Assessment of in vitro thyroid function tests in 100 consecutive patients referred to a thyroid clinic.

Abstract

SUMMARYIn a series of 100 consecutive patients referred to a thyroid clinic the total serum thyroxine (serum T‐4) measured by competitive protein binding analysis had a much higher diagnostic accuracy than either the serum protein‐bound iodine (PBI) or the triiodothyronine serum uptake (T3 serum uptake) when used alone. In contrast to previous reports, the free thyroxine index (FTI) calculated from the serum PBI and the T3 serum uptake gave disappointing results. The FTI calculated from the serum T‐4 and the T3 serum uptake correlated well with thyroid status, but equally good results were given by the effective thyroxine ratio (ETR) which, although expensive, has the advantage of being a single procedure.Primary hypothyroidism can be diagnosed with a high degree of accuracy from a single venepuncture by demonstrating a low total serum thyroxine and an elevated plasma thyrotrophin (TSH). There were no false negative results. Fortysix of the fifty euthyroid patients had plasma TSH levels less than 12 μU/ml. The four euthyroid patients with TSH levels greater than 12 μU/ml had evidence of impaired thyroid reserve.