| Total Testosterone | 10–35 nmol/L | Starting point — sum of all circulating testosterone (bound + unbound). First-line screening marker. | Fatigue, low libido, muscle loss, depression, brain fog | Acne, polycythaemia, testicular atrophy (exogenous T) |
| Free Testosterone | ~180–500 pmol/L | Biologically active fraction not bound to SHBG or albumin. Drives testosterone's physiological effects on target tissues. | Low T symptoms despite 'normal' total T — common with elevated SHBG | Less common; seen with very low SHBG or high total T |
| SHBG | 18–54 nmol/L | Sex Hormone-Binding Globulin — carries testosterone in blood, rendering it inactive. Essential companion to total T. | More free T available — seen in obesity, insulin resistance, anabolic steroid use | Reduces free T — common with ageing, hypothyroidism, liver conditions |
| LH (Luteinising Hormone) | 1.7–8.6 IU/L | Pituitary signal that stimulates testicular testosterone production. Critical for classifying hypogonadism type. | Secondary hypogonadism (pituitary/hypothalamic failure) — low LH + low T | Primary hypogonadism (testicular failure) — high LH + low T |
| FSH (Follicle Stimulating Hormone) | 1.5–12.4 IU/L | Pituitary signal for sperm production. Elevated FSH with low T suggests primary testicular failure. | Central suppression (opioids, anabolic steroids, pituitary lesion) | Testicular failure — poor prognostic sign for fertility |
| Oestradiol (E2) | 40–130 pmol/L | Primary oestrogen in men — converted from testosterone via aromatase. Essential for bone, mood, libido and cardiovascular health. | Bone loss, impaired libido, joint pain — can result from aromatase inhibitor overuse | Gynaecomastia, low libido, weight gain — common with obesity and high body fat |
| Prolactin | 86–324 mIU/L | Elevated prolactin suppresses LH/FSH and testosterone. Should always be checked when hypogonadism is suspected. | Rarely clinically significant | Suppresses testosterone, impairs libido — can indicate pituitary adenoma |
| TSH + Free T3/T4 | TSH: 0.4–4.0 mIU/L | Thyroid hormones regulate metabolism and interact with testosterone. Hypothyroidism closely mimics low T symptoms. | Hyperthyroidism signs — weight loss, palpitations, heat intolerance | Hypothyroidism — fatigue, weight gain, cold intolerance, low libido |
