Measurement of free thyroxine and free triiodothyronine in thyrotoxicosis and hypothyroidism.

Free T4 and free T3 concentrations in thyrotoxic and hypothyroid patients at presentation differentiated those with thyroid disease from euthyroid control subjects and were closely correlated with T4/TBG ratio. A further group of thyrotoxic patients was followed serially during treatment with carbimazole. Measurements of free T4 and free T3 against closely reflected thyroid status with changes in the hormone measurements being paralleled by changes in T4/TBG ratio.