When to check Thyroid-Stimulating Hormone (TSH) levels in a pregnant patient with a history of hypothyroidism on thyroid replacement therapy with Levothyroxine (thyroxine)?

From the Guidelines

Pregnant patients with a history of hypothyroidism on levothyroxine therapy should have their TSH levels checked every 6 to 8 weeks during pregnancy. 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 requirement for levothyroxine in treated hypothyroid women frequently increases during pregnancy, and therefore, serum TSH concentration should be monitored regularly and the levothyroxine dose modified as needed. Some key points to consider when monitoring TSH levels in pregnant patients with hypothyroidism include:

• The potential benefits of thyroid hormone replacement in pregnant women with elevated TSH levels, despite the lack of published intervention trials assessing the benefits of thyroid hormone replacement in this special population 1
• The importance of restoring the serum TSH concentration to the reference range to minimize the risks of thyroid insufficiency to the fetus
• The need for close monitoring of TSH levels during pregnancy, as the requirement for levothyroxine may increase during this time
• The minimal risks associated with appropriately managed levothyroxine therapy in pregnancy, making it a justifiable treatment option for pregnant women with hypothyroidism 1

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 Monitor TSH every 4 weeks until a stable dose is reached and serum TSH is within normal trimester-specific range.

Key Points:

• Check TSH levels as soon as pregnancy is confirmed
• Monitor TSH levels at minimum during each trimester of pregnancy
• Maintain serum TSH in the trimester-specific reference range
• Monitor TSH every 4 weeks until a stable dose is reached 2

From the Research

Timing of TSH Checks in Pregnant Patients with Hypothyroidism

• TSH levels should be checked when pregnancy is confirmed, and then every 4-8 weeks to monitor the need for adjustments in levothyroxine dosage 3.
• The American Thyroid Association recommends that TSH levels be maintained between 0.4-4.1 microU/mL during pregnancy 3.
• Levothyroxine requirements may increase as early as the fifth week of gestation, with a mean increase of 47% during the first half of pregnancy 4.
• It is recommended to increase the levothyroxine dose by approximately 30% as soon as pregnancy is confirmed, and then monitor serum TSH levels and adjust the dose accordingly 4, 5.

Factors Influencing TSH Checks and Levothyroxine Dosage

• The etiology of hypothyroidism plays a significant role in determining the timing and magnitude of thyroid hormone adjustments during pregnancy 3.
• Women with primary hypothyroidism may require smaller cumulative dose increases of levothyroxine compared to those with hypothyroidism resulting from treated Graves' disease or goiter 3.
• Preconception TSH levels can predict the need for increasing levothyroxine dosage during pregnancy, with higher preconception TSH levels requiring larger dose increases 6.

Target TSH Levels and Levothyroxine Dosage

• Target TSH levels during pregnancy are ≤2.5 mIU/L for the first trimester and ≤3 mIU/L for the subsequent trimesters 5, 7.
• The initial levothyroxine dose for newly discovered subclinical hypothyroidism during pregnancy can be estimated based on basal TSH levels, with a recommended dose of 1.20 μg/kg/day for TSH ≤ 4.2 mIU/L and 1.42 μg/kg/day for TSH > 4.2-10 mIU/L 7.
• For overt hypothyroidism, a higher initial levothyroxine dose of 2.33 μg/kg/day may be required 7.