Conversion Chart: Natural Desiccated Thyroid, Levothyroxine, and Liothyronine/T3
The following is a general conversion chart, showing the dosage equivalents between different natural desiccated thyroid drugs (NDT), levothyroxine (synthetic T4), and liothyronine (synthetic T3).
Here's something to understand: The US Pharmacopeia (USP) sets quality, purity, strength, and identity standards for medicines, food ingredients, and dietary supplements. USP has developed "Reference Standards" for all drugs. According to the USP Reference Standards for Natural Desiccated Thyroid, 65 mg (1 grain) of natural desiccated thyroid needs to contain no less than 90 percent and not more than 110 percent of the labeled amounts of 38 mcg of T4 and 9 mcg of T3.
Nature-Throid and WP Thyroid use the USP Reference Standard of 65 mg for a 1 grain tablet weight. (Armour and generic NP Thyroid list their pills as 60 mg for 1 grain.)
This also means that if you have a prescription written for 60 or 90 or 120 mg of NDT, you can fill it with Armour or NP Thyroid, but not Nature-Throid or WP Thyroid. If your prescription is written for 1, 1 1/2 or 2 grains of NDT, you should be able to fill it with any NDT drug.
Note: This conversion chart is only a general guideline. If you switch from levothyroxine to NDT, your optimal dose of NDT is likely to differ from this general conversion chart. The right dose of NDT for you depends on many factors, including absorption, T4-to-T3 conversion, and other factors.