Free thyroxine measurement by one-step method in the plasma of treated and untreated hypothyroid infants

The one-step radioimmunoassay of free thyroxine (f-T4) using a gamma-coated kit was used in this study. In controls the mean plasma levels (pmol/l) were 20.8 (range 13.5-37) during the first days of life, 17.4 (11.5-27) from 1 week to 1 year, and 17.0 (10-24.5) after 1 year. In preterm newborns it was correlated with the length of gestation. Among 23 untreated hypothyroid infants aged 15-22 days, f-T4 was undetectable in those with athyreosis, while in cases with dysgenetic thyroid it was variable, correlated to the width of the gland (r = 0.77, p less than 0.01). In 44 hypothyroid patients treated with l-T4, a highly significant positive correlation was found after the 1st month of treatment between plasma f-T4 and the daily l-T4 dose (r = 0.46, p less than 0.01), and a negative one between f-T4 and plasma TSH (r = -0.59, p less than 0.001). It is concluded that measurement of f-T4 offers a valuable means for control of diagnosis and treatment in congenital hypothyroidism, especially useful for avoiding both under- and overtreatment. Its correlations suggest that it is the most reliable hormonal measurement in the follow-up of thyroid children.