Clinical importance of reversibility in primary goitrous hypothyroidism

Iodide-sensitive Graves' hyperthyroidism and the strategy for resistant or escaped patients during potassium iodide treatment

The effectiveness of potassium iodide (KI) (100 mg/day) was evaluated in 504 untreated patients with Graves' hyperthyroidism (GD). Initial response to KI within 180 days, the effect of additional methylmercaptoimidazole (MMI) or radioactive iodine (RI) in resistant or escaped patients, and long-term prognosis were evaluated. Serum fT₄ levels became low or normal in 422 patients (83.7%, KI-sensitive group) without serious side effects. Among these patients, serum TSH levels became high (n = 92, hypothyroid) or normal (n = 78) in 170 patients (33.7%) (KI-sensitive with a recovered TSH response, Group A), but remained suppressed in 252 patients (50.0%) (KI-sensitive with TSH suppression, Group B). Serum fT₄ levels decreased but remained high in 82 patients (16.3%) (KI-resistant, Group C). Older patients, or those with small goiter and mild GD were more KI-sensitive with a recovered TSH response than others. Escape from KI effect occurred in 0%, 36% and 82% in Group A, B and C, respectively. Patients in Group B and C were successfully treated with additional low-dosage MMI or RI. After 2-23 years' treatment (n = 429), remission (including possible remission) and spontaneous hypothyroidism were significantly more frequent in Group A (74.3% and 11.1%, respectively,) than in Groups B (46.3% and 2.8%, respectively) or C (53.6% and 1.5%, respectively) (p < 0.0001). In conclusion, a high KI sensitivity with a recovered TSH response was observed in about a third of the patients in GD associated with a better prognosis. Additional MMI or RI therapy was effective in escaped or KI-resistant patients with suppressed TSH level.

Clinical factors predicting the successful discontinuation of hormone replacement therapy in patients diagnosed with primary hypothyroidism

BACKGROUND

Although reversible in some patients, primary hypothyroidism is considered a permanent condition requiring lifelong hormone therapy. This study aimed to investigate the factors predicting the successful discontinuation of levothyroxine (L-T4) therapy in patients with primary hypothyroidism.

METHODS

A retrospective study was performed in primary hypothyroidism patients who met inclusion criteria: patients who maintained stable L-T4 therapy for more than 1 year, following gradual dose reduction of L-T4 based on the clinical decision (L-T4 tapering); patients receiving either no L-T4 or a fixed minimum dose for more than 1 year after L-T4 tapering. Reduction in L-T4 dosage by 12.5-50 μg within 3 months was considered as L-T4 tapering. Serum free T4, TSH, and clinical symptoms were evaluated before, during and after tapering. Logistic regression and decision tree analyses were performed to predict the successful discontinuation of L-T4.

RESULTS

Among 382 patients, 22.5% and 58.4% showed successful discontinuation (T4-Discontinued) and dose reduction (T4-Reduced) of L-T4 therapy, while other did not obtained any reduction of L-T4 dose (T4-Unchanged). The median number of tapering visit was 1.0 (range, 1.0-4.0). In T4-Discontinued group, the TSH level and the positive rate of anti-thyroperoxidase at the time of L-T4 initiation were lower, the duration of L-T4 therapy was shorter, and the maintenance dose of L-T4 at the time of tapering was lower than those in the T4-Unchanged group. In ultrasonography, normal parenchyma was preserved in the T4-Discontinued group while others showed higher rates of heterogeneous or hypoechoic parenchymal changes. Among those different characteristics, the longer duration of L-T4 therapy and the higher maintenance dose of L-T4 at the time of tapering significantly predicted the failure of discontinuation of L-T4 in multivariate analysis. A decision tree showed that patients with a duration of L-T4 therapy >4.6 years had lower success rate of discontinuation.

CONCLUSION

Shorter duration of L-T4 therapy and lower L-T4 dose at the time of tapering are the predictable factors for successful L-T4 tapering in stably maintained primary hypothyroidism patients.

