What is serum testosterone?

Serum testosterone refers to the concentration of testosterone—the primary biologically active androgen—in the bloodstream, measured in a blood sample. Testosterone is predominantly synthesized in the Leydig cells of the testes in males and, to a lesser extent, in the ovaries and adrenal glands in females. It plays essential roles in the development and maintenance of male secondary sexual characteristics, muscle mass, bone mineral density, red blood cell production, libido, and mood regulation. Serum testosterone exists in three main forms: bound to sex hormone–binding globulin (SHBG), bound to albumin, and unbound (free testosterone). Only the free and albumin-bound fractions are considered biologically available, as SHBG-bound testosterone is largely inactive. Clinically, total serum testosterone is routinely measured via immunoassay or liquid chromatography–tandem mass spectrometry (LC-MS/MS)—the latter being the gold standard for accuracy, particularly at low concentrations. Interpretation requires consideration of diurnal variation (peak levels typically occur in the early morning), age-related decline, and clinical context—including symptoms of hypo- or hyperandrogenism.