Reference Intervals of Thyroid Function Tests Assessed by Immunoassay and Mass Spectrometry in Healthy Pregnant Women Living in Catalonia.

Background: Recent guidelines recommend establishing a local reference interval (RI) for thyroid function. We aimed to establish trimester-specific RIs for thyrotropin (TSH) and free thyroxine (FT4) in a cohort of healthy pregnant women in Catalonia (Spain).

Methods: A prospective observational study was conducted with 332 healthy pregnant women, from the first trimester (1T) to delivery.