Adding this (I’ve put on here in a few places) here as this is the most succint description of the confounding of SHBG vs injection frequency requirement I’ve found (attribution to cataceous on ExcelMale):
The issue with this study is that they are basing MCR on total T rather than free T. The standard equation used is:
Production_rate = MCR * Hormone_concentration
But as I argued above, the proportionality applies to free testosterone, not total. So the equation should be:
Production_rate = MCR * Hormone_concentration = MCR * FT = MCR * f(SHBG, T)
The reason it might appear to work anyway is because at constant SHBG, free T is nearly proportional to total T. So you get:
Production_rate = MCR * f(SHBG, T) ~= MCR * f1(SHBG) * T = MCRx * T
The problem is that their measured clearance rate, MCRx, is actually dependent on both the underlying metabolism (MCR) and SHBG. Unfortunately they don’t separate out the two, which potentially weakens their conclusions. The results are further muddied by the drop in SHBG—mainly in younger men—over the course of the experiment. It’s frustrating, because they did measure free testosterone, apparently by an accurate method, along with baseline and final SHBG values—so they did have the raw data needed to separate out the various effects.
Sorry I can’t link the post on here. SHBG is sometimes strongly correlated to other variables that do control testosterone elimination rate but does not control it itself (causation).