Gestational hypehyroidism occurs due to

A common cause of overactive thyroid in pregnant women is Graves' disease. This disease occurs when your immune system becomes overactive and forms antibodies (immune proteins) that attack the thyroid. This causes the gland to enlarge and make too much thyroid hormone. Most women with Graves' disease find out they have it and get treatment before they become pregnant. Women with severe nausea and vomiting or those expecting twins may develop temporary hypehyroidism. Called transient gestational thyrotoxicosis, this hypehyroidism is due to high levels of a pregnancy hormone called human chorionic gonadotropin or hCG. Because it resolves by week 14 to 18 of pregnancy, women do not need antithyroid drugs to treat this condition. Ref - pubmed.com