Dose requirements of Thyroid T3/T4 during pregnancy

Thyroid hormone therapy is the use of synthetic thyroid hormones to raise abnormally low levels of thyroid hormones. Often used to treat an underactive thyroid that is secreting little or no thyroid hormones, thyroid hormone therapy is usually administered in pill form. The most commonly prescribed thyroid hormone replacement is pure synthetic thyroxine (T4).

Thyroid assessment may be difficult during pregnancy because of the complex changes in thyroid function that occur during this time. There are several active thyroid preparations that are used for replacement therapy of hypothyroidism. There is no consensus of the appropriate dose of the thyroid hormone thyroxine (T4) needed for replacement during pregnancy for hypothyroid patients.

Pregnant patients with hypothyroidism due to thyroidectomy, removal of the thyroid (usually for thyroid carcinoma), were studied to determine the dose requirements of thyroxine (T4) and desiccated thyroid during their pregnancies. There were eight pregnancies in six women. In four patients with five pregnancies, free T4 in the blood decreased during pregnancy but increased above the normal range after delivery. Serum thyroid-stimulating hormone (TSH) increased during pregnancy but returned to an undetectable level after delivery with one exception. The T3-to-T4 ratio, which is related to the conversion of T4 to triiodothyronine (T3), was lower in the patients treated with T4 than in normal controls, regardless of pregnancy.

The ratio decreased during pregnancy and a relative deficiency of T3 during pregnancy was suspected. Replacement therapy for pregnant patients with hypothyroidism after removal of the thyroid should include an increased dose of T4. The dose of desiccated thyroid does not need to be changed. Pregnant patients with hypothyroidism due to total thyroidectomy are well suited for evaluating thyroid hormone replacement during pregnancy. Blood TSH levels should be monitored occasionally during pregnancy and the dose of T4 should be increased in cases of an elevated TSH level.

For women taking thyroxine who are planning to conceive is adviced to increase their dose of thyroxine by 30% at the confirmation of the pregnancy. TSH should be monitored every 8-10 weeks during pregnancy with further dose adjustments as necessary. The thyroxine dose returns to the pre-pregnancy dose after delivery whether the mother is breastfeeding or not.