The Endocrine Society guidelines define male hypogonadism as a clinical syndrome where the diagnosis is based on symptoms or signs and unequivocally low serum testosterone (T) levels. Generally accepted by most clinicians, diagnosis of male hypogonadism is clinically relevant because T replacement can restore libido and correct sexual dysfunction, increase lean and decrease fat mass, increase in bone mineral density and vitality. Low serum T is associated with increased all cause mortality. Low serum T levels are commonly associated with aging, obesity, metabolic syndrome, type 2 diabetes and other chronic illness. Because symptoms of T deficiency are non-specific and variable, the diagnosis of male hypogonadism requires serum T concentration measurements. Serum T shows diurnal variation, and samples for T measurements should be drawn in the morning because comparison is made with reference ranges derived from morning samples of adult men. Methods to determine serum T including immunoassays based assays and platforms and liquid chromatography tandem mass spectrometry. The latter is regarded as the gold standard for serum T measurement. For the diagnosis of male hypogonadism all these methods are generally adequate. There are wide between laboratory and method differences. The Endocrine Society is spear-heading a consensus among professional societies, references laboratories and manufacturers of platform instruments and kits to achieve highly accurate clinical T testing. Clinicians should be familiar with the adult male references ranges from the laboratory and how these are derived. Free or bioavailable T estimations are usually not necessary unless there is a suspicion that the patient may have altered SHBG binding. Either a direct measurement or a calculated free or bioavailable which may be estimated by total serum T and SHBG levels. Recent studies show that different thresholds of serum T predict symptoms of hypogonadism, e.g. sexual function requires lower serum T thresholds than vigorous activity.