Hypothesis: Persistently normal TSH levels may be used to recognize patients with transient forms of hypothyroidism and to suggest treatment withdrawal

There are no text-book recommendations on when or if treatment should or could be stopped in patients with a diagnosis of hypothyroidism, and these patients usually receive lifelong thyroxine therapy (despite the fact that some of them may have forms of transient hypothyroidism that will later recover function). Since TSH fluctuations during thyroxine treatment are common and a lack of this fluctuation might be used to identify patients who no longer need thyroxine treatment, we hypothesize that by offering patients with persistently controlled TSH levels a withdrawal trial of thyroxine treatment we may identify those who no longer need life-long treatment.

Erythrocyte zinc concentration as an indicator to distinguish painless thyroiditis-associated transient hypothyroidism from permanent hypothyroidism

Red blood cell (RBC) zinc (Zn) concentration reflects a patient's mean thyroid hormone level over the preceding several months. The aim of this study was to examine whether RBC Zn level can be used as an indicator to distinguish painless thyroiditis-associated transient hypothyroidism (TH) from permanent hypothyroidism (PH). RBC Zn level was measured in 30 untreated PH patients with Hashimoto's thyroiditis and 7 untreated TH patients with painless thyroiditis in whom preceding transient thyrotoxicosis had been confirmed. RBC Zn concentration was significantly lower in TH patients than that in PH patients. There was a positive correlation between RBC Zn and serum TSH, and the latter was clearly lower in TH patients than that in PH patients. However, RBC Zn level was again significantly lower in TH patients than PH patients despite of the comparable serum TSH levels in both groups when RBC Zn was evaluated in patients with serum TSH levels of less than 50 mU/L. Thus TH patients could be identified with RBC Zn measurement, allowing us avoidance of unnecessarily prolonged T4 administration to them.

Goitrous autoimmune thyroiditis in a pediatric population: a longitudinal study

BACKGROUND

Patients with autoimmune thyroiditis can present with thyroid function that varies from euthyroidism to frank hypothyroidism or occasionally hyperthyroidism. Although there is a risk of progression from the euthyroid or subclinical hypothyroid state to frank hypothyroidism, the rate of progression is not known.

OBJECTIVES

Subjects with diffuse goiter and autoimmune thyroiditis were followed up to observe the rate of deterioration in thyroid function from euthyroid and subclinical hypothyroid states to hypothyroidism.

METHODS

Patients who presented with goiter and autoimmune thyroiditis were grouped as those with euthyroidism, subclinical hypothyroidism, and overt hypothyroidism on the basis of levels of thyroxine and thyrotropin at presentation. Patients were followed up for a minimum duration of 24 months with periodic monitoring of thyroid function.

RESULTS

Ninety-eight consecutive subjects (aged of 8-18 years) with a diagnosis of autoimmune thyroiditis and diffuse goiter were studied. At presentation, in 24 subjects (24.5%) thyroid function was normal (euthyroidism), 32 (32.6%) had subclinical hypothyroidism, and the remaining 42 subjects (42.9%) had hypothyroidism. All of the subjects with hypothyroid were maintained euthyroid on thyroxine during follow-up. Hypothyroidism developed in 3 of 24 patients with euthyroidism and in 4 of 32 patients with subclinical hypothyroidism.

CONCLUSIONS

Subjects with goitrous autoimmune thyroiditis need periodic monitoring of thyroid function. Development of thyroid dysfunction is insidious and may not be accompanied by symptoms and clinical signs. In pediatric and adolescent age groups it is imperative to correct thyroid dysfunction to achieve optimal growth and development.

