to have TSH-binding inhibitory immunoglobulin (TBII), and her child had agenesis of the thyroid. This women had Graves disease in remission for 2 years before delivery. None had TSH-stimulated cAMP response inhibitory immunoglobulin (TSII). Ten of the 24 congenital hypothyroid children had transient neonatal hypothyroidism, seven had agenesis of the thyroid, six had dyshormonogenesis and one had a sublingual thyroid. As none of the mothers who had children with transient neonatal hypothyroidism had blocking antibodies at the time of study, the aetiology of the transient neonatal hypothyroidism remains unclear. These data suggest that maternal TSH-blocking antibodies do not play a role in most cases of sporadic congenital hypothyroidism.

Congenital hypothyroidism

Because of the lack of signs and symptoms in the first weeks of life, the most important tool for the early diagnosis of congenital hypothyroidism is a newborn screening program.

Fetal and neonatal hyperthyroidism and hypothyroidism due to maternal TSH receptor antibodies

Autoimmune thyroid disease is a generic term that includes Graves' disease and Hashimoto's thyroiditis. In the former, there is overactivity of the thyroid due to the action of a thyroid-stimulating antibody (TSAb). Pathogenesis of Hashimoto's thyroiditis is largely cell-mediated immune destruction of the thyroid. Nonetheless, there may be either a goiter or an atrophic gland. There is evidence that in some patients the lack of goiter is associated with the presence in the blood of an antibody that inhibits the binding of TSH to its receptor. This TSH-binding inhibiting antibody (TBIAb), therefore, prevents TSH from stimulating the thyroid and constitutes an acceptable explanation for an agoitrous state. Collectively, TSAb and TBIAb, both of which are IgG, are known as TSH receptor antibodies (TRAb).

Thyroid growth immunoglobulins in feline hyperthyroidism

Feline hyperthyroidism bears a strong clinical and pathologic resemblance to toxic nodular goiter in humans. To evaluate whether the observed thyroid growth might be due to circulating thyroid antibodies, as has been postulated in humans, we studied the effect of purified immunoglobulin (Ig) G preparations on a rat thyroid follicular (FRTL-5) cell line. When compared with control, hyperthyroid cat IgG caused significantly increased [3H]-thymidine (Tdr) incorporation into DNA (p less than 0.02) and stimulated cellular proliferation 15-fold. Stimulation of 3H-Tdr incorporation tended to be biphasic and could be inhibited completely by a potent, specific TSH receptor blocking antibody. Hyperthyroid cat IgG also significantly inhibited 125I-bTSH binding to porcine thyroid membranes, an effect that could be reproduced using electrophoretically pure IgG and normal cat thyroid membranes. Unlike its effect on growth, hyperthyroid cat IgG did not stimulate intracellular cAMP, and there was no correlation between thyroid function in vivo and IgG growth-promoting activity in vitro. These data suggest that elevated titers of thyroid growth IgGs, probably acting through the TSH receptor, are present in feline hyperthyroidism and may play a role in goiter formation. Unlike growth, the thyroid hyperfunction observed is not IgG dependent. Further study of feline hyperthyroidism may contribute important insights into human nodular goiter and into the mediation of thyroid growth in general.

Thyroid atrophy in myxedematous endemic cretinism: possible role for growth-blocking immunoglobulins

We have examined the ability of IgGs obtained from 8 endemic cretins to inhibit TSH-stimulated thyroid cell growth in culture. Clinical and laboratory evidence for hypothyroidism was present in six subjects; the two remaining patients had borderline low serum T4, normal T3 and exaggerated TSH response to TRH. In six patients 2 mg IgG exhibited an inhibitory effect in the cellular growth expressed by a diminished incorporation of 3H-thymidine into the DNA of TSH-stimulated FRTL-5 cells (range: 26-87% inhibition). Seven patients presented clinically with thyroid atrophy of relatively small thyroid enlargements for the degree of chronic iodine deficiency that was present in the area. The remaining subject had a large multinodular goiter and IgG purified from this patient had no inhibitory effect in the FRTL-5 cellular growth. A direct relationship was noted between the degree of thyroid growth inhibition (%) and the basal serum TSH concentration. We conclude that the presence of thyroid growth inhibiting immunoglobulin may be related to the absence of thyroid growth or even thyroid atrophy in endemic cretins.