Inverse Log-Linear Relationship Between TSH Free T4

[Mito]

Small changes in free T4 result in a large change in TSH. For example a free T4 change from 1.0 ng/dL to 0.5 ng/dL will result in a TSH rise from 0.5 mIU/mL to 50 mIU/mL.

So large changes in TSH result in very small changes in free T4. For example, a rise in TSH from 1.0 mIU/mL to 5.0 mIU/mL reflects a drop in free T4 from 1.0 ng/dL to just 0.9 ng/dL.

Biochemical Testing of the Thyroid: TSH is the Best and, Oftentimes, Only Test Needed – A Review for Primary Care
http://clinchem.aaccjnls.org/content/clinchem/57/1/122.full.pdf

 

[Mito]

”The way that the feedback occurs is that the circulating T4 is inside the cells of the pituitary converted to T3. That is what suppresses the production of TSH.”

lYou really are not looking at whether the free T3 is suppressing the TSH. 90% of that suppression comes from circulating T4 that's converted to T3 inside the thyroid gland. “

“There are some indications that high free fatty acids might also decrease cellular uptake, but not much is known about what governs cellular uptake.”

 

[Mito]