Thyroid hormone and TSH levels change during pregnancy yet most laboratories use reference ranges derived from subjects who are not pregnant and male. Does this lead to an underestimation of thyroid disorders in pregnancy?
We have derived our own reference range for Free T4 and TSH levels in pregnancy. The hormones were measured in 82 euthyroid mothers at booking and 68 at 28 weeks of pregnancy. Mean (95%CI: reference range of +2SD) FT4 levels were: Booking 14.3(13.914.7: 10.318.3) mmol/L; 28 weeks 11.8(11.412.2: 10.3-18.3) mmol/L; a significant difference (P<0.001). Mean (95%CI: reference range +2SD) TSH levels: Booking 0.86(0.71.1: 01.82) mu/L; 28 weeks 1.09(0.81.4: 0.032.15) miu/L; a significant difference (P<0.01). The mean and reference ranges for standard laboratory tests were: FT4=14.02 and 9.0 to 20.0 mmol/L; and TSH =1.7 and 0.2 to 3.0 miu/L. Compared to the laboratory range the study FT4 at 28 weeks was significantly lower (P<0.001) and TSH lower at both booking and 28 weeks (P<0.001).
In 59 treated hypothyroid patients TSH levels were: Booking - normal =31 (52.5%) and raised =22 (37.3%); at 28 weeks normal =47 (79.7%) and raised =7 (11.9%). Applying the study reference range a further 6 (10.2%) had raised TSH levels at booking and 5 (8.4%) at 28 weeks.
We have found a high number of mothers on thyroxine therapy to be hypothyroid at booking. Furthermore by using a specific reference ranges for TFTs in pregnancy the number of hypothyroid tests in treated mothers may increase.