GD 1: Sum activity of peripheral type 1 deiodinase (D1).
As can be seen, TSH and fT4 are horizontal and fT3 levels increase with increased GD1 level. Reflecting that increase and decrease of GD1 only affects T3/fT3 values. The green area is the reference range for GD
In the euthyroid state, others and we have observed a considerable interindividual variability in FT4, TSH and their regulatory set points.15 39 40
Possible molecular mechanisms may include either different individual concentrations and affinities of TRs, variations in the distribution of iodothyronine transporters or a polymorphism in the type 2 deiodinase, which in the presence of a homozygous allele weakens the negative feedback of FT4 on TSH in humans.40 41