Hypothyroidism (under active thyroid) during pregnancy affects about 0.5% of women. Thyroid hormone is needed for babies brain development. In the first trimester of pregnancy the baby is completely dependent on the mother for the production of thyroid hormone. The baby then starts to produce its own thyroid hormones but it still requires some supply from the mother. Iodine intake of 250 micrograms/day during pregnancy is needed to maintain enough thyroid hormone production. This is present in most western diets without supplementation.
Untreated hypothyroidism is associated with pregnancy complications such as low iron level (anaemia), pre-eclampsia, small babies, preterm birth and postpartum haemorrhage. Woman with hypothyroidism should have blood tests done to check thyroid funcion once they have a positive pregnancy test, as thyroid hormone requirements increase during pregnancy. This means that you may need to increase the dose your medication. Most women will have their dose increased by 25-50% by their doctor in early opregnancy. Thyroid function tests should be checked every 6-8 weeks during pregnancy to ensure that the level remains normal throughout pregnancy.
Antenatal vitamin supplements that contain iron and calcium can interfere with the absorption of thyroid hormone. Thus you should not take these at the same time of day. After delivery women can go back to her pre-pregnancy dose of medication. You should have your thyroid level checked at your 6 week postnatal appointment.
Many fertility clinics check thyroid function tests. Women with subclinical hypothroidism (TSH levels between 2.5 mIU/L and 4 mIU/L) and positive thyroid peroxidase TPO antibodies benefit from thyriod medication.This improves fertility rates, especially in unexplained infertility, and reduce miscarriage rates. For women without fetility concerns and those without a personal or family history of underactive thyroid there is no need to check thyroid hoemone levels. If in doubt ask your doctor for advice.