The Relationship Between TSH and Free T4 in a Large Population Is Complex and Nonlinear and Differs by Age and Sex

Abstract

The TSH-free T₄ relationship is not inverse log-linear but can be described by 2 overlapping negative sigmoid curves. At physiological free T₄ concentrations, TSH is higher in men and in older people, whereas the TSH response to hypothyroidism is more robust in younger people. These results advance understanding of the TSH-free T₄ relationship, which is central to thyroid pathophysiology and laboratory diagnosis of thyroid disease.