Natural course of symptomless autoimmune thyroiditis.

Out of 18 subjects with symptomless autoimmune thyroiditis (S.A.T.) 5 (28%) became hypothyroid within 4 to 39 months of observation, whereas 13 (72%) remained euthyroid up to atleast 28 to 50 months. In all those who developed hypothyroidism the basal serum level of thyroid-stimulating hormone (T.S.H.) was already initially above normal (normal range 1-6--6-9 muU/ml) and 4 had markedly elevated concentrations (less than 19 muU/ml). All those subjects developing hypothyroidism also had initially an exaggerated response to thyrotropin-releasing hormone (T.R.H.) (upper normal limit, delta T.S.H. 30 muU/ml), and in 4 the response was much exaggerated (delta T.S.H. less than 70 muU/ml). In 3 of these subjects the basal T.S.H. and the response to T.R.H. were reassessed before starting the substitution therapy and in all there was a further increase in both values. The basal serum T.S.H. was initially also slightly increased in 2 and the response to T.R.H. slightly above normal in 4 subjects who remained euthyroid. The basal T.S.H. level became normal in both cases with elevated values; and the response to T.R.H. declined to a normal level in 3 of the latter 4 subjects, but showed a further increase in 1. The thyroglobulin antibody (TgA) titres were initially significantly elevated in 15 subjects and the thyroid microsomal antibodies (MsA) in 1. The TgA titres decreased during the observation period in all but 1 subject and a similar trend was observed as regards the MsA titres. It is concluded that within a few years of observation a substantial number of subjects with S.A.T. will be hypothyroid. A definitively increased basal serum T.S.H. level and a markedly exaggerated response to T.R.H. in the symptomless stage of the disease is connected with a high risk of late hypothyroidism.