I've dug up a few references to using Free T to diagnose hypogonadism, rather than TT.
The lower bound for Free T is currently a bit of a grey area. I've seen numbers between 6 and 8 ng/dL (about 210 to 280 pmol/L), although there seems to be more consensus at the bottom end of the range.
Free testosterone may be a better tool to diagnose hypogonadism
That's the conclusion of research by Dr Leen Antonio based on a European study into aging males.
She found that men with low Free T (in this case below 6.3 ng/dL / 220 pmol/L), but normal T, exhibited more symptoms than those with low TT (under 300 ng/dL / 10.5 nmol/L) and higher Free T.
The conclusion is that clinicians should be using Free T to make a diagnosis. This might explain why some people with higher TT levels can be symptomatic, whereas those with low TT might feel fine.
There's an overview of the research at Medscape, and Endocrine Abstracts has the abstract.
The following are guidelines for diagnosing hypogonadism from Free T. Note that calculated values don't always agree with direct measurements.
- The ISA, ISSAM, EAU, EAA and ASA recommend 65 pg/mL (226 pmol/L) is used as the lower bound for calculated free testosterone. (Source)
- The ISMH suggest that a free testosterone level of 5.2 ng/dL / 180 pmol/L is the cut-off for hypogonadism, and replacement therapy isn’t needed if it’s above 7.2 ng/dL / 250 pmol/L. (Source)
- The Endocrine Society recommendations don’t give an explicit value, but recommend using the lab's ranges. For a direct measurement this is typically 4.9 to 8.9 ng/dL / 170 to 310 pmol/L. (Source)
- The Nebido Testosterone Management Tool recommends a calculated Free T value of 8 ng/dL / 270 pmol/L as the cut-off. This is from an article in The American Journal of Medicine. (Source)
There seem to be a few postings from people with TT some way over 300 ng/dL who are symptomatic, and so the above might give hints of where to look.