DIFFERENCES IN THE THYROXINE, TRIIODOTHYRONINE AND REVERSE TRIIODOTHYRONINE CONTENTS OF FETAL PIG-TISSUES RELATIVE TO GESTATIONAL-AGE

The concentrations of thyroxine (T-4), 3,5,3'-triiodothyronine (T-3) a nd 3,3',5'-triiodothyronine (reverse T-3) were measured in the liver, kidney and skeletal muscles of pig fetuses from day 32 to 113 of age. The iodothyronines were found to be present already between days 32 an d 39, that is before the onset of fetal thyroid function. In the liver , kidneys and skeletal muscles T-4 reached maximal concentrations at m idgestation (days 56 and 93). The T-3 level increased with gestational age in all tissues examined but, as opposed to T-4, its maximal value was reached shortly before birth. The rT(3) level was the highest bet ween days 85 and 100 in the liver and kidney; however, in the skeletal muscles it did not change with gestation. The presented profiles of c hange of T-4 and rT(3) in the examined tissues did not correspond to t he blood hormone pattern already observed in pig, whereas changes in t issue T-3 concentrations were similar to those in the serum.