The relationship between TSH and free T4 in a large population is complex and nonlinear and differs by age and sex
2014
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.