Basal (B) and peak (P) serum levels of thyroxine (T4), free thyroxine (FT4), triiodothyronine (T3), free triiodothyronine (FT3), and TSH were measured before and after oral TRH (40 mg) administration in 79 subjects affected with asymptomatic autoimmune thyroiditis (AAT) and in 69 normal subjects. The area under the curve (AUC) and peak values of T4, FT4, T3 and FT3 were considered as parameters of thyroid hormone reserve. Intrathyroidal iodine (ITI) was measured by the X-ray fluorescence method. The AAT subjects were divided into three groups on the basis of their basal and peak TSH values. In group I, these parameters were similar to those in the normal controls; in group II, basal TSH remained normal but peak TSH was significantly increased, and in group III both values were significantly increased. Group I differed from the controls by a decrease in P FT4 and AUC FT4, whereas in groups II and III B FT4 was also significantly lowered. T3 levels were similar in all groups except in group III, in whom they dropped. ITI was already lower in group I than in the controls. Its decline went further in groups II and III. An inverse correlation with significant r values was evidenced between log B and P TSH on one hand and log B FT4, P FT4 and AUC FT4 on the other. When group III was excluded, log P TSH was positively correlated with log B T3, P T3, AUC T3, and AUC F T3.(ABSTRACT TRUNCATED AT 250 WORDS)