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Nelson B. Watts, MD

Arch Intern Med. 1989;149(2):309-312.

Abstract
• To determine if a sensitive assay for thyrotropin (thyroid-stimulating hormone [TSH]) would be useful as a primary test to determine optimal doses of levothyroxine, an enzyme immunoassay for TSH that discriminated hyperthyroid from euthyroid subjects with no overlap was compared with radioimmunoassays for TSH and thyroxine (T₄) and the calculated free thyroxine index (FTI) in 100 patients receiving stable doses of levothyroxine. The basal TSH level, determined with the sensitive assay, predicted the TSH response to thy rotropinreleasing hormone (TRH); all patients whose basal TSH level was less than 0.35 mU/L had absent or subnormal responses, and all patients with TSH levels of 0.35 mU/L or greater had normal responses. In patients with normal TSH levels, the T₄ level and FTI were superfluous (normal) or misleadingly high; conversely, 65% of patients with low TSH levels had normal T₄ levels or FTIs, or both. An assay for TSH with sufficient sensitivity to discriminate between abnormally low values and the low range of normal can be used as the primary test for monitoring levothyroxine treatment. The T₄ or FTI measurements are not needed if the TSH level is normal, but they should be done in patients with low TSH levels.

(Arch Intern Med 1989;149:309-312)

Author Affiliations
From the Department of Medicine (Endocrinology), Emory University School of Medicine, Atlanta.

Footnotes
Accepted for publication March 2, 1988.

Presented in part at the 62nd Annual Meeting of the American Thyroid Association, Washington, DC, Sept 16, 1987.

Reprint requests to The Emory Clinic, 1365 Clifton Rd NE, Atlanta, GA 30322 (Dr Watts).

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