Switching from Armour Thyroid to Levothyroxine in Unplanned Pregnancy: Initial Dosing Strategy
Start with 100 mcg levothyroxine immediately and monitor TSH in 4 weeks, as this dose provides adequate T4 replacement while avoiding the risks of desiccated thyroid during pregnancy.
The 100 mcg starting dose is appropriate and should not be increased initially for several critical reasons specific to pregnancy and the switch from desiccated thyroid 1, 2, 3.
Why 100 mcg is the Correct Starting Dose
The conversion from Armour Thyroid to levothyroxine requires careful consideration of T3 content. Desiccated thyroid contains both T4 and T3, with T3 causing supranormal serum spikes during absorption that can cause palpitations 4. When switching to levothyroxine monotherapy, you're eliminating the exogenous T3 component and relying entirely on physiologic T4-to-T3 conversion 4.
For pregnant patients with pre-existing hypothyroidism, levothyroxine monotherapy is the only appropriate treatment because T3 supplementation provides inadequate fetal thyroid hormone delivery 1. The fetus depends on maternal T4 crossing the placenta, particularly in the first trimester when fetal neurodevelopment is most vulnerable 1.
Pregnancy-Specific Dosing Considerations
Levothyroxine requirements increase by 25-50% during pregnancy in women with pre-existing hypothyroidism 1, 3. However, since this patient has never taken levothyroxine before, starting at 100 mcg allows you to:
- Establish a baseline response to pure T4 therapy without the confounding T3 component from Armour Thyroid 4
- Avoid iatrogenic hyperthyroidism, which increases risks of atrial fibrillation, osteoporosis, and cardiac complications 1
- Monitor the patient's individual conversion efficiency from T4 to T3 4
Target TSH should be <2.5 mIU/L in the first trimester to prevent adverse pregnancy outcomes including preeclampsia, low birth weight, and permanent neurodevelopmental deficits 1. Check TSH and free T4 every 4 weeks until stable, then at minimum once per trimester 1, 3.
Critical Monitoring Protocol
Recheck TSH and free T4 in 4 weeks (not the standard 6-8 weeks for non-pregnant patients) because pregnancy requires more aggressive monitoring 1, 3. If TSH remains above 2.5 mIU/L, increase by 12.5-25 mcg increments 1, 3.
The patient should take levothyroxine on an empty stomach, 30-60 minutes before breakfast for optimal absorption, which is particularly critical during pregnancy 1. Prenatal vitamins containing iron or calcium must be taken at least 4 hours apart from levothyroxine 1.
Why Not Start Higher
Starting above 100 mcg risks overtreatment because:
- Approximately 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, increasing serious complication risks 1
- TSH suppression below 0.1 mIU/L during pregnancy carries significant risks including atrial fibrillation and bone loss 1
- You don't yet know this patient's individual levothyroxine requirement since she's never taken it before 3
Common Pitfalls to Avoid
Never wait for symptoms to develop before checking TSH - fetal harm can occur before maternal symptoms appear 1. Avoid TSH targets >2.5 mIU/L in the first trimester, as even subclinical hypothyroidism is associated with adverse pregnancy outcomes 1.
Do not continue Armour Thyroid during pregnancy due to lack of standardization in T3 content and the risk of supranormal T3 levels during absorption 4, 5. The combined T4/T3 formulation is inappropriate for pregnancy because adequate fetal thyroid hormone delivery requires levothyroxine monotherapy 1.
Rule out concurrent adrenal insufficiency before increasing the dose, as starting or increasing thyroid hormone before corticosteroids can precipitate adrenal crisis 1. This is particularly important in patients with autoimmune thyroid disease who have increased risk of concurrent autoimmune adrenal insufficiency 1.