Development of the independent function of fetal thyroid glands in the dog in connection with iodothyronine concentrations in pregnant bitches, fetal fluids, and fetal serum

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Thuroczy J, Szilagyi J, Muller L, et al.

Domest Anim Endocrinol 2017;58:14-18.

Thyroxine (T4) and triiodothyronine (T3) concentrations in pregnant and nonpregnant bitches were measured. The allantoic and amniotic fluid samples were collected separately in the third week of pregnancy, and fetal blood samples were collected in the fourth week of pregnancy. There was no difference between T4 results in the pregnant and nonpregnant animals, but the measured serum concentrations exceeded the healthy range for normal adults. Serum T4 concentrations were lower in the fetus than those in adults (P < 0.01). Fetal T4 concentrations continuously increased and reached 13.38 +/- 6.19 nmol/L before birth. The fetal serum T4 concentrations were lower than the T4 concentrations in allantoic and amniotic fluid until the seventh week, and the fetal serum T3 concentrations were lower than those in fetal fluids throughout the pregnancy (P < 0.01). Maximum T3 concentrations in allantoic and amniotic fluid exceeded the concentrations in the fetal and maternal serum. It is conceivable that the considerable differences between maternal and fetal serum T4 concentrations in healthy animals are explained by the T4 impermeability of the placenta. Extremely high maternal T4 (193.5 nmol/L) in 1 bitch was associated with T4 concentrations under the detection limit in the fetal fluids and serum suggesting an inhibitory effect. The T4 concentrations in all the fetal fluids and serum were under the detectable concentration that can be defined by 3.0 nmol/L in that bitch. We have demonstrated that fetal thyroid glands start functioning independently at the same time as thyroid cell formation in the dog, but the overproduction of maternal T4 may have a suppressive effect on fetal iodothyronine production.