Thyrotrophin and free thyroxine trimester-specific reference intervals in a mixed ethnic pregnant population in the...

Physiological alterations in the homeostatic control of thyroid hormones cause changes in thyroid function tests in pregnant women. A lack of method, trimester and population-specific reference intervals for free thyroxine (FT4) and thyrotrophin (TSH) makes interpretation of FT4 and TSH levels in pregnancy difficult. We established trimester-specific reference intervals for TSH and FT4 in a mixed ethnic population of pregnant women attending two antenatal clinics in the United Arab Emirates.

TSH and FT4 result from 1140 women with uncomplicated singleton pregnancy were available. The 95% reference intervals were determined for TSH and FT4 for each trimester for Arab women from the United Arab Emirates and other Arab countries and Asian women.

Suppressed TSH levels in the first trimester recovered to non-pregnant levels in the third trimester. There was a significant difference in TSH levels between trimesters 1 and 2, and 2 and 3 (p<0.0005). There was no significant difference in the TSH levels between the various ethnic groups. Mean FT4 levels decreased with each progressive trimester in all groups. There were significant differences in FT4 levels between all three trimesters (p<0.005), especially between the first and second trimesters. FT4 differed significantly between UAE nationals and Asians in the first and second trimesters (p<0.005).

In general, the findings were in keeping with earlier reports. Use of trimester-specific reference intervals should help in the appropriate interpretation of thyroid hormone results in the mixed UAE population.