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What You Need to Know About Your Natural Thyroid's "API"

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.

Some API is reportedly manufactured in China, where pharmaceutical regulations and environmental protections are lax, and product quality and purity are questionable. In some cases, API manufacturers outside the U.S. are sourcing their animals from undisclosed third countries—information not shared with patients.

A 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 used to make 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.

Compounded Natural Desiccated Thyroid

A compounded medication is a drug that is mixed and prepared as a custom mix, specifically to meet your needs. Compounded natural desiccated thyroid is a prescription drug.

Like manufactured drugs, creating a compounded drug also involves mixing an API with excipients.

To make compounded NDT, a compounding pharmacy mixes Thyroid USP -- the API -- with fillers (excipients), and this mixture is then put into capsules.

As noted, the source of the API is an important variable, and compounding pharmacies get their Thyroid USP from different API manufacturers around the world. That means that you can find it challenging to learn which API your compounding pharmacy is using, where the API is being manufactured, or where the pigs are raised.

If you want full disclosure on the API used in compounded NDT, you do have an option. A network of compounding pharmacies in the U.S. uses the API sourced from Specialty Process Labs (SPL). (SPL’s Thyroid USP is the same API used to make Nature-Throid and WP Thyroid brand NDT.) All the ingredients used in the API from SPL are sourced solely within North America, and the API itself is manufactured in the United States. These pharmacies provide compounded prescription hypothyroid medication that never leaves a highly regulated and monitored setting.

An Alternative to Nature-Throid and WP Thyroid Tablets

Patients taking the Nature-Throid and WP Thyroid brands of NDT have faced difficulties getting their medication in 2020 and 2021, due to FDA interference and extended FDA recalls that have taken these drugs out of distribution.

Unfortunately, many patients who, out of necessity switched to Armour® or NP Thyroid®, have reported a poor response to the substitutions and difficulty managing their hypothyroidism.

If you're a patient who feels best on Nature-Throid® or WP Thyroid® and you're not functioning well on a substitute, the API used by these other brands may be causing the problem.

If you want to get an NDT with a formulation closest to Nature-Throid® or WP Thyroid®, consider getting compounded NDT that uses the same API.

Foothills Pharmacy, a pharmacy based out of Arizona, provides high-quality compounded NDT -- using SPL’s API -- and other thyroid medications by mail-order across the U.S. For more information, see You can also get a list of compounding pharmacies that use SPL’s API Thyroid USP here at Natural Thyroid Guide at


Find out everything you need to know about natural desiccated thyroid drugs and hypothyroidism treatment in the 2021 Natural Thyroid Guide, a free book.

1 Comment

Wojciech Majda
Wojciech Majda
May 26, 2022

Hello, interesting article. I have a reason to believe, that WP Thyroid and Nature-Throid were also sourcing the Thyroid Powder from China until 2018. It's because they used to be good products until FDA shut down importation from that factory in Sichuan. Since 2018 the quality went down, so something has changed. I think that they change to an America source of Thyroid powder after 2018.

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