Five kits for estimating free thyroxin concentration in serum evaluated, and correlated with other indices to thyroid status.

I evaluated Amerlex (Amersham International), Coat-A-Count (Diagnostic Products Corporation), Gammacoat (Clinical Assays), Immophase (Corning Medical), and Liquisol (Damon Diagnostics) radioimmunoassay kits for determinations of free thyroxin concentration in sera. Kit results for free thyroxin and the free thyroxin index and thyroxin/thyroglobulin ratio were compared for patients with thyroid disease and for subjects with normal and altered thyroglobulin binding capacity. Values for free thyroxin, free thyroxin index, and thyroxin/thyroglobulin clearly distinguished hyperthyroid and hypothyroid patients from euthyroid subjects. Each free thyroxin kit showed that fewer euthyroid patients with increased or decreased thyroglobulin binding capacity had values outside of normal limits than was indicated by kits for either free thyroxin index or thyroxin/thyroglobulin. The free thyroxin kits not only provided a better overall index of thyroid status in euthyroid patients who had alterations in thyroglobulin binding capacity than either of the indirect assessments of free thyroxin, but also supplied similar diagnostic information in thyroid disease.