The prevalence of subclinical hypothyroidism in adults with low-normal blood thyroxine levels.

The purpose of this study was to determine whether subclinical hypothyroidism, defined as the state in which there is an elevation of thyrotropin blood level (TSH) with a low-normal blood thyroxine level (T4), can be found in clinically healthy subjects. We identified, by screening of adults in an ambulatory patient care facility, 67 persons with T4 at the lower range of normal (4.5-6.0 micrograms/dL) who were free of acute nonthyroidal illness and who were not on medications known to affect T4, other than long term thyroxine therapy. TSH was determined in all 67 persons, whose ages ranged from 25 to 87 years. Nineteen persons in this population had a history of thyroid disease and 48 did not. Elevation of TSH was found in 13 subjects (19%), seven of whom had a history of thyroid disease. The prevalence of TSH elevation was 50% in those with T4 of 4.5- less than 5.0 micrograms/dL, 26% in those with T4 of 5.0-5.5 micrograms/dL, and 8% in those with T4 of greater than 5.5-6.0 micrograms/dL (p less than 0.02). Three of these 13 patients had been on thyroxine therapy at the time of the survey, and their thyroxine doses were adjusted, whereas five others were started on thyroxine therapy subsequent to the survey. Treatment doses as low as 0.05 mg/day increased the T4 and returned the TSH to normal in six patients. In two of five patients who went untreated, there was a substantial fall in T4 and/or a rise in TSH.(ABSTRACT TRUNCATED AT 250 WORDS)