Management of Armour Thyroid (Desiccated Thyroid) in Pregnancy
Pregnant women on Armour Thyroid should ideally be switched to levothyroxine (T4) monotherapy before or early in pregnancy, as levothyroxine provides more predictable dosing and better maternal-fetal outcomes; if continuation of desiccated thyroid is necessary, the dose typically does not require adjustment during pregnancy, unlike levothyroxine which requires a 30-47% increase.
Preferred Approach: Switch to Levothyroxine
The standard of care for hypothyroidism in pregnancy is levothyroxine monotherapy, not desiccated thyroid preparations like Armour Thyroid 1, 2.
Levothyroxine requirements increase by approximately 30-47% during pregnancy, with increases beginning as early as the 5th week of gestation and plateauing by week 16 2.
Women on levothyroxine should increase their dose by approximately 30% (or take 9 doses per week instead of 7) as soon as pregnancy is confirmed, followed by monthly TSH monitoring 1, 2.
If Continuing Armour Thyroid
Key Dosing Difference
Desiccated thyroid (Armour Thyroid) does not require dose increases during pregnancy, in contrast to levothyroxine 3.
In a study of women after total thyroidectomy, those treated with desiccated thyroid maintained stable serum TSH throughout pregnancy at equivalent pre-pregnancy doses, while levothyroxine-treated patients required dose increases 3.
The serum free T4 decreased to low-normal values during gestation with desiccated thyroid but remained stable enough that TSH did not rise 3.
Monitoring Protocol
Measure TSH at minimum every 4-6 weeks throughout pregnancy to ensure adequate thyroid hormone replacement 4.
Target TSH should be ≤2.5 mIU/L in the first trimester and ≤3.0 mIU/L in the second and third trimesters 4.
Check both TSH and free T4 levels, as desiccated thyroid contains both T4 and T3 in fixed ratios that may not reflect physiologic needs 5, 3.
Potential Concerns with Desiccated Thyroid
The T3 content in desiccated thyroid may cause supraphysiologic T3 peaks that are not ideal during pregnancy, though the FDA label notes minimal placental transfer of thyroid hormones 5.
The T3-to-T4 ratio may not optimally support fetal neurodevelopment, as the fetus relies primarily on maternal T4 for its own thyroid hormone production 3.
Desiccated thyroid preparations have variable potency between batches, making consistent dosing more challenging than synthetic levothyroxine 1.
Pre-Conception Optimization
TSH should be optimized to ≤2.5 mIU/L before conception in any woman with thyroid dysfunction or thyroid antibodies 4.
Consider switching from Armour Thyroid to levothyroxine before pregnancy to allow for easier dose titration during gestation 1, 2.
Postpartum Management
After delivery, thyroid hormone requirements typically decrease, necessitating dose reduction back to pre-pregnancy levels 6.
Re-measure TSH every 4-6 weeks postpartum until stable within the normal reference range (0.4-4.0 mIU/L) 6.
Minimal amounts of thyroid hormones are excreted in breast milk, so continuation during lactation is safe 5.
Critical Pitfalls to Avoid
Do not use non-pregnant TSH reference ranges during pregnancy—pregnancy-specific ranges are lower (first trimester <2.5 mIU/L, second/third trimester <3.0 mIU/L) 4.
Untreated or inadequately treated hypothyroidism causes serious fetal harm, including impaired neurological development, preterm birth, and low birth weight 4.
Do not assume that stable pre-pregnancy thyroid function eliminates the need for increased monitoring during pregnancy 4.
The FDA label for Armour Thyroid emphasizes individualized dosing based on laboratory findings and patient response, but does not provide pregnancy-specific guidance 5.