Development of subclinical hyperthyroidism due to Graves' disease in a hypothyroid woman who had undergone hemithyroidectomy for adenomatous goiter and radiotherapy for nasopharyngeal cancer

A variety of thyroid disorders develop following external radiation to head and neck cancers. Hypothyroidism is the most common clinical consequence of the radiotherapy and lifelong thyroid hormone replacement is required in many cases. Patients who received both hemithyroidectomy and the external radiation to the neck are at especially high risk for permanent hypothyroidism. Here we report an unusual case with radiation-induced hypothyroidism who had undergone hemithyroidectomy for adenomatous goiter 8 years before the radiotherapy for nasopharyngeal cancer and subclinical hyperthyroidism due to Graves' disease developed during thyroxine replacement therapy. Thus subclinical hyperthyroidism due to Graves' disease can develop in patients with radiation-induced hypothyroidism even if they have undergone hemithyroidectomy for thyroid nodules. Therefore careful and periodic evaluation of thyroid function is required even on adequate thyroxine replacement therapy.

Effects of kelp supplementation on thyroid function in euthyroid subjects

OBJECTIVE

To study the effects of ingestion of two different doses of supplemental kelp on the thyroid function of healthy euthyroid subjects.

METHODS

We conducted a double-blind prospective clinical trial involving 36 healthy euthyroid subjects, who were randomly assigned to receive placebo (4 alfalfa capsules per day), low-dose kelp (2 kelp capsules and 2 alfalfa capsules per day), or high-dose kelp (4 kelp capsules per day) for 4 weeks. Thyrotropin (thyroid-stimulating hormone or TSH), free thyroxine, and total triiodothyronine were assessed at weeks 0, 4, and 6. Response to thyrotropin-releasing hormone stimulation, urinary iodine excretion, and basal metabolic rate were determined at weeks 0 and 4.

RESULTS

TSH concentrations did not differ significantly between week 0 and week 4 in the placebo group (P = 0.16) but increased significantly in both the low-dose kelp (P = 0.04) and high-dose kelp (P = 0.002) groups. Free thyroxine concentrations decreased slightly but significantly after 4 weeks of placebo but were unchanged in the low-dose and the high-dose kelp groups. In contrast, total triiodothyronine levels did not differ significantly after 4 weeks of placebo or low-dose kelp therapy but were significantly decreased after high-dose kelp therapy (P = 0.04). Similarly, the thyrotropin-releasing hormone stimulation test showed no significant change in poststimulation TSH after 4 weeks in the placebo or low-dose kelp groups but revealed a significantly increased response after high-dose kelp therapy (P = 0.0002). The 24-hour urinary iodine excretion showed dose-dependent increases in the two kelp study groups. Basal metabolic rate did not change significantly in any study group during the 4-week study period. All thyroid laboratory values returned to baseline 2 weeks after cessation of kelp supplementation, except for TSH in the high-dose kelp group, which was significantly decreased.

CONCLUSION

Short-term dietary supplementation with kelp significantly increases both basal and poststimulation TSH. These findings corroborate previous studies on the effects of supplemental iodide given to euthyroid subjects for a similar period. Further studies are needed to determine whether long-term kelp supplementation would cause clinically significant thyroid disease.

Iodine-Induced hypothyroidism

Iodine is an essential element for thyroid hormone synthesis. The thyroid gland has the capacity and holds the machinery to handle the iodine efficiently when the availability of iodine becomes scarce, as well as when iodine is available in excessive quantities. The latter situation is handled by the thyroid by acutely inhibiting the organification of iodine, the so-called acute Wolff-Chaikoff effect, by a mechanism not well understood 52 years after the original description. It is proposed that iodopeptide(s) are formed that temporarily inhibit thyroid peroxidase (TPO) mRNA and protein synthesis and, therefore, thyroglobulin iodinations. The Wolff-Chaikoff effect is an effective means of rejecting the large quantities of iodide and therefore preventing the thyroid from synthesizing large quantities of thyroid hormones. The acute Wolff-Chaikoff effect lasts for few a days and then, through the so-called "escape" phenomenon, the organification of intrathyroidal iodide resumes and the normal synthesis of thyroxine (T4) and triiodothyronine (T3) returns. This is achieved by decreasing the intrathyroidal inorganic iodine concentration by down regulation of the sodium iodine symporter (NIS) and therefore permits the TPO-H202 system to resume normal activity. However, in a few apparently normal individuals, in newborns and fetuses, in some patients with chronic systemic diseases, euthyroid patients with autoimmune thyroiditis, and Graves' disease patients previously treated with radioimmunoassay (RAI), surgery or antithyroid drugs, the escape from the inhibitory effect of large doses of iodides is not achieved and clinical or subclinical hypothyroidism ensues. Iodide-induced hypothyroidism has also been observed in patients with a history of postpartum thyroiditis, in euthyroid patients after a previous episode of subacute thyroiditis, and in patients treated with recombinant interferon-alpha who developed transient thyroid dysfunction during interferon-a treatment. The hypothyroidism is transient and thyroid function returns to normal in 2 to 3 weeks after iodide withdrawal, but transient T4 replacement therapy may be required in some patients. The patients who develop transient iodine-induced hypothyroidism must be followed long term thereafter because many will develop permanent primary hypothyroidism.

