How to manage hypothyroxinemia with a target Thyroid-Stimulating Hormone (TSH) level matching the trimester-specific level during pregnancy?

From the Guidelines

To manage hypothyroxinemia with a target Thyroid-Stimulating Hormone (TSH) level matching the trimester-specific level during pregnancy, levothyroxine treatment should be initiated promptly to restore the serum TSH concentration to the reference range, as recommended by the JAMA study 1. The goal of treatment is to maintain TSH levels within the trimester-specific targets, which are:

• First trimester: 0.1-2.5 mIU/L
• Second trimester: 0.2-3.0 mIU/L
• Third trimester: 0.3-3.0 mIU/L Key considerations for managing hypothyroxinemia during pregnancy include:
• Monitoring TSH and free T4 levels every 6-8 weeks during pregnancy, as recommended by 1
• Adjusting the levothyroxine dose as needed to maintain TSH within the target range and free T4 in the upper half of the normal range
• Increasing the levothyroxine dose by 25-30% as soon as pregnancy is confirmed, as thyroid hormone requirements increase during pregnancy
• Taking levothyroxine on an empty stomach, 30-60 minutes before breakfast, and separating it from prenatal vitamins by at least 4 hours The potential benefits of levothyroxine therapy in pregnancy, as noted by 1, include reducing the risk of fetal wastage and subsequent neuropsychological complications in the offspring due to thyroid insufficiency. Overall, the management of hypothyroxinemia during pregnancy requires careful monitoring and adjustment of levothyroxine treatment to ensure adequate thyroid hormone levels for fetal development and maternal health.

From the FDA Drug Label

For pregnant patients with pre-existing hypothyroidism, measure serum TSH and free-T4 as soon as pregnancy is confirmed and, at minimum, during each trimester of pregnancy. In pregnant patients with primary hypothyroidism, maintain serum TSH in the trimester-specific reference range The recommended daily dosage of levothyroxine sodium tablets in pregnant patients is described in Table 3. Pre-existing primary hypothyroidism with serum TSH above normal trimester-specific range: Pre-pregnancy dosage may increase during pregnancy. Increase levothyroxine sodium dosage by 12.5 to 25 mcg per day. Monitor TSH every 4 weeks until a stable dose is reached and serum TSH is within normal trimester-specific range.

To manage hypothyroxinemia with a target Thyroid-Stimulating Hormone (TSH) level matching the trimester-specific level during pregnancy:

• Monitor serum TSH and free-T4 as soon as pregnancy is confirmed and during each trimester.
• Adjust the levothyroxine sodium dosage to maintain serum TSH in the trimester-specific reference range.
• For pre-existing primary hypothyroidism, increase the dosage by 12.5 to 25 mcg per day if serum TSH is above the normal trimester-specific range.
• Monitor TSH every 4 weeks until a stable dose is reached and serum TSH is within the normal trimester-specific range 2.

From the Research

Managing Hypothyroxinemia During Pregnancy

To manage hypothyroxinemia with a target Thyroid-Stimulating Hormone (TSH) level matching the trimester-specific level during pregnancy, the following steps can be taken:

• Adjust levothyroxine dosage according to TSH levels measured throughout pregnancy, with most adjustments made in the first trimester 3
• Consider an empirical dose increase of 30-50% as soon as pregnancy is confirmed for women already on levothyroxine 4
• Titrate levothyroxine doses against TSH, which have trimester-specific ranges, to achieve a TSH target of below 2.5mIU/L in the first trimester and below 3mIU/L in subsequent trimesters 4, 5

Levothyroxine Dosage Requirements

The required levothyroxine dosage may vary depending on the severity of hypothyroidism and the trimester of pregnancy:

• For subclinical hypothyroidism (SCH) with TSH ≤ 4.2 mIU/L, an initial dose of 1.20 μg/kg/day may be sufficient 5
• For SCH with TSH > 4.2-10 mIU/L, an initial dose of 1.42 μg/kg/day may be required 5
• For overt hypothyroidism (OH), an initial dose of 2.33 μg/kg/day may be necessary 5
• Increasing levothyroxine dose for women with uncontrolled hypothyroidism in the first trimester of pregnancy may decrease the risk of pregnancy loss 6

Monitoring TSH Levels

Regular monitoring of TSH levels is crucial to ensure that the target levels are achieved:

• TSH levels should be measured before conception, after the first missed menstrual period, and in the second and third trimesters of pregnancy 3
• Women with TSH levels greater than 2.5 mU/L in the first trimester may have an increased risk of miscarriage, and those with TSH levels exceeding 4.5 mU/L may have a higher risk of miscarriage 7