LIGHT Trial: Levothyroxine for Gestational Hypothyroidism in Recurrent Pregnancy Loss

Investigators will perform a multi-center, randomized, placebo, double-blind clinical trial of levothyroxine (LT4) in patients aged between 18 years and 40 yearls who are diagnosed with isolated hypothyroxemia (defined as lower FT4 level with normal TSH) and have a previous history of recurrent miscarriages (defined as two or more pregnancy loss). Investigators will assess the effects of standard dose of 50ug/d levothyroxine therapy from the day after randomization to the day of delivery on the pregnancy outcomes, including live birth, neonatal complications, and adverse pregnancy outcomes, etc.