High prevalence and little change in TSH receptor blocking antibody titres with thyroxine and antithyroid drug therapy in patients with non-goitrous autoimmune thyroiditis

OBJECTIVE

We have reevaluated the prevalence and pathogenetic importance of TSH receptor blocking antibodies (TRBAb) in autoimmune hypothyroidism, and investigated the changes in TRBAb activities during thyroxine and antithyroid drug treatment.

DESIGN

Serum TSH binding inhibitor immunoglobulin (TBII) and thyroid stimulation blocking antibody (TSBAb) were measured serially in all patients with non-goitrous autoimmune thyroiditis (AT) and measured monthly during methimazole treatment in 6 patients.

PATIENTS

Ninety patients with non-goitrous AT and 95 patients with goitrous AT were entered consecutively into this study. All patients with non-goitrous AT were treated with thyroxine and followed at intervals of 6 months for 2 years initially and then yearly intervals. The duration of follow-up was 1-8 years. Six patients from the TRBAb-positive non-goitrous AT group who were treated with thyroxine were randomly selected and given additional treatments with methimazole (40 mg per day) for 6 months.

MEASUREMENTS

Serum TBII was measured by a radioreceptor assay, TSBAb by using FRTL-5 cells, and antithyroid peroxidase and antithyroglobulin antibodies by radioimmunoassay.

RESULTS

The prevalences of TBII and TSBAb is non-goitrous AT were 47.8 and 58.9%, respectively, which were significantly higher than those in goitrous AT (6.3% for TBII, 10.5% for TSBAb). All but one patient showed persistent TBII and TSBAb activities during the thyroxine treatment for up to 8 years. A high dose of methimazole (40 mg per day) did not affect the titres of TBII and TSBAb in 5 out of 6 patients with non-goitrous AT tested. However, antithyroid peroxidase and antithyroglobulin antibodies activities were significantly decreased during the methimazole treatment.

CONCLUSION

The high prevalence of TSH receptor blocking antibodies (TRBAb) suggests that TRBAb may play a major role in the development of hypothyroidism and thyroid atrophy in the vast majority of patients with non-goitrous autoimmune thyroiditis. Most TRBAb activities are stable for at least 8 years and are now affected by thyroxine and antithyroid drug treatment.

Thyroxine therapy

The availability of sensitive thyrotropin assays allows effective biochemical monitoring of both replacement and suppressive therapy with thyroxine. Whatever target organ is examined, there is tissue thyrotoxicosis if the serum thyrotropin concentration is low, even if the serum triiodothyronine and thyroxine concentrations are normal. Although suppression of thyrotropin secretion is recommended in the treatment of patients with thyroid carcinoma, the aim of thyroxine-replacement therapy in patients with primary hypothyroidism should be to maintain the serum thyrotropin concentration in the normal range. The most convincing argument for the treatment of subclinical hypothyroidism is progression to overt hypothyroidism at a rate of 5 to 20 percent per year.

Screening for thyroid diseases in an iodine sufficient area with sensitive thyrotrophin assays, and serum thyroid autoantibody and urinary iodide determinations

OBJECTIVE

The present study was designed to investigate the prevalence of thyroid dysfunction and its relation to thyroid autoantibodies and urine iodide concentration in apparently healthy people residing in Sapporo, a city of northern Japan, where the iodine intake is high.

