Changes in thyroxine, 3,3',5-triiodothyronine and 3,3'5'-triiodothyronine content in the thyroid gland and in serum to thyroid tissue iodothyronine ratios during ontogenesis in the fetal pig

Abstract

The concentrations of thyroxine (T4), 3,3',5-triiodothyronine (T3) and 3,3',5'-triiodothyronine (reverse T3; rT3) in thyroid gland tissue and serum of the fetal pig (n = 68) from day 39 to 113 of gestation were measured. Tracer quantities of iodothyronines, displaying the onset of thyroid hormone activity, were found in the thyroid tissue on day 39, i.e. before the appearance of a measurable quantity of iodothyronines in the serum. The T4 and T3 thyroidal content showed the first rise between days 56 and 76. Then, T3 was increasing sharply from day 92 till birth, while T4 content was decreasing from about day 76 to a low value between day 92 and 105, and then showing an increase shortly before birth. The rT3 content was the highest on day 39 and then it was steadily decreasing to reach a nadir on about day 76. Measurable amounts of thyroid hormones (TH) in the serum were observed not earlier than on day 46 of gestation. Near birth, the tissues of the pig fetus are in a milieu characterised by the highest blood TH concentrations. The serum to thyroid concentration ratio for rT3 and T4 was generally below 1.0 until the last trimester of gestation, when it was over 5.0 for rT3 and over 4.0 for T4. By contrast, the T3 serum to thyroid ratio was below 0.5 throughout the gestation. The results show that the fetal pig thyroid displays a low rT3 and T4 content, but the marked T3 elevation observed near term supports the view that a high production and secretion of T3 near term may be a critical factor for normal postnatal adaptation to extrauterine cooling in the pig.