[Clinical importance of the level of the free fraction of thyroid hormones].

The physiologic importance of the free fraction of thyroid hormones is well accepted for some time. The determination of free thyroid hormones plasma levels by column chromatography and radio-immunoassays has been tested in our laboratory since november 1980. Six hundred and ten patients have been submitted to a precise anamnesis, a clinical examination and a thyroid investigation by scintigraphy and echography. We have compared the following parameters: T4 and fT4, T3 and fT3, TSH, TBG in euthyroidism and in the pathologic status of hyperthyroidism, we have found an excellent correlation (r greater than 0,9) between the values of T4 and T3 and their free fraction. The determination of fT3 and fT4 seems to be specially interesting in comparison of the T4 and T3 because of the low incidence of false abnormal results in those patients regarding the other parameters. Hyperthyroidism is characterized by a lack of correlation between total and free hormones; the levels of the free fraction being widely increased. The determination of the free hormones levels improves the sensitivity and the accuracy of the diagnosis of hyperthyroidism. It permits an early diagnosis of the disease, an earlier detection of the recurrence and makes easier the follow-up of the treated patients. In hypothyroidism, there is a so light decrease in the value of fT3 and fT4 that the TSH estimation remains the method of choice to assure the diagnosis. Thus, we can conclude that the determination of fT3 and fT4 by column chromatography and radio-immunoassays is more discriminating in euthyroidism and hyperthyroidism. In hypothyroidism TSH and fT4 are the most sensitive parameters.