DESIGN AND SUBJECTS

Serum TSH and thyroid autoantibodies, and urine iodide were measured in 4110 people (2931 men and 1179 women) (age 45.6 +/- 10.3 years (mean +/- SD)) who were recruited at the hospital for medical examinations.

RESULTS

The thyroid autoantibodies were positive in 6.4% of males and 13.8% of females with an age-related increase. Of the people with positive antibodies, 87.2% had normal TSH values (0.15-5.0 mU/l) as measured by a sensitive assay. The prevalence of unsuspected hyperthyroidism as defined by suppressed TSH values was 0.61%, of which 64% was diagnosed as Graves' disease based on positive thyrotrophin receptor antibody results. The prevalence of unsuspected hypothyroidism, as evidenced by supranormal TSH, was 0.68% for males and 3.13% for females with an age-related increase. Of those with hypothyroidism, 45.5% were autoantibody positive. The overall prevalence of Hashimoto's thyroiditis was 13.11% for females and 6.15% for males. The urine iodide levels of hypothyroidism with a positive autoantibody of 38.5 (17.7-83.9) mumol/l and a negative autoantibody of 34.9 (17.9-67.9) mumol/l were both significantly higher than that of normal subjects (26.9 (14.6-49.6) mumol/l) (P < 0.01). When iodine intake was restricted for 6-8 weeks for hypothyroid subjects, the elevated TSH and thyroglobulin and low free T4 levels were reversed in the autoantibody negative but not in the positive group.

CONCLUSIONS

This study provides further information on the prevalence of thyroid dysfunction and autoimmune thyroid diseases in an iodine sufficient area. In addition, it suggests that more than half of the patients with unsuspected hypothyroidism were negative for autoantibodies and that the excessive iodine intake may be involved in causing latent hypothyroidism.

Disappearance of thyrotropin-blocking antibodies and spontaneous recovery from hypothyroidism in autoimmune thyroiditis

BACKGROUND

Hypothyroidism may result from the production of antibodies that block the actions of thyrotropin. How often these thyrotropin-blocking antibodies are a cause of hypothyroidism and whether their production may cease, causing hypothyroidism to disappear, have not been extensively studied.

METHODS

We determined the frequency with which thyrotropin-blocking antibodies were present in 172 hypothyroid patients with goitrous autoimmune thyroiditis (Hashimoto's disease) and 64 hypothyroid patients with atrophic autoimmune thyroiditis (idiopathic primary hypothyroidism). For 6 to 11 years we then followed 21 of these patients who were found to have thyrotropin-blocking antibodies. They received levothyroxine therapy for 3.5 to 8 years, after which it was discontinued. At frequent intervals during this time we measured the patients' serum concentrations of thyroxine, triiodothyronine, thyrotropin, and thyrotropin-blocking antibodies (measured as immunoglobulins that inhibit thyrotropin binding and immunoglobulins that inhibit thyrotropin bioactivity).

RESULTS

Thyrotropin-blocking antibodies were detected in 9 percent of the patients with goitrous autoimmune thyroiditis and in 25 percent of those with atrophic autoimmune thyroiditis. Among the 21 patients studied serially while receiving levothyroxine, thyrotropin-blocking antibodies disappeared in 15 (group 1), 7 of whom had goiter initially, and persisted in 6 (group 2), none of whom had goiter initially. Levothyroxine therapy was subsequently discontinued in these 21 patients. Six of those in group 1 (four with goiter) remained euthyroid (mean follow-up after discontinuation of therapy, 2.1 years), and nine became hypothyroid again within 3 months. All six patients in group 2 remained hypothyroid.

CONCLUSIONS

Hypothyroidism in some patients with autoimmune thyroiditis may be due to thyrotropin-blocking antibodies. The production of thyrotropin-blocking antibodies may subside, producing remissions of hypothyroidism. Chronic autoimmune thyroiditis may therefore cause transient as well as permanent hypothyroidism.

