I always wondered why I needed to be dosed with levothyroxine so my T4 was slightly above range for me even to be able to function. This article explains why:
thyroidpatients.ca/2020/05/...
In a nutshell, the thyroid doesn't secrete 20% T3 and 80% T4, that is an AVERAGE which is meaningless given the wide spread of the individual results making up the data.
In this study (Pilo, 1990), T3 secretion by the thyroid ranged from 6.5% T3 from the thyroid gland to 42%!!
Yet it is the average that is always quoted.
I must be at the 42% T3 produced by the thyroid end of this range. This is borne out by my being homozygous for one of the DI01 gene mutations and heterozygous for another.
There are lots of other articles written by Tania Sona Smith that I find insightful and fascinating.