Laboratory based study of undetectable thyroid stimulating hormone.

The clinical importance of an undetectable thyroid stimulating hormone (TSH) concentration (less than 0.2 mU/l) was studied in a consecutive series of 2573 requests for routine thyroid function tests. Two hundred and seventeen (8.4%) patients had an undetectable TSH concentration, and of these 39 (18%) had otherwise normal thyroid hormone concentrations and no history of thyroid disease. In a follow up study 71 patients (34 outpatients and 37 inpatients) with undetectable TSH concentration associated with otherwise normal thyroid hormone concentrations were randomly selected during routine reporting of thyroid function test results. None of these patients had a history of thyroid disease. Sex hormone binding globulin concentrations were increased in five out of 50 of these patients and antithyroid antibodies were detectable in four out of 49, suggesting that in most cases the isolated undetectable TSH concentration was not associated with thyroid dysfunction, particularly hyperthyroidism. Isolated undetectable TSH concentration was observed in both inpatients and outpatients and was not associated with any particular clinical condition. Repeat specimens were received in 54 of the 71 patients and TSH concentration remained persistently undetectable in 35 of these.