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Carbimazole in pregnancy

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  • Carbimazole in pregnancy

    I am currently taking 10mg carbimazole once a day and I am planning on becoming pregnant in a few months. My doctor told me that before I become pregnant I should switch meds to PTU for the first trimester and then back to carbimazole for the remainder of the pregnancy. However, I read here that dr Hale answered a question asked by a doctor regarding a patient of his who was 25 weeks pregnant and taking carbimazole and dr Hale said carbimazole is not safe to take during any trimester. I’m really confused now, o would really appreciate if someone could explain to me what is the current recommendation?
    thank you

  • #2
    Rowan6,

    "Carbimazole is converted into methimazole in the body, and methimazole is available in the U.S. for treatment of hyperthyroidism. There is an increased risk for liver toxicity and changes in the infection fighting cells of the blood with methimazole treatment in adults.[2] There has been reported a cluster of fetal malformations including skull abnormalities, unusual facial features, nasal and esophageal abnormalities, absence of the nipples, hearing loss, and developmental delay with methimazole use.[3]Due to the possible association of congenital malformations with methimazole, PTU is recommended for treatment of hyperthyroidism during pregnancy. Carbimazole is an antithyroid medication used in the treatment of hyperthyroidism also known as Graves' disease. Hyperthyroidism without treatment can cause pre-eclampsia, preterm labor, and miscarriage in pregnant women and intrauterine growth retardation, stillbirth, cardiac failure, and hyper/hypothyroid states in the fetus. Therefore, treatment is needed to correct hyperthyroidism. Frequent monitoring of mother?s thyroid status during pregnancy is important to prevent hypothyroidism in the fetus as this condition leads to neuropsychological damage.[1] Carbimazole is converted into methimazole in the body, and methimazole is available in the U.S. for treatment of hyperthyroidism. There is an increased risk for liver toxicity and changes in the infection fighting cells of the blood with methimazole treatment in adults.[2] There has been reported a cluster of fetal malformations including skull abnormalities, unusual facial features, nasal and esophageal abnormalities, absence of the nipples, hearing loss, and developmental delay with methimazole use.[3] There have also been reports of healthy babies with no congential defects treated with methimazole during pregnancy. Wing et al compared propylthiouracil (PTU) and methimazole during pregnancy and found no increase in fetal anomalies with either drug.[4] PTU has been cited as the drug of choice during pregnancy due to the absence of fetal teratology.[5] In summary, due to the possible association of congenital malformations with methimazole, PTU is recommended for treatment of hyperthyroidism during pregnancy." (Medications and Mothers' Milk database, Dr Thomas Hale, PhD).


    1.##Clark SM, Saade GR, Snodgrass WR, Hankins GD. Pharmacokinetics and pharmacotherapy of thionamides in pregnancy. Ther Drug Monit. Aug 2006;28(4):477-483.
    2.##Rivkees SA, Mattison DR. Propylthiouracil (PTU) Hepatoxicity in Children and Recommendations for Discontinuation of Use. Int J Pediatr Endocrinol. 2009;2009:132041.
    3.##Wolf D, Foulds N, Daya H. Antenatal carbimazole and choanal atresia: a new embryopathy. Arch Otolaryngol Head Neck Surg. Sep 2006;132(9):1009-1011.
    4.##Wing DA, Millar LK, Koonings PP, Montoro MN, Mestman JH. A comparison of propylthiouracil versus methimazole in the treatment of hyperthyroidism in pregnancy. Am J Obstet Gynecol. Jan 1994;170(1 Pt 1):90-95.
    5.##Briggs G, Freeman, R., and Yaffe, S. Drugs in Pregancy and Lactation. Vol Seventh Ed. Philadelphia PA: Lippincott, Williams, and Wilkins; 2005.

    Sandra Lovato R.N.
    InfantRisk Center
    806-352-2519

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