BIOAVAILABLE TESTOSTERONE (ALBUMIN, SHBG, TESTOSTERONE)

The Bioavailable Testosterone test is a blood-test profile that measures albumin, Sex Hormone-Binding Globulin (SHBG) and total testosterone. These results are used to assess the proportion of circulating testosterone that is readily available to body tissues. Depending on the laboratory method, bioavailable testosterone may be measured directly or calculated using validated equations based on total testosterone, SHBG and albumin.

Most testosterone in the bloodstream is attached to proteins. A substantial proportion is bound tightly to SHBG, while much of the remainder is bound more weakly to albumin. Only a small fraction circulates completely unbound as free testosterone. Bioavailable testosterone generally refers to free testosterone plus the albumin-bound fraction because testosterone separates relatively easily from albumin and can become available to tissues.

Total testosterone alone may not accurately reflect biologically available hormone when SHBG or albumin concentrations are abnormal. Measuring all three components can provide a more complete assessment in patients whose symptoms and total testosterone result do not agree. This profile may support evaluation of suspected testosterone deficiency in males and androgen excess in females.

The test cannot independently diagnose hypogonadism, infertility, polycystic ovary syndrome or another endocrine disorder. Testosterone results must be interpreted with symptoms, age, sex, collection time, medicines, medical history and additional hormone tests.

Benefits of the Test

• Measures total testosterone, SHBG and albumin in one profile.
• Provides an estimate or measurement of biologically available testosterone.
• Offers additional information when total testosterone does not match the patient's symptoms.
• Helps account for variations in testosterone-binding proteins.
• Supports assessment of suspected testosterone deficiency in males.
• May support evaluation of androgen excess in females.
• Can assist the investigation of reproductive and sexual-function symptoms.
• May help monitor selected patients receiving hormone-related treatment.
• Provides information useful in conditions that alter SHBG or albumin.

Why Doctors Recommend This Test

A doctor may recommend this profile when a male patient has symptoms of testosterone deficiency but the total testosterone result is borderline, normal or inconsistent with the clinical findings. Possible symptoms include reduced sexual desire, erectile difficulty, reduced morning erections, infertility, loss of muscle mass, reduced body hair, low bone density, fatigue or delayed puberty.

SHBG binds testosterone strongly and affects how much testosterone remains free or loosely attached to albumin. A person may have a normal total testosterone concentration but reduced bioavailable testosterone when SHBG is increased. Conversely, low SHBG may result in a low total testosterone measurement even when the biologically available fraction is less severely reduced.

SHBG may be influenced by age, sex, thyroid function, liver disease, obesity, insulin resistance, pregnancy, nutritional status and medicines. Estrogen-containing treatment may increase SHBG, while androgen use and some metabolic conditions may reduce it. Measuring SHBG can therefore clarify an unexpected total testosterone result.

Albumin is produced by the liver and transports hormones, medicines and other substances in the blood. Because testosterone binds weakly to albumin, the albumin concentration is used in many calculations of bioavailable and free testosterone. Significant liver disease, kidney disease, inflammation, malnutrition or other conditions may alter albumin and affect the calculation.

In females, the profile may support evaluation of increased facial or body hair, persistent acne, scalp hair thinning, irregular periods or other signs of androgen excess. Possible causes include PCOS, medicine or supplement use and, less commonly, ovarian or adrenal disorders. The test cannot diagnose PCOS by itself.

The panel may also be used to monitor selected patients receiving testosterone treatment, anti-androgen treatment or other hormone therapy. The required sampling time depends on the medicine, formulation, dose and treatment schedule, so patients should follow the treating doctor's instructions.

Preparation Before Test

A venous blood sample is collected, usually from a vein in the arm. For adult males undergoing evaluation of possible testosterone deficiency, morning collection is generally preferred because testosterone commonly reaches its highest concentration earlier in the day. The doctor or laboratory may provide a specific collection window.

Fasting requirements can vary. Testosterone may be affected by food intake, sleep and acute illness, and some clinical protocols recommend a fasting morning sample. Follow the instructions supplied by the doctor or Focus Diagnostics, especially when glucose, lipid or other fasting investigations are ordered simultaneously.

Get normal sleep before testing when possible and avoid unusually strenuous exercise immediately before collection. Acute illness, inadequate sleep, major physical stress and severe calorie restriction may temporarily reduce testosterone and produce a result that does not represent the usual hormonal state.

Inform the doctor about all prescribed medicines, non-prescription medicines, vitamins, protein products and supplements. Testosterone, anabolic steroids, estrogen, hormonal contraception, glucocorticoids, opioids, anticonvulsants, thyroid medicines and other treatments may alter testosterone, SHBG or albumin.

