Is switching a pregnant woman with hypothyroidism from 90mcg of Armour Thyroid (desiccated thyroid) to 100mcg of levothyroxine (T4) appropriate given her elevated Thyroid-Stimulating Hormone (TSH) level of 5.56, low free T4 of 0.8, and free T3 of 2.5?

Switching from Armour Thyroid to Levothyroxine in Pregnancy is Appropriate and Necessary

Switching from 90mg Armour Thyroid (desiccated thyroid) to 100mcg levothyroxine is the correct decision for this pregnant patient with inadequately controlled hypothyroidism (TSH 5.56), and the dose should be increased immediately to achieve TSH <2.5 mIU/L. 1, 2

Why Levothyroxine Monotherapy is Mandatory in Pregnancy

• Levothyroxine (T4) is the only appropriate thyroid hormone replacement during pregnancy because T3 (contained in Armour Thyroid) provides inadequate fetal thyroid hormone delivery, as the fetal brain depends on maternal T4 that crosses the placenta and is converted to T3 locally 3, 2
• Desiccated thyroid products like Armour Thyroid contain both T4 and T3 in a fixed ratio that does not match physiologic needs during pregnancy, and the T3 component does not adequately support fetal neurodevelopment 3, 2
• Untreated or inadequately treated maternal hypothyroidism increases risk of preeclampsia, low birth weight, miscarriage, and permanent neurodevelopmental deficits in the child 1, 2, 4

Current Thyroid Status Indicates Inadequate Treatment

• TSH of 5.56 mIU/L is significantly elevated and requires immediate intervention, as the target TSH during pregnancy should be <2.5 mIU/L in the first trimester and within trimester-specific reference ranges thereafter 1, 2, 4
• Free T4 of 0.8 (assuming ng/dL units, which is low-normal to low) combined with elevated TSH confirms inadequate thyroid hormone replacement 3, 5
• Free T3 of 2.5 (assuming pg/mL units) is less relevant for monitoring, as TSH and free T4 are the primary markers for adequacy of levothyroxine therapy in pregnancy 3, 5

The 100mcg Levothyroxine Dose is Likely Insufficient

The initial switch to 100mcg levothyroxine is a reasonable starting point but will likely require further dose increases:

• For new-onset hypothyroidism with TSH ≥10 mIU/L, the recommended starting dose is 1.6 mcg/kg/day, but for TSH 5-10 mIU/L (as in this case), 1.0 mcg/kg/day is appropriate 1
• However, patients switching from desiccated thyroid to levothyroxine during pregnancy typically require 25-50% higher doses than their pre-pregnancy levothyroxine equivalent because pregnancy increases thyroid hormone requirements 1, 6
• The conversion from Armour Thyroid to levothyroxine is approximately 60mg Armour = 100mcg levothyroxine, so 90mg Armour ≈ 150mcg levothyroxine equivalent 3
• Given the elevated TSH, the dose should be increased by 12.5-25 mcg immediately (to 112.5-125 mcg daily) rather than waiting 4-6 weeks 7, 1

Monitoring Protocol During Pregnancy

• Check TSH and free T4 every 4 weeks until TSH is stable within trimester-specific reference range (ideally <2.5 mIU/L in first trimester) 1, 2, 5
• Adjust levothyroxine dose by 12.5-25 mcg increments based on TSH results, with more aggressive titration acceptable given the pregnancy context 7, 1
• Once TSH is stable, continue monitoring at minimum once per trimester throughout pregnancy 1
• Immediately after delivery, reduce levothyroxine dose back to pre-pregnancy levels and recheck TSH 4-8 weeks postpartum 1

Critical Pitfalls to Avoid

• Do not wait 6-8 weeks for the next TSH check in pregnancy—use 4-week intervals until TSH is optimized, as fetal harm can occur before maternal symptoms appear 3, 1, 2
• Never discontinue levothyroxine during pregnancy, even if TSH normalizes, as requirements typically increase throughout gestation 1, 2
• Avoid targeting TSH >2.5 mIU/L in the first trimester, as even subclinical hypothyroidism is associated with adverse pregnancy outcomes 3, 2, 4
• Do not rely on free T3 levels for monitoring adequacy of levothyroxine therapy during pregnancy—TSH and free T4 are the appropriate markers 3, 5, 8

Specific Dosing Recommendation for This Patient

Increase levothyroxine to 125 mcg daily immediately (25 mcg increase from current 100 mcg dose) given:

• TSH 5.56 mIU/L is significantly above the pregnancy target of <2.5 mIU/L 1, 2
• The patient was previously on 90mg Armour Thyroid (≈150 mcg levothyroxine equivalent), suggesting higher baseline requirements 6
• Pregnancy increases levothyroxine requirements by 25-50% in most women with pre-existing hypothyroidism 1, 6
• Recheck TSH and free T4 in 4 weeks and adjust by additional 12.5-25 mcg increments as needed until TSH <2.5 mIU/L 7, 1