Thyroxine turnover studies in Jamaicans

ABSTRACT

Radioactively labelled thyroxine (T4 - 125 I) has been used to carry out studies on thyroxine turnover in Jamaicans subjects. The decrease in plasma radioactivity following intravenous injection of labelled T4 has two components. The slow component lasting from 1 to 10 days post injection may be mathematically analysed to give the following data (a) thyroxine distribution space; (b) total extra thyroidal thyroxine; (c) fractional turnover rate of the hormone and (d) thyroxine secretion rate. The results obtained in six normal Jamaican subjects agree well with those found by workers in North America and Europe. The fast component (10 minutes to 4 hours post injection) has been analysed in 8 normal, 2 hypothyroid and 1 thyrotoxic subjects using simultaneously injected albumen 125I and thyroxine 131I. This technique has previously been used by Oppenheimer et al (1967). It has until recently been thought that the cellular uptake of thyroxine was dependent on the concentration of diffusible (non protein bound) thyroxine in plasma. The results presented here show cellular uptake values 100 times greater than can be explained on this basis. These data have been analysed by a computer programme designed by Dr. J. B. Shepherd, Department of Physics, U.W.I. (AU)