Painless thyroiditis complicated by acromegaly.

The serum thyroid stimulating hormone (TSH) level is decreased in acromegalic patients. Although this phenomenon is thought to be caused by the enhanced secretion of somatostatin which suppresses TSH production, it has not yet been proven. We describe a 60-year-old woman with acromegaly who showed a low concentration of TSH. We diagnosed her as painless thyroiditis based on an increased level of thyroglobulin, depressed radioactive iodine uptake (RAIU), normal vascularity and mild swelling of the thyroid, and normal T3, T4, free T3 and free T4 levels. To our knowledge, this is the second reported case of acromegaly complicated by painless thyroiditis. The differential diagnosis between central hypothyroidism and painless thyroiditis is so important. Since it is difficult to diagnose precisely based on only the data of a low level of TSH and normal levels of thyroid hormones, we consider that measurement of thyroglobulin and RAIU is necessary when the complication of painless thyroiditis is suspected.