Thyroid function parameters in normal pregnancies in an iodine sufficient population

•TSH levels in normal pregnancies do not differ from levels seen in adults.•The 10th-percentile value for fT3 in pregnancy is 3.9 pmol/l.•The 10th-percentile value for fT4 in pregnancy is 11.3 pmol/l.•A drop of TSH levels early in pregnancy is not a general phenomenon.

BackgroundThe aim of this retrospective observational study was to describe thyroid function parameters (fT3, fT4 and TSH) in the course of normal pregnancies.MethodsData were obtained between 2006 and 2007 at the University Hospital in Innsbruck, Austria. The starting point was the identification of women who had had a normal birth as recorded in the birth registry of Tyrol. Thyroid function parameters were determined using methods implemented at the Department of Nuclear Medicine in Innsbruck.ResultsThe fT3 and fT4 values were normally distributed. Grouping the results by trimester revealed the following values: 4.93 ± 0.59, 4.54 ± 0.48, and 4.27 ± 0.45 pmol/l for fT3; and 15.23 ± 2.43, 13.79 ± 1.99, and 13.32 ± 0.2.01 pmol/l for fT4, respectively. The values corresponding to the 10th-percentile were 3.9 pmol/l for fT3 and 11.3 pmol/l for fT4, respectively. TSH values showed a typical left skewed distribution, thus the mean values were calculated after log transformation of the data. The corresponding mean trimestral values for TSH were 1.46 ± 1.29, 1.68 ± 1.23, and 1.70 ± 2.22 mIU/l, respectively.ConclusionIn an iodine sufficient population, thyroid function parameters in normal pregnancies do not differ from those in non-pregnant women. Our previously defined reference range for TSH of 0.3 to 3.5 mIU/l is equally valid for normal pregnancies.General significanceThe question of cognition and IQ development of children has been proposed to be associated with thyroid function. The addition of data regarding normal thyroid function during pregnancy will contribute to this research.