Debate: Level of TSH at which thyroid hormone replacement should be initiated in gestation

In this debate, two clinicians discuss the merits and limitations of levothyroxine treatment for pregnant women with thyrotropin (TSH) concentrations 2.5-10 mIU/L. Recently published clinical practice guidelines have raised the treatment threshold, suggesting treatment for women with thyrotropin (TSH) ≥4.0 mIU/L and normal thyroxine concentrations only in the first trimester and recommending against treatment for women with TSH concentrations between 2.5 and 4.0 mIU/L. The evolution of these recommendations and the uncertainty regarding the overall balance of benefits and harms across the spectrum of mild thyroid dysfunction during pregnancy may lead to challenges in clinical practice, underscoring the importance of clear, practical guidance to support informed and consistent clinical decision-making. Dr. Bliddal argues that individualized levothyroxine treatment may remain reasonable in selected pregnant women with TSH concentrations of 2.5-4.0 mIU/L, and supports consideration of treatment in women with first trimester subclinical hypothyroidism and TSH concentrations between 4.0-10.0 mIU/L, emphasizing biological heterogeneity, limitations of fixed thresholds, and remaining evidence gaps. Dr. Maraka argues against routine levothyroxine treatment in these women, highlighting the uncertainty regarding clinical benefit and the potential risks of overtreatment.