Is NP thyroid (thyroid hormone replacement medication) safe to use during pregnancy?

Is NP Thyroid Safe During Pregnancy?

NP Thyroid (desiccated thyroid extract) is not recommended during pregnancy; levothyroxine is the preferred and guideline-supported thyroid hormone replacement for pregnant women with hypothyroidism. 1, 2

Why Levothyroxine is the Standard of Care

The ACOG Practice Bulletin explicitly states that hypothyroidism in pregnant women should be treated with levothyroxine in sufficient dosage to return TSH to normal range. 1 The FDA drug label for levothyroxine confirms that clinical experience in pregnant women treated with oral levothyroxine has not reported increased rates of major birth defects, miscarriages, or adverse maternal or fetal outcomes. 2

Critically, the FDA emphasizes that levothyroxine should not be discontinued during pregnancy, and hypothyroidism diagnosed during pregnancy should be promptly treated. 2 This recommendation specifically applies to synthetic levothyroxine (T4), not to desiccated thyroid products like NP Thyroid.

Why Desiccated Thyroid Products Are Problematic

NP Thyroid contains both T4 and T3 in fixed ratios derived from animal thyroid glands. The key issues during pregnancy include:

• Lack of pregnancy-specific safety data: Guidelines and FDA labeling specifically reference levothyroxine monotherapy, not combination T4/T3 or desiccated thyroid products. 1, 2

• Unpredictable T3 levels: The fixed T4:T3 ratio in desiccated thyroid does not match physiologic needs during pregnancy, when T4 requirements increase substantially (often by 30-50%). 3

• Difficulty with dose titration: Pregnancy requires frequent dose adjustments every 4 weeks until TSH stabilizes, then monitoring every trimester. 1 The fixed combination in NP Thyroid makes precise T4 adjustment impossible without also changing T3 levels inappropriately.

Risks of Inadequate Thyroid Hormone Replacement

Untreated or inadequately treated maternal hypothyroidism carries serious consequences:

• Increased risk of spontaneous abortion, gestational hypertension, and preeclampsia 2
• Stillbirth and premature delivery 2
• Low birth weight in neonates 1, 4
• Adverse effects on fetal neurocognitive development 2, 5
• In severe iodine deficiency, risk of congenital cretinism with mental retardation and neuropsychological defects 1, 4

Treatment Algorithm for Hypothyroidism in Pregnancy

For women currently taking NP Thyroid who become pregnant:

1. Switch immediately to levothyroxine monotherapy at an equivalent or slightly higher dose (pregnancy increases T4 requirements). 1, 2

2. Target TSH between 0.5-2.5 mIU/L during pregnancy, which is lower than the non-pregnant reference range. 5

3. Monitor TSH every 4 weeks during dose titration until stable, then every trimester. 1

4. Maintain free T4 in the high-normal range using the lowest effective levothyroxine dose. 1, 4

5. Expect dose increases: Most women with hypothyroidism require 30-50% higher levothyroxine doses during pregnancy. 3

6. Return to pre-pregnancy dose immediately after delivery, as postpartum TSH levels return to preconception values. 2

Preconception Planning

Women with hypothyroidism planning pregnancy should ideally achieve optimal thyroid function before conception, with a target preconception TSH below 1.2 mIU/L. 6 This is the time to transition from NP Thyroid to levothyroxine if not already done.

Critical Pitfall to Avoid

Never discontinue thyroid hormone replacement during pregnancy, even if switching formulations. 2 The transition from NP Thyroid to levothyroxine should be seamless, with immediate initiation of levothyroxine when stopping the desiccated thyroid product. Maternal hypothyroidism, even if subclinical, poses greater risks to fetal development than the theoretical concerns of medication exposure. 6, 4