Labs can be expensive, so I will list the most essential ones for diagnosis and treatment. I will also point out that there are different types of assays for each test which can effect their accuracy.

For initial diagnosis, in order to rule out secondary hypogonadism, a FSH, LH, and prolactin level should be ordered. If the prolactin is elevated, a pituitary MRI is necessary. After treatment with TRT is initiated, these tests do not need to ever be ordered again as they are not relevant to testosterone therapy.

There are a few different testosterone tests available: total testosterone (TT), bioavailable testosterone (BAT), and free testosterone (FT). Of all of these, the most important one is FT.

Free testosterone can be measured by equilibrium dialysis or ultrafiltration, which are difficult to perform and largely unavailable but reliable. In contrast, the radioimmunoassay for free testosterone is widely available but unreliable. So what this means is that if your FT is from a lab that uses equilibrium dialysis or ultrafiltration, then that is the only test you need to diagnose and manage hypogonadism.

However, if your lab uses the less reliable radioimmunoassay for FT (which most do), then you will benefit from getting TT and sex-hormone binding globulin (SHBG). Because total testosterone and SHBG assays are readily available and cheap, calculating bioavailable testosterone (free testosterone and testosterone weakly bound to albumin) might be a better choice based on what your options are.

Hemoglobin and hematocrit (usually included on a complete blood count) and a lipid panel should be done as a baseline and at follow-up.