Test for recovery from hypothyroidism during thyroxine therapy in Hashimoto's thyroiditis

Hypothyroid patients with Hashimoto's thyroiditis usually receive lifelong thyroxine therapy. Some are known to recover thyroid function, but identification of these patients during continued thyroxine therapy has been impossible. 92 patients with hypothyroidism after Hashimoto's thyroiditis and 70 normal controls were studied. All controls but not patient before thyroxine was started had a normal thyroid response to thyroid stimulating hormone (TSH), circulating concentrations of which were increased by administration of 500 micrograms thyrotropin releasing hormone (TRH). During treatment with thyroxine, 22 patients recovered thyroid responsiveness to TSH, and when treatment was stopped these patients have remained euthyroid for 1-8 years, whereas all 70 who did not recover thyroid TSH responsiveness became hypothyroid within 3 months. Over 20% of patients with hypothyroidism after Hashimoto's thyroiditis may recover satisfactory thyroid function, and can be identified during thyroxine treatment by their thyroid response to TSH in a TRH test.

A sensitive thyroid stimulating hormone assay for screening of thyroid functional disorder in elderly Japanese

The use of a screening test for thyroid functional disorder by sensitive thyroid stimulating hormone assay in the elderly was investigated. The basal thyroid stimulating hormone levels predicted the response of thyroid stimulating hormone to thyrotropin releasing hormone; it was suppressed in 99 (99.0%) of 100 hyperthyroid patients. Therefore, not only primary hypothyroidism but also hyperthyroidism can be excluded when the serum thyroid stimulating hormone levels are normal. An epidemiological study was then performed on 2,421 (76.7%) of the Japanese general population aged 40 or over recruited from the residents in Hisayama town and also in 122 residents between 20 and 40 years of age. Additional free T4 measurement was necessary in about 10% of the residents with abnormal TSH levels to confirm the diagnosis of hyperthyroidism or distinguish latent from overt hypothyroidism. There was a significant correlation between age and serum thyroid stimulating hormone levels after logarithmic conversion (r = 0.1533, P less than .001). The prevalence of thyroid dysfunction found in 1,026 males and in 1,395 females aged 40 or over was, respectively: hyperthyroidism, less than 0.1% and 0.2%, latent (subclinical) hypothyroidism, 3.2% and 5.5%, and overt hypothyroidism, 0.4% and 0.7%. We conclude that the screening with this sensitive thyroid stimulating hormone assay and additional free T4 measurement is useful for detection of patients with thyroid functional disorder.

Recovery of thyroid function in primary hypothyroidism

In recent years transient primary hypothyroidism has reported with increasing frequency. Physicians are often unsure whether withdrawal of thyroid hormone to identify the transient hypothyroidism is indicated and cost-effective and in which patients this should be done. To study these questions, thyroid hormone therapy was withdrawn from 63 patients with proven primary hypothyroidism at 6 months and again at 1 and 3 years to determine if there was recovery of thyroid function. Of the 49 patients with primary hypothyroidism (PH) that was not attributable to such causes as drug therapy, surgery, iodine-131 therapy, or silent or subacute thyroiditis, only two patients recovered thyroid function. In the other 14 patients, hypothyroidism developed within 6 months postpartum. Nine of these 14 recovered thyroid function. Therefore, it appears that when PH is not related to certain specific causes or states, it is likely to be permanent. Furthermore, withdrawal of thyroid hormone therapy to assess recovery of thyroid function is unnecessary and not cost-effective.

TSH dependent elevation of serum thyroglobulin in reversible primary hypothyroidism

Serum levels of thyroglobulin (Tg) were measured using immunoradiometric assay in 18 patients with primary hypothyroidism, whose serum levels of thyroid stimulating hormone (TSH) were higher than 40 mU/l and anti-Tg antibodies were negative. In 12 patients, serum Tg levels were extremely elevated above the upper limit of normal (30 ng/ml) and the levels were more than 800 ng/ml in 10 of them. In all of these 12 patients, thyroid function recovered spontaneously with only iodide restriction and the serum Tg levels declined concomitantly with the decrease in serum TSH concentrations, events suggesting the TSH dependency of this Tg elevation. In the other 6 patients without elevated Tg levels, thyroid function did not recover and replacement therapy with L-thyroxine had to be given. Data from our study show that the TSH dependent Tg secretion is observed in reversible type primary hypothyroidism and that it may proceed vigorously even though thyroid hormone production is subnormal. Measurement of serum Tg may be valuable for predicting the prognosis of primary hypothyroidism.

