Sensitivity, specificity, and cost-effectiveness of the sensitive thyrotropin assay in the diagnosis of thyroid disease in ambulatory patients.

The sensitivity and specificity of two sensitive thyrotropin assays were compared with those of other standard thyroid function tests in 544 ambulatory subjects who were clinically euthyroid, thyrotoxic, or hypothyroid. Both sensitive thyrotropin assays had the highest sensitivity and specificity (95%/89% and 92%/95%), following by estimated free thyroxine (T4) level (82% and 94%), calculated free T4 index (78% and 93%), and free triiodothyronine index (86% and 88%). Sensitivity of the two thyrotropin assay kits in the diagnosis of thyrotoxicosis was 86% and 95%, and that in the diagnosis of hypothyroidism was 92% and 94%. Other tests were nearly as sensitive in the diagnosis of thyrotoxicosis but not hypothyroidism. A cost analysis of a testing strategy that used either total T4, free T4 index, or sensitive thyrotropin assay alone as the first-line thyroid test disclosed that to establish the patient's thyroid metabolic status would have cost $11,093, $14,536, and $24,902, respectively, using each test first. We suggest that, at current prices, routine use of the thyrotropin assay as a first-line test in ambulatory patients is not as cost-effective as the free T4 index.