Around 1-2% of pregnant women receive levothyroxine treatment for overt hypothyroidism. This condition, which commonly has an autoimmune cause, is defined as a low plasma free thyroxine (T4) concentration and a raised plasma thyroid-stimulating hormone (TSH) concentration. Around another 2.5% of pregnant women have subclinical ('compensated') hypothyroidism, which is defined as a raised plasma TSH concentration with a normal free T4 concentration. Here we review the evidence on whether such problems affect pregnancy outcome or the offspring and update our previous advice on the management of hypothyroidism in pregnancy; we do not cover postpartum thyroid disorders.