Buehler and Sullivan have expressed concern in the use of Liquisol (Damon Diagnostics) free T4 assay in patients with nonthyroidal illness (NTI) as described in our report. The interpretation of thyroid function tests in NTI must be undertaken with the knowledge that the "true" state of thyroid function in these persons is unknown, although most observers agree they are clinically euthyroid. The contention that the Liquisol assay provides "excellent clinical correlation," providing their normal reference ranges are used, overlooks a critical clinical situation. Using either the reference range we determined in our own control subjects or using the reference range given with the Liquisol kit literature (0.8 to 2.4 ng/dL), the assay failed to identify both patients in our report with primary hypothyroidism and associated NTI. Recently, others have shown that Liquisol assay for free T4 failed to identify 50% of persons with uncomplicated hypothyroidism.1
Slag MF, Morley JE, Elson MK, Shafer RB. Free Thyroxine Levels in Critically III Patients-Reply. JAMA. 1982;248(7):829. doi:10.1001/jama.1982.03330070016011
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