Tapazole (methimazole)

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There are no contraindication interactions for this drug.

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There are no contraindication contraindications for this drug.

There are no severe contraindications for this drug.

There are no moderate contraindications for this drug.

There is no enhanced messaging for this drug.

There are no severe adverse reactions for this drug.

There are no less severe adverse reactions for this drug.

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Methimazole readily crosses the placental membranes and may cause fetal harm, particularly when administered in the first trimester of pregnancy. The drug can also cause fetal goiter and hypothyroidism (cretinism) when administered to a pregnant woman.

In April 2010, the US Food and Drug Administration (FDA) reported a review of postmarketing data analyzing the potential for birth defects associated with use of propylthiouracil or methimazole during pregnancy. FDA found that congenital malformations were reported approximately 3 times more often with prenatal exposure to methimazole compared with propylthiouracil (29 cases with methimazole; 9 cases with propylthiouracil). In addition, there was a distinct and consistent pattern of congenital malformations associated with the use of methimazole that was not found with propylthiouracil. Approximately 90% of the congenital malformations with methimazole were craniofacial malformations (e.g., scalp epidermal aplasia (aplasia cutis), facial dysmorphism, choanal atresia). In most of the cases, there were multiple malformations that frequently included a combination of craniofacial defects and GI atresia or aplasia. These specific birth defects were associated with the use of methimazole during the first trimester of pregnancy but were not found when the drug was administered later in pregnancy. In contrast, FDA did not find a consistent pattern of birth defects associated with the use of propylthiouracil and concluded that there is no convincing evidence of an association between propylthiouracil use and congenital malformations, even with use during the first trimester.

Despite the potential fetal hazard, antithyroid agents are still considered the therapy of choice for the management of hyperthyroidism during pregnancy. Since methimazole may be associated with the rare development of fetal abnormalities, such as aplasia cutis, craniofacial malformations (facial dysmorphism, choanal atresia), and GI malformations (esophageal atresia with or without tracheoesophageal fistula, umbilical abnormalities), propylthiouracil is the preferred agent when an antithyroid drug is indicated during organogenesis in the first trimester of pregnancy or just prior to the first trimester of pregnancy. Patients receiving methimazole should be switched to propylthiouracil if pregnancy is confirmed in the first trimester. Because of the potential adverse maternal effects of propylthiouracil (e.g., hepatotoxicity), however, it may be preferable to switch from propylthiouracil to methimazole for the second and third trimesters (i.e., after the first trimester). If the patient is switching from propylthiouracil to methimazole, thyroid function should be assessed after 2 weeks and then every 2-4 weeks thereafter. It is not known if the risk of methimazole-induced aplasia cutis or embryopathy outweighs the risk of propylthiouracil-induced hepatotoxicity.

If methimazole is used during pregnancy for the management of hyperthyroidism, the manufacturer states that a sufficient, but not excessive, dosage of methimazole is necessary; the lowest possible dosage of methimazole to control the maternal disease should be used. Some clinicians state that antithyroid drug therapy should be initiated or adjusted to maintain maternal free thyroxine (T4) concentrations at or just above the upper limit of normal (ULN) of the nonpregnant reference range, or to maintain total T4 concentrations at 1.5 times the ULN or the free T4 index in the ULN, while using the lowest possible dosage of antithyroid drugs. In women receiving antithyroid drugs during pregnancy, free T4 and TSH concentrations should be monitored approximately every 2-6 weeks. The manufacturer states that because thyroid dysfunction diminishes in many women as pregnancy proceeds, a reduction in dosage of antithyroid therapy may be possible, and, in some patients, use of antithyroid therapy can be discontinued 2-3 weeks before delivery.

Patients should be advised to contact their clinician immediately about their therapy if they are or plan to become pregnant while receiving an antithyroid drug. If methimazole is used during pregnancy or if the patient becomes pregnant while receiving the drug, the patient should be advised of the potential hazard to the fetus; in addition, when considering antithyroid drug use during pregnancy, the patient should be informed of the risks of methimazole-associated fetal malformations, as well as the risks of propylthiouracil-associated hepatotoxicity.

Methimazole is distributed into milk. However, several studies found no effect on clinical status in nursing infants of women receiving methimazole, particularly if thyroid function is monitored at frequent (weekly or biweekly) intervals. A long-term study of 139 thyrotoxic lactating women and their infants failed to demonstrate toxicity in infants who are breast-fed by women receiving methimazole.

Methimazole generally is compatible with breast-feeding, and moderate dosages of the drug (i.e., 20-30 mg daily) appear to be safe during breast-feeding. Because of concerns regarding severe hepatotoxicity (i.e., hepatic necrosis in either woman or child) associated with maternal use of propylthiouracil, some clinicians consider methimazole to be the preferred antithyroid drug in nursing women. If an antithyroid drug is used in nursing women, some clinicians recommend that the drug be administered after a feeding and in divided doses, and that thyroid function be monitored in nursing infants.

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