The effect of thyroid dysfunction and fasting on placenta inner ring deiodinase activity in the rat

The placenta contains iodothyronine 5-deiodinase activity (P5-Dase) that probably acts on iodothyronines in the fetal circulation to convert T4 to rT3 and T3 to 3,3'-T2. Since thyroid status and fasting have profound effects on iodothyronine deiodinases in other tissues, the present studies were performed to determine if these perturbations affected P5-Dase. Control and treated rats were mated and killed near term on the 20th day of gestation. P5-Dase was determined in placenta homogenates enriched with dithiothreitol by measuring the conversion of T4 to rT3. In four of five studies, P5-Dase was similar in dams that underwent thyroidectomy (Tx) on day 7 of gestation and sham Tx dams. P5-Dase was not altered in dams that were treated with methimazole (MMI) to induce maternal and fetal hypothyroidism. Treatment of dams with supraphysiological doses of T4, beginning on the seventh day of gestation, did not significantly affect P5-Dase. In three of four studies, P5-Dase was similar in fed dams to values in dams fasted for the last 5 days of pregnancy. Placenta iodothyronine 5'-deiodinase activity (P5'-Dase) was also measured in some studies. P5'-Dase was not decreased in Tx rats and was modestly decreased in MMI-treated rats. However, the effect of MMI was not reversed by the administration of supraphysiological doses of T4, Tx, MMI treatment, and fasting all decreased hepatic T4 5'-deiodinase activity in pregnant rats. These results strongly suggest that thyroid status and fasting do not alter P5-Dase activity.