Reference intervals in evaluation of maternal thyroid function during the first trimester of pregnancy

Insufficient function of the thyroid gland can cause low concentrations of thyroid gland hormones which are necessary for the proper development of the foetus brain, especially during the beginning of pregnancy. We determined the reference interval for TSH in the first trimester of pregnancy and the decision point for anti-TPO antibodies' positivity.

We tested 5520 women from the central part of the Czech Republic during the first trimester of pregnancy for serum TSH, anti-TPO and FT4 by chemiluminometric immunoanalysis on an ADVIA Centaur system (Siemens). The reference interval for TSH during the first trimester of pregnancy was determined using the log transformation, and then summarized as the geometrical mean (95% CI); following which, a suitable decision point for anti-TPO positivity was set.

The reference interval for TSH was determined to be 0.06-3.67 mU/l. The suppression of TSH was found in 2.93% of the women; a raised concentration of TSH had been found in 4.48% of the women. For anti-TPO, the cut-off was established at 143 kU/l; 11.2% of the pregnant women were found to be anti-TPO positive.

Determination of the reference interval for TSH during early pregnancy is one of the basic requirements when implementing a general examination of the thyroid gland at the beginning of pregnancy. The decision level for positivity of anti-TPO is more than double the manufacturer's reference interval.