Hyperthyroidism in the first trimester of pregnancy

Thyrotoxicosis in a pregnant patient in the first trimester. Pharmacotherapy requires caution: thiamazole is linked to teratogenicity (aplasia cutis, developmental defects); propylthiouracil is the agent of choice in the first trimester per ATA 2017.

ICD-10 O99.21 drug

Also known as: Thyrotoxicosis in pregnancy

Marketing claims without evidence base

The drug is promoted for these uses outside international guidelines. Each entry below is analyzed against AEMPS, FDA, EMA, Cochrane and major RCTs.

FThiamazoleNot recommended · Antithyroid preparations. Sulfur-containing imidazole derivativesIn the first trimester, thiamazole is not used as first-line therapy. ATA 2017 recommends switching to propylthiouracil at confirmation of pregnancy and continuing it through the first trimester. This is due to the risk of thiamazole embryopathy: aplasia cutis of the scalp, choanal and esophageal atresia, anterior abdominal wall defects. After the first trimester, a switch back to thiamazole is acceptable, as propylthiouracil carries higher hepatotoxicity risk.

How to read tiers A–F

Evidence tiers belong to the drug-indication pair, not to the drug as a whole: the same substance can be A for one condition and F for another.

AStrong evidence – first or second line in international guidelines
BRecommended in selected cases
CStudy data contradict each other
DBenefit not proven, no documented harm
FDocumented harm, or risk without benefit