Spontaneous Hypothyroidism in Symptomless Autoimmune Thyroiditis. A Long‐term Follow‐up Study

SUMMARY A follow‐up study was performed on subjects with symptomless autoimmune thyroiditis in order to obtain information on the natural course of the disease. Seven out of the twenty‐two subjects with symptomless autoimmune thyroiditis became hypothyroid within 4–48 months of observation, whereas fifteen subjects were still euthyroid after an observation period of 26–102 months. The yearly incidence of hypothyroidism in symptomless autoimmune thyroiditis was 7–3%. The basal serum thyrotrophin (TSH) level as well as the response in serum TSH to thyrotrophin releasing hormone (TRH) was initially elevated in all subjects who developed hypothyroidism. The annual incidence of hypothyroidism was 26% in the subjects with an initially elevated TSH level. Serum TSH was initially normal in all but one of the eleven subjects with symptomless autoimmune thyroiditis in whom it was measured, and who remained euthyroid. In this group the TRH response was initially slightly exaggerated in three out of ten subjects. It was not performed in five subjects. During the follow‐up, serum TSH and its response to TRH also became normal in these cases. An elevated basal serum TSH value is thus highly indicative of subsequent hypothyroidism, and these subjects with symptomless autoimmune thyroiditis should be carefully followed‐up.