Background Thyroid hormone measurements are often performed in pregnant women, as hypo- and hyperthyroidism can severely affect the fetus with lifelong consequences. Free T4 measurements are well known for the analytical challenges, due to the low concentrations and the subtle equilibrium between the free T4 and the bound T4 (to thyroid-binding globulin (TBG) and albumin) in serum. Pregnant women have increased TBG concentrations and consequently free T4 measurement might be even more challenging in their serum samples. As accurate free T4 measurements in serum samples of pregnant women are crucial, we measured serum free T4 concentrations using several commercially available immunoassays and the reference method (ID-LC-MS/MS) in both pregnant women and healthy controls.

Methods 30 samples from both pregnant women (PW) and healthy controls (HC) were collected to measure free T4 using the reference methode (ID-LC-MS/MS) and using 5 commercially available automated immunoassays (Alinity (Abbott), Atellica (Siemens), Unicel DXI (Beckmann Coulter), Cobas (Roche) and Lumipulse (Fujirebio)). Method comparisons (Bland Altman plots, Passing and Bablok analyses and correlation coefficients) were performed.

Results The free T4 immunoassays deviated +6.9 to +24.6% more from the LC-MS/MS in PW samples than in HC samples.

Discussion and conclusion Our results indicate that free T4 results in pregnant women measured using immunoassay are often falsely increased, leading to an overestimation of the thyroid function. Physicians and laboratory specialists should be aware of this phenomenon to avoid drawing wrong conclusions about thyroid function in pregnant women.