Immunoreactive calcitonin content in foetal thyroid glands and in placentae of rats.

Immunoreactive calcitonin (I-CT) was assayed in rat thyroid glands from foetuses with a gestational age of 15.5-22 days. The mean content of the hormone was 455.0 +/- 35.8 ng/gland. A positive linear correlation was found between the I-CT content in the thyroid glands and the gestational age of the foetuses. I-CT was also present in rat placentae from 10.5-21.5 days old pregnancies. A mean placental concentration of 4.5 +/- 1.2 ng I-CT/mg wet tissue was found. The highest hormone concentrations were observed in placentae from 10.5 day old foetuses, displaying a progressive fall during the further course of pregnancy. It is concluded that the rat foetal thyroid gland produces I-CT beginning from a gestational age of 15.5 days'. The need for further investigations into a possible role of CT in transplacental calcium transport is also indicated.