I just stumbled upon this statement on Ray Peat's website. This statement really surprised me, as it seems to imply that the T4:T3 ratio in NDT is completely wrong and increases the risk of T4 to rT3 conversion:
The naturally found concentrations in the thyroid gland of animals of T4 and T3 are usually in a ratio of 4:1 or 5:1 and this ratio is most commonly found in NDT products like Armour. However, in hypothyroid people this high ratio of T4 to T3 can lead to rapid inactivation of T4 into reverse T3 (rT3) and thus worsen symptoms of hypothyroidism. According to Ray Peat, in hypothyroid states a much more desirable ratio would be 3:1 or even 2:1.
The human thyroid gland does not produce T4 and T3 in a 2:1 ratio, as far as I know, so how can this ratio be the best one for hypothyroid people?