management of Hyperthyroidsm in pregnancy, which antithyroid drug is safer in pregnancyHyperthyroidism is a condition in which thyroid gland starts to secrete an increased amount of thyroxine in blood. This, in turn, produces several effects in the body, including, palpitations, weight loss, increased appetite, heat intolerance, sleeplessness, generalized weakness and abortions. There are various causes that are responsible for this increased production of thyroxine by the thyroid gland. We will not go into this detail in this article but have explained it in a separate article, the link of which is stated below.
Thyroid Diseases, an overview
Hyperthyroidism in pregnancy should be treated, because it may result in fetal abnormalities. The thyroid hormone T3 and T4 crosses the placenta in small amounts. The amount of diffusion across the placenta decreases with the age of pregnancy. But still it may cross in significant amounts. The thyrotropin-releasing hormone and thyroid stimulating immunoglobulins (which are responsible for maternal hyperthyroidism) and beta blockers (which are given in the treatment of hyperthyroidism) readily cross the placenta. So beta-blockers should be avoided in a pregnant thyrotoxic woman. There are three drugs that can be used to treat hyperthyroidism, Carbimazole, Methimazole and Propylthiouracil. Carbimazole and Methimazole cross the placenta and may cause fetal hypothyroidism and abnormalities. The risk is higher with the use of Methimazole. Propylthiouracil has shown to have a much safer profile in pregnancy and can be used. But lowest possible dose should be given.
If a pregnant woman is severely thyrotoxic, then subtotal or near-total thyroidectomy may be used as an option in selective patients.
Antithyroid drugs are secreted in breast milk, so breastfeeding should be avoided if the mother is taking antithyroid drugs.
In the fetus, the thyroid gland follicles and colloid start to appear in the 10th – 12th week of pregnancy. So before 10 weeks, antithyroid drugs don’t do any harm to the fetus but after that one should be very careful.

References:
    1. Drug therapy for hyperthyroidism in pregnancy: safety issues for mother and fetus. Atkins P, Cohen SB, Phillips BJ.  URL: www.ncbi.nlm.nih.gov/pubmed/11005705
    2. American Thyroid Association: How should hyperthyroidism in pregnancy be managed? URL: www.thyroid.org/thyroid-guidelines/hyperthyroidism/resultss/

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