Optimising levothyroxine ...

In pregnancy, subclinical hypothyroidism (SCH) diagnosis relies on elevated TSH levels above trimester-specific reference ranges while T4 levels remain within the normal range. Guidelines, such as the European Thyroid Association's 2014 publication and the American Thyroid Association's 2017 recommendations, emphasize the need for locally-derived TSH reference ranges, with suggested upper limits at 2.5-3.5 mIU/L for different trimesters. In the absence of specific local ranges, the upper limit is advised to be around 4 mIU/L for the first trimester, allowing for adjustments throughout pregnancy. Factors like weight, ethnicity, and location must also be considered for accurate TSH interpretation.