Oscillations in reverse triiodothyronine levels in serum of healthy infants aged 0 to 130 hours.

rT3 was measured by RIA in sera from 273 normal healthy infants between 0--130 h of age. The curve for the mean rT3 level plotted against age was polyexponential in shape and showed superimposed oscillations with a period of about 16 h, similar to the curves for T4, T3, and TSH previously found for the same group of subjects (J Clin Endocrinol Metab 47: 61, 1978). The ratio of concentrations of rT3 to T4 remained approximately constant, with a mean value of 0.016 over the age range of 3--130 h. These observations suggest that the variations in rT3 levels in newborn serum during the first few days of life are largely accountable by variations in T4 substrate concentrations.