I've just found out how in the USA at least and probably elsewhere, the T4/T3 content of NDT is controlled. Obviously raw thyroids will change in content batch to batch and I first thought that batches would be mixed to get the right hormone content. It seems however that to control T4 and T3 content in NDT, synthetic T4 or T3 is added to produce the right product. So NDT is NOT purely natural but is a mix of natural hormone + synthetic hormone.
How NDT content of T4 and T3 is controlled (USA) - Thyroid UK
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