Increase Levothyroxine Dose
For a pregnant woman with pre-existing hypothyroidism on levothyroxine, the dose should be increased immediately upon pregnancy confirmation, even if current TSH and T4 levels appear normal, because thyroid hormone requirements increase substantially during pregnancy starting as early as 5 weeks gestation. 1, 2
Rationale for Dose Increase
Thyroid hormone requirements increase by approximately 30-50% during pregnancy, with the increase beginning as early as the fifth week of gestation and plateauing by week 16 1, 3, 2
The FDA label specifically recommends for pregnant patients with pre-existing primary hypothyroidism to increase levothyroxine sodium dosage by 12.5 to 25 mcg per day as soon as pregnancy is confirmed 1
An empirical dose increase of 30-50% should be implemented immediately upon pregnancy confirmation, rather than waiting for TSH to rise, because maintaining euthyroidism from the onset of pregnancy is critical for fetal neuropsychological development 3, 2
Physiologic Basis
Maternal thyroid hormone is essential for fetal brain development, particularly in the first trimester before the fetal thyroid is functional 2
The increased requirement is driven by:
- Increased thyroid-binding globulin from estrogen
- Increased maternal blood volume
- Placental transfer of thyroid hormone to the fetus
- Placental metabolism of thyroid hormone 2
Monitoring Protocol
Monitor TSH every 4 weeks after dose adjustment until a stable dose is reached and serum TSH is within normal trimester-specific range 1
The goal is to maintain TSH in the trimester-specific reference range, with many experts targeting TSH <2.5 mIU/L in the first trimester 4, 3
Free T4 should be maintained in the high-normal or upper half of the normal range 4, 1
Critical Pitfall to Avoid
Do not wait for TSH to become elevated before increasing the dose - this reactive approach allows a period of maternal hypothyroidism that can adversely affect fetal neurodevelopment 2
Women with pre-conception TSH levels between 1.2-2.4 mIU/L have a 50% chance of requiring dose increases during pregnancy, while those with TSH <1.2 mIU/L only have a 17.2% chance, emphasizing that "normal" TSH at baseline does not preclude the need for dose adjustment 5