Circadian thyrotropin variations are preserved in normal pregnant women.

Serum thyrotropin (TSH) concentration circadian rhythm is abolished in many endocrine and non-endocrine diseases. In the present study we have measured serum TSH concentration over 24 h every 2 h in second and third trimester pregnant women. During the 24-h period, serum free thyroxine and free triiodothyronine concentrations did not change significantly. In contrast, serum TSH concentrations demonstrated significant circadian variations both in the second and third trimester pregnant women (p < 0.02 and p < 0.005, respectively). In summary, second and third trimester pregnancy is associated with a normal circadian TSH rhythm.