Serum levels of T4 [thyroxine], T3 [triiodothyronine], and TSH [thyrotropin] and peak TSH after thyrotropin releasing hormone, thyroliberin, administration were determined in 60 female subjects affected with asymptomatic autoimmune thyroiditis (AAT), in 27 normal subjects and in 8 myxedematous patients used as controls. The AAT subjects were divided into 3 groups on the basis of their basal and peak TSH values. In the 1st group, these parameters were similar to those of the normal controls; in the 2nd (grade II AAT), basal TSH remained normal but peak TSH was significantly increased; and in the 3rd, both parameters were significantly increased. Serum T4 levels decreased progressively from group 1 to group 3, but all T4 values were within the normal range. T3 levels were similar in all groups. Peak and basal TSH values were highly correlated, except in grade II AAT. There seems to be a graded process of subclinical hypothyroidism in AAT; a progressive pituitary TSH reserve is modulated by a progressive decrease in T4 levels still within the normal range.