The mention of thyroid hormone in riboflavin conversion is something you will find in a good textbook on nutrition and biochemistry, as it is well-established knowledge. Conversion also depends on ATP. Riboflavin deficiency also retards T4 to T3 conversion, so it is a bit circular. ACTH will increase conversion in the adrenal cortex, liver and kidney, so secondary hypoadrenalism may be another source of problems. Aldosterone is also suspected of influencing conversion in the kidneys. I suspect many of the people who are sick enough to seek answers on this forum are unable to do conversion at a useful rate, and a side-by-side trial would show if that's true.