[Incidence of abnormal thyroid function in patients with low titers of thyroid microsomal antibodies].

The incidence of abnormal thyroid function related to autoimmune disorders was examined in a district of Shimane Prefecture in 1988. Thyroid microsomal antibodies (MCPA) in sera were measured in a general population of 1,646, including 678 males and 968 females, aged 57.8 +/- 14.8 (mean +/- SD) yr. MCPA titer was defined as high (less than 1:128), low (1:16, 1:32, 1:64) or negative (less than 1:16) according to the highest dilution of test serum capable of agglutinating gelatin particles coated with the appropriate antigen. Test of MCPA revealed high titers in 141 subjects (group A), low in 43 subjects (group B) and negative in 1,462 (group C). Twenty-four patients with overt thyroid disorders were found in groups A and B: five with Graves' disease, two with Hashimoto's thyroiditis and 15 with goiter in group A, and two with goiter in group B. In the remaining 119 subjects in group A and 41 subjects in group B, serum free T4 (FT4) and TSH levels were measured. According to abnormalities in the levels of serum FT4 and/or TSH, their thyroid function was divided into the following 3 subgroups: 1) hyperthyroid (FT4 greater than 2.0 ng/dl, TSH less than 0.4 mu U/ml), 2) latent hypothyroid (0.8 less than FT4 less than 2.0 ng/dl, TSH greater than 5.0 mu U/ml) and 3) hypothyroid (FT4 less than 0.8 ng/dl, TSH greater than 5.0 mu U/ml). The hyperthyroid group consisted of two patients in group A and one in group B. Ten latent hypothyroid patients were found in group A and two in group B. Hypothyroidism was found in four patients in group A. The incidence of abnormal thyroid function was not different between group A (16 out of 119, 13.4%) and group B (three out of 41, 7.3%). Graves' disease and primary myxedema were found in one patient each in group B; these patients had no subjective symptoms but showed low titers of MCPA. These findings suggest that not only high titers of MCPA but also low titers of MCPA are closely related to abnormal thyroid function.