The replacement therapy problem in hypothyroidism

There is increasing evidence from studies of heart rate, liver enzyme activity, bone density and urinary sodium excretion that standard replacement therapy doses of thyroxine which suppress TSH secretion are associated with changes in target organ function similar to, but less marked than, those recorded in overt hyperthyroidism. There is also evidence that in subclinical hypothyroidism it is not only the pituitary thyrotroph which recognizes a minor reduction in serum thyroid hormone levels within the normal range. Although there is no proof that slight 'overtreatment' with thyroxine or non-treatment of subclinical hypothyroidism is detrimental to the patient in the long term, the appropriate studies have not been performed. It would seem good clinical practice, however, to treat all grades of thyroid failure and to ensure, if possible, that the dose of thyroxine is adjusted to maintain a normal and detectable TSH level when measured by a sensitive assay system. It must be conceded, however, that with the vagaries of human nature there is always likely to be greater morbidity from patients with hypothyroidism failing to take their medication regularly, than from failure by the medical attendant to make minor adjustments to the dose of thyroxine.

Autoimmunity and hypothyroidism

Hypothyroidism can be induced by various diseases. An autoimmune cause accounts for approximately 90% of adult hypothyroidism, mostly due to Hashimoto's disease. The majority of Hashimoto patients are women aged between 20 and 60 years old and nearly 10% show overt hypothyroidism. With time euthyroid patients progress to hypothyroidism and thus the prevalence of hypothyroidism is higher in elderly patients. Especially at 3 to 8 months postpartum, the prevalence of hypothyroidism is very high, up to 2-4%, but more than 90% of these cases are transient. Autoimmune destructive mechanisms, such as antibody dependent cytotoxicity, K and NK cell cytotoxicity, T lymphocyte cytotoxicity and lymphokine cytotoxicity, have been studied in vitro, but the most important factor in vivo is still unknown. A recent finding is that thyroid stimulation blocking antibody (TSBAb) may induce primary atrophic hypothyroidism. This antibody not only blocks TSH-induced cAMP production but also blocks TSH-induced DNA synthesis and iodine uptake in cultured thyroid cells. The prevalence of TSBAb in patients with primary atrophic hypothyroidism varies in different studies, from 0 to 47%. Reports on the relationship between TSBAb and TSH-binding inhibitory immunoglobulin (TBII) detected by radioreceptor assay are conflicting. The prevalence of TSBAb in patients with goitrous hypothyroidism is also controversial, varying from 0 to 20%. Transient hypothyroidism is observed frequently in the postpartum period and in the post-thyrotoxic phase of pregnancy-unrelated silent thyroiditis. Maternal TSBAb causes transient neonatal hypothyroidism when the activity is more than 1500 i.u./litre. The blocking and stimulatory types of anti-TSH receptor antibodies may both react with the same epitope(s) of TSH-receptor related antigens but the exact mechanisms that lead to the different effects are unknown. In some patients, including those with Graves' disease, stimulating and blocking antibodies co-exist and thyroid function may change from hyperthyroidism to hypothyroidism, or vice-versa, depending on the balance of stimulatory and blocking activities. Hypothyroidism in Graves' disease after treatment is thought to be induced in two ways: autoimmune thyroid destruction and the predominant appearance of TSBAb. Dietary iodine restriction is helpful in allowing recovery from hypothyroidism in more than half of the patients with spontaneously occurring primary hypothyroidism in Japan. Submaximal doses of T3 may be useful in differentiating transient from persistent hypothyroidism, since spontaneous recovery is detected by an increase of serum T4.(ABSTRACT TRUNCATED AT 400 WORDS)