I haven't read the studies fully, but what I get from briefly skimming the abstracts is that there are bacteria in your body which hydrogenate PUFA into SFA.
Hydrogenation of polyunsaturated fatty acids by human colonic bacteria - Howard - 2002 - Letters in Applied Microbiology - Wiley Online Library
Polyunsaturated fatty acid saturation by gut lactic acid bacteria affecting host lipid composition
Biohydrogenation of C20 polyunsaturated fatty acids by anaerobic bacteria
So could the composition of bodily bacteria be why many people eat PUFA their whole lives but have no estrogenic/hypothyroid symptoms? And the people who don't have the beneficial bacteria can't hydrogenate their dietary sources of PUFA, so the unsaturated fats accumulate in the body. If that is true, the question is how do you get these beneficial bacteria.
Hydrogenation of polyunsaturated fatty acids by human colonic bacteria - Howard - 2002 - Letters in Applied Microbiology - Wiley Online Library
Polyunsaturated fatty acid saturation by gut lactic acid bacteria affecting host lipid composition
Biohydrogenation of C20 polyunsaturated fatty acids by anaerobic bacteria
So could the composition of bodily bacteria be why many people eat PUFA their whole lives but have no estrogenic/hypothyroid symptoms? And the people who don't have the beneficial bacteria can't hydrogenate their dietary sources of PUFA, so the unsaturated fats accumulate in the body. If that is true, the question is how do you get these beneficial bacteria.