Problems in Estimating Thyroxine Secretion Rates in Cattle

Abstract

Experiments with dairy cows evaluated thyroxine secretion rates calculated on plasma disappearance rates of iodine from radioactive thyroxine, chemical thyroxine, and protein-bound iodine. Variations in estimated thyroxine secretion rates occurred in the same cows due to different rates of disappearance and, apparently, different volumes of distribution of test doses between different organic iodine sources. After a large intravenous dose of thyroxine, iodine from radioactive thyroxine and chemical thyroxine disappeared from plasma faster than protein-bound iodine. High iodine diets reduced disappearance rates of protein-bound iodine more than that of thyroxine. Feeding 100 milligrams iodine daily caused normal thyroxine determined in plasma protein-bound iodine to decline from 88 to 59%. Injecting 20 milligrams of 1-thyroxine intravenously caused thyroxine iodine to exceed protein-bound iodine for 1 day on high iodine diets and 2 days on low iodine diets. Protein-bound iodine cannot be used as a precise index of plasma thyroxine. Most accurate estimations of thyroxine secretion rates are made by injecting a minimum dose of radioactive thyroxine to determine rate of disappearance and thyroxine space and determine plasma thyroxine by a method specifically for chemical thyroxine.