In an attempt to assess a possible relationship between pituitary size and TSH secretion, the volume of sella turcica was measured in 570 subjects, 26 primary hypothyroid patients, and 34 thyrotoxic patients. The volume of sella turcica, measured by a 3-dimensional approach, increased progressively with age until 20 years of age and was rather constant thereafter in normal subjects. In thyrotoxic patients, the volume of sella turcica was normal in spite of decreased plasma TSH concentration. In contrast, 81% of primary hypothyroid patients had an abnormal enlargement of the sella turcica. The magnitude of an increase of sella turcica inversely related with a decrease in serum T4 and T3 concentrations. On the other hand, the magnitude of an increase of sella turcica correlated well with an increase of circulating TSH. We suggest that an increase of sella turcica indirectly reflects an increase in pituitary size and TSH-secreting capacity, possibly due to hypertrophy and hyperplasia of TSH cells in primary hypothyroid patients.