Grades of subclinical hypothyroidism in asymptomatic autoimmune thyroiditis revealed by the thyrotropin-releasing hormone test.

Serum levels of T4 T3, and TSH and peak TSH after TRH administration were determined in 60 female subjects affected with asymptomatic autoimmune thyroiditis (AAT), in 27 normal subjects, and in 8 myxoedematous patients used as controls. The AAT subjects were divided into 3 groups on the basis of their basal and peak TSH values. In the first group (grade I AAT), these parameters were similar to those of the normal controls; in the second (grade II AAT), basal TSH remained normal but peak TSH was significantly increased; and in the third (grade III AAT), 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 exist 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.