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The API – The Active Pharmaceutical Ingredient

A drug’s Active Pharmaceutical Ingredient – the API -- is the active ingredient that performs the desired action. The API in NDT drugs is Thyroid USP, a powder made from dried and processed porcine (pig) thyroid glands.  

The quality and purity of the powder in Thyroid USP depend on the API’s manufacturer, the regulations in the country where the API is manufactured, and the geographic source of the pigs used to produce it.

The drug manufacturer mixes the API with excipients. Excipients are the fillers, binders, coatings, and dyes that help a drug stay together as a pill or help contain and deliver the medication to your system.


Unlike the API, excipients are not chemically active.

The mixture of the Thyroid USP and excipients is then formed into tablets. These tablets include standardized amounts of the two key thyroid hormones, T4 (thyroxine) and T3 (triiodothyronine). Some formulations of tablets also have an added outer coating.

Manufacturers make their NDT using APIs from different manufacturers. Specifically:

Nature-Throid® and WP Thyroid® – The API for both drugs is sourced from Specialty Process Labs (SPL) – – a US-based API manufacturer. All the animals are sourced from North America. 

Armour® Thyroid – The API manufacturer is undisclosed. The geographic source of the animals is undisclosed.

NP Thyroid® The API manufacturer is undisclosed. The geographic origin of the animals is undisclosed.

The excipients used in each brand of NDT are also unique to each brand’s specifications and formulation.

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