[Free thyroxin as a determinant for thyroid function. Evaluation of direct determination of fT4 compared to the oral...

The diagnostic value of a direct assay of free thyroxine has been evaluated by comparing the concentration of free T4 with the results of an oral TRH test. 147 patients were referred to an endocrine outpatient department because of suspected thyroid dysfunction. Based on clinical investigation and on the results of an oral TRH test, 75 patients were euthyroid, 12 hyperthyroid and 12 hypothyroid. Preclinical hypothyroidism (3 h- TSH value above 35 mE/l and euthyroid peripheral thyroid hormone levels) was found in 25 patients. In 23, preclinical hyperthyroidism (3 h- TSH value after TRH below the sensitivity of the assay, i.e. less than 1 mE/l) together with euthyroid peripheral thyroid hormone values was found. Free thyroxine was assayed using the solid phase technique (Clinical Assay); intraassay coefficients of variation varied between 4.5 and 7.6% and interassay coefficients of variation between 8 and 13%. A normal range for free thyroxine was found between 8.4 and 26.4 pmol/l, the log mean being 14.9 pmol/l. In 12 hyperthyroid patients the free T4 concentration was above 49.4 pmol/l, while the mean of 12 hypothyroid patients was below 4.4 pmol/l. The mean free T4 concentrations in the syndromes of preclinical hypo- and hyperthyroidism, which by definition are only recognizable by the TSH increase in response to TRH, were within the normal range. Using the sensitive TRH test as a comparison, the present investigation thus confirms the reliability of direct assay of free T4 for the diagnosis of thyroid function in clinical practise.(ABSTRACT TRUNCATED AT 250 WORDS)