Animal-derived thyroid products, including Armour Thyroid, NP Thyroid, WP Thyroid, and Nature-throid, are not FDA-approved to treat hypothyroidism; however, they existed before the Food, Drug, and Cosmetic Act of 1938 and, therefore, were grandfathered into the FDA's list of prescription drug medications without going through the formal approval process. The products are produced from dried, ground thyroid glands, typically from pigs. The FDA contends that due to a lack of safety and efficacy data, these products may be unsafe because of variations in purity and potency and the presence of contaminants. The agency sent letters to manufacturers on August 6, 2025, notifying them of their intent to take action against unapproved animal-derived thyroid medications. The FDA is also encouraging healthcare providers to transition patients from animal-derived products to synthetic levothyroxine.

An estimated 1.5 million Americans receive prescriptions for animal-derived combination products each year. However, all major professional organizations recommend synthetic levothyroxine products (e.g., Synthroid) for treating hypothyroidism. Reasons they are preferred over combination products include:

• Combination products (e.g. Armour Thyroid) have a T4 to T3 ratio of 4:1, while physiologic ratios secreted by the thyroid gland are around 14:1.
• Combination therapy leads to supraphysiologic levels of T3, which may lead to symptoms of thyrotoxicosis
• T3 has a shorter half-life than T4, which can lead to fluctuations in T3 levels, with a peak occurring shortly after dosing
• There is substantially more data from trials on the use of levothyroxine compared to combination therapy

In my experience, patients who are on animal-derived combination products have already tried synthetic levothyroxine and feel that the combination products control their symptoms better. However, blinded studies comparing levothyroxine to combination products have not found a difference in symptom control. Despite this, it can be difficult to change patient perceptions, especially with regard to subjective symptoms. I anticipate significant pushback when trying to switch these patients to levothyroxine products.

• FDA notification on animal-derived thyroid products
• Hypothyroidism review
• Evaluating the effectiveness of combined T4 and T3 therapy or desiccated thyroid versus T4 monotherapy in hypothyroidism: a systematic review and meta-analysis, BMC Endocr Disord (2024)