This is because of unmet biological need for T3.
Looking into history,
In 1965, 85% of thyroid prescriptions were for natural thyroid products.
Then came Johnny come lately,Levothyroxine and it was felt there was peripheral conversion of T4 to T3.
T4 replacement suppressed TSH and normalized T4 and T3 levels but at the expense of elevated T4 to T3 ratio.
Other markers for bonfire thyroxine replacement did not suggest correction of lipids,or signs of hypothyroidism.
With elevated T4 to T3 ratio impairs systemic T3 ptoduction by down regulating deiodinase pathway.
With only T4 replacement only,T3 levels do not come to hypothyroid state.
A specific gene in some hypothyroid patientsDIO2, a type 2 iodothronine deiodinase gene improves hypothyroid symptoms with replacement of T 4 And T3.
Dr Gregory W Rutecki M.D at Cleveland clinic wrote this paper.