Monitor THYROID disease during pregnancy

Overview

Uncontrolled thyroid disease during pregnancy has risks, such as miscarriage, preterm birth, low birth weight, and cognitive defects. Consider these tips to limit adverse fetal outcomes...

Management

HYPOthyroidism

Pregnant women need higher levothyroxine doses due to faster drug metabolism, T4 transfer to the baby, etc. Generally aim for a TSH less than 2.5 mIU/L during pregnancy and when a woman is planning to become pregnant. Once pregnancy is confirmed, increase the levothyroxine dose by 25% to 30%. For example, have these women take an extra levothyroxine dose two days/week "see table 1". Monitor TSH every 4 weeks during the first half of pregnancy and at least once between 26- and 32-weeks gestation.

Table (1)

Levothyroxine Dosing Guidance in Pregnancy.

After delivery, return to the woman's pre-pregnancy levothyroxine dose and check TSH in about 6 weeks. Some women will need to go back to their pregnancy dose if their TSH goes up.

Don't use desiccated thyroid (Armour Thyroid, etc) or add liothyronine (Cytomel). These don't provide enough T4 to the fetal brain. Both Armour Thyroid and Cytomel are NOT available in Egypt

Tell women that levothyroxine absorption is better when taken about 4 hours apart from their prenatal vitamin. But if that's not convenient, tell them it's most important to take their medications at CONSISTENT times.

HYPERthyroidism

Use propylthiouracil (Thyrocil) for women trying to get pregnant and during their first trimester. But consider switching to methimazole for the second and third trimesters and while breastfeeding. Use a dose ratio of about 1:20 for methimazole to propylthiouracil when changing from one drug to another. For example, Methimazole 15 mg/day (e.g., given once daily or divided up to three times daily) converts to a propylthiouracil dose of about 300 mg/day (e.g., usually divided two or three times daily).

This helps limit maternal liver toxicity from propylthiouracil and birth defects from methimazole during early fetal development. With either drug, use the lowest dose that keeps the free T4 level near the top of the normal range, around 1.8 ng/dL, to limit fetal drug exposure. Consider a beta-blocker, usually propranolol (Inderal) or metoprolol (Seloken Zoc) for 2 to 6 weeks to limit hyperthyroid symptoms until antithyroid drugs kick in. Don't routinely screen all pregnant women for thyroid disease. But check a TSH if women have symptoms of hypo- or hyperthyroidism.

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References

1. Practice Bulletin No. 148: Thyroid disease in pregnancy. Obstet Gynecol. 2015 Apr;125(4):996-1005.
2. Alexander EK, Pearce EN, et al. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid. 2017 Mar;27(3):315-389.
3. Garber JR, Cobin RH, et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Thyroid. 2012 Dec;22(12):1200-35.
4. Bahn RS, Burch HB, et al. Hyperthyroidism and other causes of thyrotoxicosis: management guidelines of the American Thyroid Association and American Association of Clinical Endocrinologists. Endocr Pract. 2011 May-Jun;17(3):456-520.

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