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August 1980

Detecting Unsuspected Thyroid Dysfunction by the Free Thyroxine Index

Author Affiliations

From the Division of Nuclear Medicine (Drs dos Remedios and Weber, Mr Schurr and Mr Tsoi) and the Department of Medical Methods Research (Dr Feldman), Kaiser-Permanente Medical Center, Oakland, Calif.

Arch Intern Med. 1980;140(8):1045-1049. doi:10.1001/archinte.1980.00330190057019

• The free thyroxine index (FTI) was used in 2,704 adults to detect unsuspected thyroid dysfunction. Among 2,581 adults found to be clinically euthyroid without thyroid medication, 2,571 had a truly normal FTI (121 to 360) and ten had a falsely abnormal FTI (seven <121, three >360). Among 25 subjects with newly diagnosed thyroid dysfunction, there were eight hyperthyroid (prevalence, 0.31%) and 14 hypothyroid (prevalence, 0.50%) subjects. The sensitivity of the FTI was 1.0, and the specificity was 0.996. The predictive value of an abnormal FTI with a prevalence of 0.81% was 67%. The cost to find a new case averaged $127. The annual incidence of symptomatic hyperthyroidism was 0.05%; of hypothyroidism, about 0.08%. We conclude that the FTI is cost-effective for case finding in thyroid dysfunction.

(Arch Intern Med 140:1045-1049, 1980)