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
2004
Brzezińska-Ślebodzińska, Ewa | Ślebodziński, A. B.
The concentrations of thyroxine (T<sub>4</sub>), 3,3',5-triiodothyronine (T<sub>3</sub>) and 3,3',5'-triiodothyronine (reverse T<sub>3</sub>; rT<sub>3</sub>) 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 T<sub>4</sub> and T<sub>3</sub> thyroidal content showed the first rise between days 56 and 76. Then, T<sub>3</sub> was increasing sharply from day 92 till birth, while T<sub>4</sub> 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 rT<sub>3</sub> 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 rT<sub>3</sub> and T<sub>4</sub> was generally below 1.0 until the last trimester of gestation, when it was over 5.0 for rT<sub>3</sub> and over 4.0 for T<sub>4</sub>. By contrast, the T<sub>3</sub> serum to thyroid ratio was below 0.5 throughout the gestation. The results show that the fetal pig thyroid displays a low rT<sub>3</sub> and T<sub>4</sub> content, but the marked T<sub>3</sub> elevation observed near term supports the view that a high production and secretion of T<sub>3</sub> near term may be a critical factor for normal postnatal adaptation to extrauterine cooling in the pig.
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