Armour Thyroid and NP Thyroid: Identical Active Ingredients with Different Inactive Excipients
Both Armour Thyroid and NP Thyroid are natural desiccated thyroid preparations derived from porcine thyroid glands that contain identical amounts of active thyroid hormones—38 mcg levothyroxine (T4) and 9 mcg liothyronine (T3) per grain—but differ only in their inactive ingredients. 1, 2
Active Hormone Content (Identical)
Both products provide the same thyroid hormone composition per grain:
- Levothyroxine (T4): 38 mcg per grain 1, 2
- Liothyronine (T3): 9 mcg per grain 1, 2
- T4:T3 ratio: Approximately 4:1 by weight 1, 2
This fixed ratio is non-physiologic and can produce supraphysiologic T3 concentrations, particularly in adolescents and other populations. 3
Inactive Ingredients (The Only Difference)
Armour Thyroid contains:
- Calcium stearate
- Dextrose
- Microcrystalline cellulose
- Sodium starch glycolate
- Opadry white 1
NP Thyroid contains:
- Calcium stearate
- Dextrose (agglomerated)
- Mineral oil
- No gluten-containing ingredients (explicitly certified gluten-free) 2
The key practical difference is that NP Thyroid is certified gluten-free, making it the preferred choice for patients with celiac disease or gluten sensitivity. 2
Clinical Equivalence and Switching
- Because the active hormone content is identical, the products are therapeutically equivalent on a grain-for-grain basis. 1, 2
- However, differences in inactive excipients can affect dissolution and absorption in individual patients, similar to switching between levothyroxine brands. 4
- When switching between Armour Thyroid and NP Thyroid, maintain the same grain dose but re-check TSH 4–8 weeks later to confirm therapeutic stability. 4
Guideline Position on Both Products
The American Association of Clinical Endocrinologists and the American Thyroid Association explicitly recommend against using either desiccated thyroid preparation (including both Armour Thyroid and NP Thyroid) for hypothyroidism treatment because no evidence demonstrates superiority over levothyroxine monotherapy. 5
Key guideline concerns applicable to both products:
- Lack of controlled trials: A 2012 PubMed search found zero prospective randomized controlled trials of desiccated thyroid for any indication across all years in the database, compared with 35 trials of levothyroxine published in just 2007–2011. 5
- Variable T3 levels: The T3 component causes substantial fluctuations in serum T3 throughout the day, making monitoring unreliable. 5
- Unpredictable potency: The T4:T3 ratio and total hormone content vary between manufacturers and animal sources (porcine vs. bovine), rendering dosing potentially unsafe. 6, 3
- Unanimous expert opinion: The recommendation against desiccated thyroid reflects consensus from all major endocrine societies. 5
Safety Considerations Common to Both
- Both products carry a theoretical risk of contamination with porcine and bovine viral or adventitious agents because they are derived from animals processed for human food consumption. 1
- Neither manufacturer has reported cases of disease transmission associated with use. 1
- Both are contraindicated for weight loss (alone or combined with other drugs) and for treatment of infertility unless accompanied by documented hypothyroidism. 1
Bottom Line
Armour Thyroid and NP Thyroid are functionally identical products with the same active hormone content per grain; the only clinically relevant difference is that NP Thyroid is certified gluten-free. 1, 2 However, current evidence-based guidelines strongly recommend levothyroxine monotherapy over either desiccated thyroid product for all patients with hypothyroidism. 5, 6