Diurnal variations in thyrotropin, prolactin and cortisol during human pregnancy

Diurnal variations in serum concentrations of thyrotropin (TSH), prolactin (PRL) and cortisol were measured in 4 women during late stages of pregnancy (gestational weeks 34-38), in 2 women at an early stage of pregnancy (gestational weeks 11 and 17) and in 2 nonpregnant women. Blood samples were obtained through a nonthrombogenic catheter using continuous withdrawal of blood by means of a portable pump changing the recipient tube at 30-min intervals. Serum TSH concentrations showed a diurnal variation in both pregnant and nonpregnant women with maximal values around midnight. Serum PRL concentrations were elevated in pregnant women but the concentrations showed marked variations indicating episodic secretion of the hormone. Serum cortisol concentrations increased in pregnant women, but the typical diurnal pattern persisted with nadir levels around midnight and marked elevations during the early morning hours. The results suggest that also during human pregnancy there is a diurnal variation in TSH and cortisol secretion, as well as an episodic secretion of PRL.