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Detecting Unsuspected Thyroid Dysfunction by the Free Thyroxine Index

Leo V. dos Remedios, MD; Paul M. Weber, MD; Robert Feldman, MD; David A. Schurr, MS; Timothy G. Tsoi
Arch Intern Med. 1980;140(8):1045-1049. doi:10.1001/archinte.1980.00330190057019.
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• 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)


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