Why wouldn't the body down-regulate the conversion of FT4 into FT3? Is that conversion just constitutively activated? Since FT3 is more potent than T4, it would make sense for the body to turn that conversion down...that was my reasoning...obviously not correct, but idk why that wouldn't be the case. Anyone have insight?
Taking excess Levothyroxine will cause, Incr fT4, Incr fT3, decr TSH, decr Thyroidal iodine uptake. Excess liothyronine intake will cause incr T3, Decr T4, decr rT3, Decr TSH, because T4 gets converted to T3 and rT3 in periphery.
A bit confused here because I thought T4 is converted to T3 INSIDE peripheral tissue. Why would serum T3 levels rise then?
According to this article, patients with hypothyroidism taking too much levothyroxine ended up having NORMAL serum T3 levels: https://pubmed.ncbi.nlm.nih.gov/26738396/#:~:text=In%20levothyroxine%20(T4)%2Dinduced,in%20over%2Dtreated%20hypothyroid%20patients.
submitted by โm-ice(370)
They explicitly state that the patient has been taking excess of his levothyroxine medication. Levothyroxine is the exogenous form of T4. Therefore, free T4 must be elevated. T4 is converted to T3 at most peripheral tissues, so T3 will also be elevated. Because the body has more thyroid hormone than needed, less TSH will be made, and the thyroid will be less active, taking up LESS iodine.