Intra-individual variability in TSH levels of healthy women during the first half of pregnancy.

OBJECTIVE: TSH is the parameter most widely accepted to assess thyroid function, especially in pregnant women. The aim of this current study was to analyze intra-individual changes in TSH during the first half of pregnancy in women with TSH levels higher than 2.5mIU/L in early pregnancy.

METHODS: An observational, prospective study was conducted on 243 healthy pregnant women in the first trimester of pregnancy. Thyroid function was assessed by testing TSH and free T4 levels. A subgroup of women with TSH levels >2.5mIU/L underwent additional tests (TSH, free T4, peroxidase antibodies). Information on dietary iodine intake and/or iodine supplements was also recorded.

RESULTS: Mean TSH level was 1.89mIU/L (range 0.024-6.48mIU/L), and mean FT4 level was 1.19ng/dL (range 0.80-1.90ng/dL). Fifty-eight women (23.8%) had TSH levels>2.5mIU/L in the first trimester of pregnancy, and additional thyroid function tests were performed in 27 women. TSH levels significantly decreased from the first to the second test (3.59±0.92mIU/L vs 2.81±1.06mIU/L respectively; p<0.01), and the decrease was significantly greater in pregnant women who used iodized salt as compared to those who did not (1.16±0.65mIU/L vs 0.19±0.93mIUI/L respectively; p<0.01). A positive correlation was found between the time elapsed to the second measurement (24.3±17.2 days; range 8-58) and the decrease in TSH levels (r=0.40; p=0.038).

CONCLUSION: TSH levels showed a continuous, uniform decrease during the first half of pregnancy in women with values slightly above the normal range. Pregnant women who used iodized salt were more likely to have decreased TSH levels in a second test.