Abstract
We compared measurement of free thyroxin (fT4) and total thyroxin (tT4) in serum as a primary screening test of thyroid function, based solely on biochemical diagnostic classification. The normal 95% reference intervals for fT4 and tT4 were equated by probit transformation, which allowed us directly to compare the efficiency of the biochemical classification. fT4 showed a small but distinct advantage over tT4 in the diagnosis of hyperthyroidism, but narrowing the reference interval proportionally for fT4 and tT4 to improve the sensitivity of the screening test resulted in the loss of this advantage. No clear advantage of fT4 over tT4 for the initial diagnosis of hypothyroidism was found at any reference interval.
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