Measurements of thyroxine (T4), triiodothyronine (T3), thyrotropin (TSH), and T3 talc uptake (T3TU) were performed on 425 hospitalized patients over 60 years of age. Unsuspected thyroid disease was found in 10 patients (2.4%); 9 were hypothyroid and 1 hyperthyroid. Another 11% of the population had abnormal T4 or TSH levels but were not proved to have thyroid disease. Repeat measurements in 10 of 40 patients with low serum T4 concentrations showed a return of the tests to normal. The low T4 levels occurred in patients who were seriously ill and were usually associated with low T3TU values. Low serum T3 concentrations were observed in 32 of the patients. Although we found TSH to be the most useful test in the diagnosis of hypothyroidism, it is a relatively expensive test and is not helpful in screening for hyperthyroidism. The present results suggest that the best single test for screening for occult thyroid disease is a serum T4 measurement. We recommend a T4 test on all hospitalized patients over age 60 years. Serum should be held in the laboratory for measurement of TSH concentration if the T4 result is below 6.5 μg/dl. A T3 resin or talc uptake and a serum T3 measurement should be performed to rule out hyperthyroidism if the serum T4 concentration is elevated. This regimen will identify virtually all cases of occult thyroid disease