Anomalous Thyrotropin Values

We studied problems associated with use of an "ultrasensitive" thyrotropin (TSH, thyroid-stimulating hormone) assay for diagnosis of hyperthyroidism. Of 955 TSH assays performed in our laboratory during four months, 135 gave TSH values less than 0.1 milli-int. unit/L. We noted low TSH values at all concentrations of free thyroxin (FT4) in plasma. Nine of 13 patients with a normal or low FT4 and no obvious endocrine explanation for a low TSH were elderly and ill. This raises questions about the pituitary function in such patients. Twenty-seven patients who had high FT4 and non-suppressed TSH were clinically euthyroid, 20 of them being on treatment with thyroxin or amiodarone. Low TSH values in a hospital environment do not always indicate hyperthyroidism, although a normal value for TSH probably indicates euthyroidism.