Is Carbimazole safe while breastfeeding?

Effects on the breastfed infant

Eleven mothers were taking oral carbimazole in dosages ranging from 5 to 20 mg daily during pregnancy and 5 to 15 mg daily during breastfeeding (extent not stated). None of the 12 infants, including one set of twins, had a serum thyroxine (T4) concentration below the lower limit of normal on day 4 of life. Thyrotropin (TSH) concentrations were normal in all infants when measured at various times over the first 21 days postpartum.[ 10 ] Four women were receiving 10 to 20 mg of carbimazole daily. Infant blood samples were obtained on days 4, 7, 10, and 42 and at 3 and 6 months postpartum. Thyroid function was normal in 3 infants. In one infant, the TSH was elevated for the first 10 days of life.[ 6 ] A mother with twins began taking carbimazole 30 mg daily 2 months postpartum. The dosage was lowered as she became euthyroid. The infants were breastfed (extent not stated) and clinical and laboratory examinations were performed over the following 4 months. There was no evidence of alterations in thyroid function.[ 7 ] Fifteen mothers received carbimazole 10 to 20 mg daily for 12 to 40 weeks during pregnancy and 9 continued to take the drug during 2 to 26 weeks of lactation. Their infants were monitored for up to 18 months. Infant thyroid function was within nornml over this period, the range of mean values for individual infants being TSH 1.4 to 5.9 millliunits/L, free T3 6.2 to 9.3 pmol/L and T4 104 to 189 nmol/L. Physical examination at intervals for 2 to 18 months was normal in all infants who received carbimazole and Griffiths mental development scales were normal in all 6 infants who were assessed at 18 months.[ 8 ] A mother was taking carbimazole 50 mg once weekly during pregnancy and postpartum. Her infant was exclusively breastfed for the first 84 days of life and had clinical and laboratory examinations performed over the first 4 months of life. Although the infant's tone and deep tendon reflexes were slightly increased and she was easily irritable, serum thyroi

Effects on milk supply

Relevant published information was not found as of the revision date.

Levels in milk

Carbimazole is a prodrug that is rapidly converted to the active drug methimazole. The methimazole dose is 61% of the carbimazole dose after conversion. Maternal Levels. Five lactating women who were 2 to 6 weeks postpartum were given a single dose of carbimazole 40 mg orally, equivalent to 24.5 mg of methimazole. Methimazole milk concentrations were measured every hour for 8 hours. The average milk concentration was 182 mcg/L at 1 hour after the dose and decreased to 83 mcg/L at 8 hours after the dose. The average total amount of methimazole excreted into milk over 8 hours was 34 mcg (range 29 to 47 mcg).[ 5 ] Four women were receiving 10 to 20 mg of carbimazole daily. Milk samples were obtained on days 4, 7, 10, and 42 and at 3 and 6 months postpartum. The highest methimazole milk level was 75 mcg/L in a woman taking carbimazole 5 mg 4 times daily. The average milk level in all samples was 43.5 mcg/L.[ 6 ] A mother with twins began taking carbimazole 30 mg daily 2 months postpartum. The dosage was lowered as she became euthyroid. Free methimazole was measured in breastmilk 10 times between weeks 2 and 16 of therapy. The mean milk methimazole concentration was 43 mcg/L (range 0 to 92 mcg/L), although the exact dosage the mother was taking during all of these measurements was not stated. The peak milk methimazole concentration occurred about 4 hours after the dose.[ 7 ] Five mothers received carbimazole 10 to 20 mg daily during pregnancy and for 2 to 26 weeks of lactation. Steady-state concentrations of carbimazole (assayed as methimazole) in milk were 6 to 51 mcg/L. The estimated quantity of carbimazole ingested by the infants in milk was 0.5 to 3.2% of the weight-adjusted maternal daily dose.[ 8 ] A woman (time postpartum not stated) given a single dose of 10 mg of carbimazole excreted a total of 0.47% of the dose in her breastmilk in 24 hours.[ 9 ] Infant Levels. A mother with twins began taking carbimazole 30 mg daily 2 months postpartum. The dosage was lowered

Alternatives LactMed lists

• Methimazole
• Propylthiouracil