How should the dose of levothyroxine (T4) be adjusted in pregnant women with hypothyroidism?

From the Guidelines

Pregnant women with hypothyroidism typically require an increase in their levothyroxine dose by approximately 25-50% during pregnancy, with adjustments made as soon as pregnancy is confirmed and monitored every 4-6 weeks during the first half of pregnancy and at least once during the second half. This recommendation is based on the potential association between high TSH and increased fetal wastage or subsequent neuropsychological complications occurring in the offspring due to thyroid insufficiency 1. The increased dose requirement occurs because pregnancy raises thyroxine-binding globulin levels, increases placental metabolism of T4, and expands blood volume, all of which reduce the availability of active thyroid hormone.

Key Considerations

• Thyroid function tests should be monitored regularly during pregnancy, with TSH goals being 0.1-2.5 mIU/L in the first trimester and 0.2-3.0 mIU/L in the second and third trimesters 1.
• Dose adjustments should be made promptly based on these results to ensure proper neurologic development of the fetus.
• Women who are adequately treated before pregnancy and those diagnosed and treated early in pregnancy have no increased risk of perinatal morbidity 1.
• It is essential to monitor women on thyroid replacement therapy and educate them about its impact on pregnancy.

Adjusting Levothyroxine Dose

• The levothyroxine dose may need to be increased by four to six weeks’ gestation, possibly by 30% or more 1.
• Routine screening for subclinical hypothyroidism is not recommended; however, women with risk factors and symptoms of thyroid diseases should be screened, and subclinical hypothyroidism should be treated 1.
• After delivery, women should return to their pre-pregnancy dose within 4-6 weeks and have thyroid function rechecked 6-8 weeks postpartum to ensure appropriate levels.

From the FDA Drug Label

Since elevations in serum TSH may occur as early as 4 weeks gestation, pregnant women taking Levothyroxine Sodium Tablets, USP should have their TSH measured during each trimester. An elevated serum TSH level should be corrected by an increase in the dose of Levothyroxine Sodium Tablets, USP

Hypothyroidism during pregnancy is associated with a higher rate of complications, including spontaneous abortion, pre-eclampsia, stillbirth and premature delivery. Maternal hypothyroidism may have an adverse effect on fetal and childhood growth and development. During pregnancy, serum T4 levels may decrease and serum TSH levels increase to values outside the normal range

The dose of levothyroxine should be adjusted in pregnant women with hypothyroidism by:

• Measuring TSH levels during each trimester
• Increasing the dose of levothyroxine if TSH levels are elevated
• Monitoring TSH levels closely to avoid undertreatment or overtreatment 2 Key points to consider:
• Hypothyroidism during pregnancy can have adverse effects on fetal and childhood growth and development
• Levothyroxine dose may need to be increased during pregnancy to maintain normal TSH levels
• TSH levels should be monitored closely during pregnancy to ensure proper levothyroxine dosing 2

From the Research

Adjusting Levothyroxine Dose in Pregnant Women with Hypothyroidism

• The dose of levothyroxine (T4) may need to be adjusted in pregnant women with hypothyroidism to maintain euthyroidism and prevent adverse fetal and maternal outcomes 3, 4.
• Studies suggest that levothyroxine requirements increase during pregnancy, with the majority of women requiring an increase in their dose, especially in the first trimester 3, 5.
• The increase in levothyroxine dose is typically around 30-50% of the preconception dose, with some women requiring a higher or lower increase 4, 5.
• The timing of the increase in levothyroxine dose varies, but most women require an increase by the 8th week of gestation, with the dose plateauing by week 16 3.
• Serum thyrotropin (TSH) levels should be monitored regularly during pregnancy to adjust the levothyroxine dose accordingly, with a target TSH level of below 2.5 mIU/L 4, 6.
• Women with a preconception TSH level closer to the normal range (less than 1.2 mIU/L) may be less likely to require an increase in their levothyroxine dose during pregnancy 6.
• It is essential to individualize the adjustment of levothyroxine dose during pregnancy, as some women may not require an increase or may even require a decrease in their dose 5, 7.