Figure 1

HP function (f1), curvature function (f2) and derivative of f2 (f3). For illustrative purposes, [FT4]sp (set point) and [TSH]sp (set point) which respectively represent the values of [FT4] and [TSH] where the euthyroid set point occurs are 15 pmol/L and 1.6 mIU/L respectively. These set point values correspond to $\phi =\frac{1}{\left[\mathit{TSH}\right]\sqrt{2}}=0.442$ and S = [TSH]exp(φ[FT 4]) = 1211.