The US Pharmacopeia (USP) -- the organization that sets standards for medicines --has Reference Standards for Natural Desiccated Thyroid that specify that:

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• 1 grain of natural desiccated thyroid is 65 mg

• 1 grain/65 mg needs to contain no less than 90% and not more than 110% of the standardized T4 and T3 content and potency.

• 1 grain/65 mg is standardized at 38 mcg of T4 and 9 mcg of T3. 

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Nature-Throid and WP Thyroid use the USP Reference Standard of 65 mg for a 1 grain tablet. The manufacturer, RLC, has voluntarily set their own tighter manufacturing standard of 95% to 105% potency. 

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Armour and NP Thyroid list their pills as 60 mg for 1 grain.

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Because Nature-Throid/WP Thyroid use the 65 mg/1 grain standard -- and Armour and NP Thyroid use the 60 mg/1 grain standard -- you can run into some difficulties when filling your prescription depending on how it is written. Technically, because of the milligram differences, Nature-Throid and WP Thyroid are not interchangeable with Armour Thyroid and generic NP Thyroid.

If your prescription is written for grains of NDT, you should be able to fill it with any brand name or generic natural desiccated thyroid drug.

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If your prescription is written for Nature-Throid or WP Thyroid in mg, technically, it can only be filled with one of these two medications. Armour Thyroid (or generic NP Thyroid) are not considered equivalent to Nature-Throid and WP Thyroid.

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If your prescription is written for Armour Thyroid (or generic NP Thyroid), they are considered equivalent to each other. 

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The following chart summarizes the grains and milligrams for the different NDT drugs.