Mercazole

[Loz]

I am 13 weeks pregnant and have just been diagnosed with hyperthyroidism. My doctor wants me to take 2.5mg of mercazole as that is what is prescribed in the second trimester in Australia. She only wants me on it for hopefully a short time however I am very concerned this could harm my baby and I keep reading conflicting information.

Could a short time on this drug on this amount still cause issues after the first trimester

[Loz]

I did ask to be prescribed PTU as I had read safer reviews for this drug however she said she would be going against recommendations since I am now in my second trimester.

[Sandra]

Loz,

This is the data we have for methimazole also known as thiamazole which is the equivalent of mercazole

Methimazole (thiamazole-mercazole)

"The Use: Antithyroid agent
Terato1: P4 - Positive evidence of fetal risk
Terato2: P4 - Positive evidence of fetal risk
T 1/2: 6-13 h
Oral: 80-95%
T Max: 1 h
Adult Dose: 5-30 mg daily.
Trade Names: Tapazole
Methimazole readily crosses the placental membranes and can induce goiter and even cretinism in the developing fetus. In addition, rare instances of congenital defects: aplasia cutis, as manifested by scalp defects; esophageal atresia with tracheoesophageal fistula; and choanal atresia with absent/hypoplastic nipples, have occurred in infants born to mothers who received methimazole during pregnancy. Methimazole can cause fetal harm when administered to a pregnant woman. Methimazole readily crosses the placental membranes and can induce goiter and even cretinism in the developing fetus. In addition, rare instances of congenital defects: aplasia cutis, as manifested by scalp defects; esophageal atresia with tracheoesophageal fistula; and choanal atresia with absent/hypoplastic nipples, have occurred in infants born to mothers who received methimazole during pregnancy. If methimazole is used during pregnancy or if the patient becomes pregnant while taking this drug, the patient should be warned of the potential hazard to the fetus. Since the above congenital defects have been reported in offspring of patients treated with methimazole, it may be appropriate to use other agents in pregnant women requiring treatment for hyperthyroidism. Methimazole used judiciously is an effective drug in hyperthyroidism complicated by pregnancy. In many pregnant women, the thyroid dysfunction diminishes as the pregnancy proceeds; consequently, a reduction in dosage may be possible. In some instances, use of methimazole can be discontinued 2 or 3 weeks before delivery." (Medications and Mothers' Milk database, Dr Thomas Hale PhD).

Sandra Lovato R.N.
InfantRisk Center

[Loz]

Hi Sandra, thank you.

[Loz]

Oops I send that without my message!
I have been taking it for a week now as I was assured by several doctors it was safe at my stage in pregnancy (13 weeks 5mg/day). I feel very concerned about this information you have given me .

What would be my alternatives? Could you give me info on PTU? Do you have information on the amounts of Mercazole or length of time/what point in pregnancy the drug was given to have these results? I feel very very worried

[Sandra]

Loz,

It looks like methimazole is most concerning in the in the first trimester while the baby is developing and the last 2-3 weeks of pregnancy. I would not be so concerned in the second trimester. I think you have time to discuss your options with your Dr.

"Human experience suggests propylthiouracil (PTU) is not expected to increase the risk of congenital anomalies; however, fetal and neonatal thyroid impairment may occur.[1] Propylthiouracil is considered to be the antithyroid agent of choice during pregnancy. [1]" (Medications and Mothers' Milk database, Dr Thomas Hale PhD).

1.##The REPROTOX? System. Georgetown University Medical Center and Reproductive Toxicology Center, Columbia Hospital for Women Medical Center, Washington, D.C. (electronic version). Thomson Reuters, Greenwood Village, Colorado, USA. Available at: [url]http://csi.micromedex.com[/url] (Jun/1/2010).

Sandra Lovato R.N.

[Loz]

Thank you so much. This is my third pregnancy and I’ve found the help I’ve received from this site wonderful. Thank you for keeping us informed :-)

[Loz]

Are either of these drugs safe in breastfeeding? Thanks

[Sandra]

Loz

Methimazole is rated an L2-limited data-probably compatible. The amount that transfers into breast milk is 5.8-14.7% of your dose. We think if the dose is kept low (no more than 30mg a day) you can continue to nurse, but recommend checking your infants thyroid function to be safe. "In a large study of over 134 thyrotoxic lactating mothers and their infants, even at methimazole doses of 20 mg/day, no changes in infant TSH, T4 or T3 were noted in over 12 months of study." (Medications and Mothers' Milk database, Dr Thomas Hale PhD).

PTU (Propylthiouracil) is rated an L2-limited data-probably compatible. The amount that passes into breast milk is 1.8% of your dose. "Propylthiouracil is the preferred antithyroid agent in breastfeeding mothers. Only small amounts are secreted into milk, and side effects to the infant are minimal. To be safe, monitor thyroid function of the infant. No changes in infant thyroid have been reported. Monitor infant thyroid function (T4, TSH) carefully during therapy." (Medications and Mothers' Milk database, Dr Thomas Hale PhD).

Sandra Lovato R.N.
InfantRisk Center