Nature of the 131I-Compounds Released Into the Thyroid Veins of Rabbits, Dogs and Cats, Before and After TSH Administration1

Abstract

Thyroid venous blood was collected from an inferior thyroid vein of anesthetized rabbits, cats and dogs previously injected with ¹³¹I. The ¹³¹I was usually given as a single injection 36–48 hr before the cannulation, although in some experiments it was administered daily for 8–13 days to approach isotopic equilibrium. Samples of thyroid venous plasma were collected before, and at intervals after, administration of TSH, and whole plasma and concentrated butanol extracts of plasma were chromatographed on filter paper. ¹³¹I peaks on the chromatograms were located by scanning and identified by reference to known carriers. ¹³¹I-T4 was always the major organic ¹³¹Icompound in thyroid venous plasma. ¹³¹I-T3 was present in much lower concentration, and ¹³¹I-T3′ and ¹³¹I-T2 were not detectable. ¹³¹IMIT and ¹³¹I-DIT also were generally below the limit of detection of the method, even though the thyroid glands themselves contained most of their ¹³¹I in the form of iodotyrosines. Chromatography of whole plasma frequently showed a significant band at the origin of the chromatogram, especially in plasma from rabbits. When TSH was administered, there occurred, after a latent period of 15–30 min, an increase in the ¹³¹I-T4 level of thyroid venous plasma. Increases of 3- to 10-fold were observed, the maximum concentration usually being attained between 1 and 2 hr. ¹³¹I-iodide also increased, but this was sometimes masked by initially high levels in the plasma. ¹³¹I-T3 usually showed an increase, and in many cases this component became detectable in thyroid venous plasma only after TSH administration. However, the other iodothyronines, ¹³¹I-T3′ and ¹³¹I-T2, and the iodotyrosines also, were not detectable even at the peak of the TSH effect. Results in animals injected with ¹³¹I daily for 1–2 weeks before collection of blood samples were similar to those in more acutely injected animals. Also, in several experiments rabbits were treated with TSH chronically for 4 or 8 days before, or together with; ¹³¹I, and the pattern of ¹³¹I-distribution in thyroid venous plasma was similar to that seen in the more acutely treated animals. Chromatograms of concentrated butanol extracts of thyroid digests from some of the animals showed ¹³¹I peaks corresponding to T4 and T3 markers, but not unequivocally to the other iodothyronines. However, radioautography did reveal faint bands corresponding to the T3′ and T2 markers. (Endocrinology74: 902,1964)