Thyroid autoimmunity in siblings: a population study.

Familial occurrence of thyroid autoantibodies and elevated TSH levels was investigated among siblings from a middle-aged rural population in Finland. The initial sample consisted of 801 subjects representing 311 sibling groups. Paired sera taken at a five-year interval were available for testing from most of them. Thyroglobulin antibodies were detected in 26% of the siblings of probands with thyroglobulin antibodies, compared with 11% among siblings of probands who did not have these antibodies. The corresponding figures for thyroid microsomal antibodies were 32% and 11%, respectively. The frequency of positive test results among the siblings was about the same irrespective of the antibody titre of the proband. When the proband had only thyroid microsomal antibodies, most (66%) of the autoantibody-positive siblings were positive in this system only. When the proband had only thyroglobulin antibodies, the pattern among siblings was less clear. Nearly half of the cases with elevated TSH level but negative in the thyroid autoantibody tests were in families in which other members had evidence of thyroid autoimmunity. The findings are compatible with the concept that a few major genes affect susceptibility to thyroid autoimmune diseases.