Serum free tri-iodothyronine (FT3), using two different radioimmunoassays, and serum albumin, using bromocresol green dye-binding method, were measured prospectively in 1374 adults investigated for thyroid disease. The results of similar pairs of measurements in 6608 serum specimens obtained from adults, but using one of the radioimmunoassays for FT3, were examined retrospectively. A close correlation was observed between the FT3 values and serum albumin concentration. Normalisation of the FT3 level using a standard albumin value of 43 g/L in the absence of chemical evidence of hypothyroidism (serum thyrotrophin <6·5 mIU/L) gave a distribution of FT3 values which closely resembled that from a normal population. The effects of acute illness and old age, which may lower the FT3 value, can be corrected by this transformation.
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