What is the estimated conversion of 90mg of Armor Thyroid (desiccated thyroid extract) to levothyroxine (T4) for a pregnant female with Hashimoto's thyroiditis and elevated Thyroid-Stimulating Hormone (TSH) levels?

Armour Thyroid to Levothyroxine Conversion in Pregnancy

For a pregnant woman with Hashimoto's thyroiditis taking 90mg of Armour Thyroid, the estimated levothyroxine equivalent is approximately 112-135 mcg daily, though this conversion is imprecise and requires immediate dose adjustment with close TSH monitoring.

The Mathematical Conversion

The traditional conversion ratio estimates that 1 grain (60mg) of desiccated thyroid extract contains approximately 38 mcg of T4 and 9 mcg of T3 1. However, this conversion is problematic for several reasons:

• 90mg of Armour Thyroid = 1.5 grains 1
• This theoretically provides 57 mcg T4 + 13.5 mcg T3 1
• Each 1 mcg of T3 is approximately 3-4 times more potent than T4 1
• Converting the T3 component: 13.5 mcg T3 × 4 = 54 mcg T4 equivalents 1
• Total levothyroxine equivalent: 57 + 54 = approximately 111 mcg 1

However, some sources use a simpler approximation where 1 grain (60mg) = 75-100 mcg levothyroxine, which would make 90mg equivalent to 112-150 mcg levothyroxine 1.

Critical Problems with This Conversion in Pregnancy

The conversion math becomes largely irrelevant in pregnancy because T3 does not adequately cross the fetal blood-brain barrier, making desiccated thyroid preparations inappropriate for pregnant women 2. The fetus depends almost entirely on maternal T4, which is converted to T3 in fetal tissues 2.

Why Immediate Switching is Essential

• Levothyroxine monotherapy is the only recommended treatment during pregnancy 2
• T3 supplementation provides inadequate fetal thyroid hormone delivery 2
• Untreated or inadequately treated maternal hypothyroidism increases risk of preeclampsia, low birth weight, and permanent neurodevelopmental deficits in the child 3, 1
• Inadequate treatment is associated with low birth weight and potential cognitive impairment in offspring 3

Practical Dosing Strategy for This Patient

Start with 125 mcg levothyroxine daily (the closest standard dose to the calculated 112-135 mcg equivalent) and check TSH within 2 weeks 1.

Target TSH Levels in Pregnancy

• First trimester: TSH should be <2.5 mIU/L 1, 4, 5
• Second and third trimesters: TSH should be <3.0 mIU/L 1, 4
• Optimal pre-conception TSH is <1.2 mIU/L to minimize the need for dose increases during pregnancy 5

Expected Dose Adjustments During Pregnancy

• Approximately 81.8% of pregnant women with Hashimoto's thyroiditis require levothyroxine dose increases during pregnancy 4
• 31.8% require increases of ≥50 mcg 4
• Levothyroxine requirements typically increase by 25-50% above pre-pregnancy doses 1
• Over 50% of women with Hashimoto's thyroiditis experience increased levothyroxine requirements postpartum compared to pre-gestational doses 6

Monitoring Protocol

Check TSH and free T4 every 4 weeks during the first half of pregnancy, then at least once between 26-32 weeks gestation 1.

Dose Adjustment Strategy

• Increase levothyroxine by 25-50 mcg if TSH exceeds trimester-specific targets 1
• Women with pattern 2 dosing (requiring multistep increases throughout pregnancy) had mean TSH levels of 2.8 mIU/L versus 1.3 mIU/L in those requiring minimal adjustment 6
• Women requiring multistep increases were 62 times more likely to need doses at least 20% above baseline at 3 months postpartum 6

Critical Pitfalls to Avoid

• Never continue desiccated thyroid preparations during pregnancy—the T3 component does not adequately support fetal neurodevelopment 2
• Do not rely solely on the mathematical conversion—individual thyroid reserve varies significantly in Hashimoto's thyroiditis 4, 6
• Do not wait for symptoms to develop before checking TSH—fetal harm can occur before maternal symptoms appear 3, 1
• Never start thyroid hormone before ruling out concurrent adrenal insufficiency, as this can precipitate adrenal crisis 1
• Avoid using TSH targets >2.5 mIU/L in the first trimester, as even subclinical hypothyroidism is associated with adverse pregnancy outcomes 3, 1, 5

Special Considerations for Hashimoto's Thyroiditis

Women with Hashimoto's thyroiditis have progressive autoimmune destruction of thyroid tissue, making their residual thyroid function highly variable 6.

• The presence of anti-TPO antibodies is associated with a 2-4 fold increase in risk of recurrent miscarriages and preterm birth 2
• Inferior thyroid artery peak systolic velocity (ITA-PSV) reflects residual thyroid function and predicts the magnitude of dose increases needed 4
• Women with lower thyroid blood flow require larger levothyroxine dose increases during pregnancy 4