The course of untreated mild hypothyroidism was followed in 67 nursing home residents (mean age 78 years). The diagnosis was based on a normal free thyroxine index (FTI) and elevated thyrotropin concentration (TSH 4.6 to 15.0 microIU/mL, nl less than or equal to 4.5 microIU/mL. FTI and FSH were measured in follow-up 42-378 (mean 161) days after the diagnosis of mild hypothyroidism had been made. In 45 patients initial TSH was less than 6.8 microIU/mL; in 23 of these subjects TSH returned to normal during the observation period, whereas in 22 TSH remained elevated. In all 22 residents whose initial TSH was greater than 6.8 microIU/mL, TSH remained elevated at follow-up. In 4 subjects whose initial TSH concentrations ranged from 5.0 to 9.6 microIU/mL, FTI fell below normal 91-141 days after the diagnosis of mild hypothyroidism was made. Clinical progression of the signs or symptoms of hypothyroidism was not detected in the 4 patients who developed hypothyroxinemia. Three demonstrated positive thyroid antibody titers, and 1 had myasthenia gravis. These observations suggest a need for replacement therapy in debilitated patients with mild hypothyroidism and evidence of thyroid autoimmunity.