Biotin supplements may interfere with certain laboratory immunoassays. Do not stop biotin or any prescribed treatment without advice from the doctor or laboratory. If testosterone treatment is being monitored, provide the product name, dose, route and exact date and time of the last dose.

Tell the healthcare professional about liver disease, kidney disease, thyroid dysfunction, diabetes, obesity, eating disorders, pituitary disease, testicular or ovarian conditions, pregnancy, menopause and recent serious illness. These conditions may influence the results or their interpretation.

Female patients should report pregnancy status, menstrual-cycle information and use of hormonal medicines when requested. The appropriate collection timing depends on the clinical reason for testing and the doctor's protocol.

Normal Reporting Time

The Bioavailable Testosterone profile is generally reported within 24 to 72 hours after the laboratory receives an acceptable serum sample. The exact turnaround time depends on whether testosterone is measured by immunoassay or mass spectrometry and whether bioavailable testosterone is directly measured or mathematically calculated.

Reporting may take longer when the sample is referred to a specialised laboratory, when a result requires dilution or repeat analysis, or when quality verification is necessary. Patients requiring the result before a consultation or treatment dose should confirm the expected reporting time with the collection centre.

A calculated bioavailable testosterone result cannot be completed until total testosterone, SHBG and albumin results are all available and verified. The report may therefore be released later than the individual albumin result.

Who Should Take This Test?

This profile may be appropriate for adult males with symptoms suggesting testosterone deficiency, particularly when a previous total testosterone result is borderline or does not correspond with the symptoms. Diagnosis usually requires compatible symptoms and consistently low testosterone measurements rather than one isolated result.

Older males or patients with conditions that alter SHBG may benefit from assessment of bioavailable or free testosterone. Such conditions can include thyroid disease, liver disease, obesity, insulin resistance and the use of certain hormonal medicines.

Females with signs of androgen excess, such as excess facial or body hair, persistent acne, scalp hair loss, deepening of the voice or irregular menstrual periods, may be advised to undergo testing. Additional investigations may include DHEAS, androstenedione, 17-hydroxyprogesterone, LH, FSH or pelvic imaging.

Patients receiving testosterone-replacement therapy, gender-affirming hormone treatment or anti-androgen therapy may require monitoring when prescribed by their treating clinician. The desired range and sample timing differ according to the treatment and should not be selected without medical guidance.

Adolescents with delayed or atypical pubertal development may sometimes require testosterone and binding-protein assessment as part of specialist endocrine evaluation. Paediatric results require age- and developmental-stage-specific interpretation.

This test is not recommended for self-diagnosing low testosterone based only on nonspecific symptoms such as tiredness. A healthcare professional should assess whether this profile and any additional pituitary, thyroid, reproductive or metabolic investigations are appropriate.

Understanding the Components

Total Testosterone: Total testosterone represents all circulating testosterone, including testosterone bound strongly to SHBG, testosterone bound weakly to albumin and the small free fraction. It is usually the initial laboratory measurement used when investigating androgen deficiency or excess.

In males, testosterone is produced primarily by the testes under stimulation from LH. In females, smaller quantities are produced by the ovaries and adrenal glands and through conversion of other androgens. Testosterone supports sexual function, reproductive health, muscle and bone maintenance, body-hair development and red-blood-cell production.

Sex Hormone-Binding Globulin: SHBG is a protein produced mainly by the liver. It binds testosterone and estradiol strongly and regulates how much hormone is available to enter tissues. An increased SHBG concentration may reduce the bioavailable testosterone fraction, while decreased SHBG may increase the proportion that is available.

Increased SHBG may occur with ageing, overactive thyroid function, estrogen exposure, pregnancy and certain liver conditions. Reduced SHBG may be associated with obesity, insulin resistance, underactive thyroid function, androgen exposure and some systemic disorders. An SHBG result does not establish the cause, and further assessment may be required.

Albumin: Albumin is the most abundant protein in blood plasma and is produced by the liver. Testosterone binds less strongly to albumin than to SHBG. Because albumin-bound testosterone can dissociate relatively easily, it is generally included with free testosterone in the bioavailable fraction.

Albumin may decrease with significant liver disease, kidney protein loss, inflammation, malnutrition and other medical conditions. An abnormal albumin result may require liver-function tests, kidney assessment or nutritional evaluation based on the patient's clinical findings.

Bioavailable Testosterone: Bioavailable testosterone generally includes unbound testosterone and testosterone weakly bound to albumin. It excludes the fraction tightly bound to SHBG. Depending on the laboratory, it may be calculated from total testosterone, SHBG and albumin or assessed using another validated analytical method.

Bioavailable testosterone is different from total testosterone and is not always identical to a directly measured or calculated free testosterone result. The report should specify the method and appropriate reference interval.

Detailed Information

The relationship between total and bioavailable testosterone depends greatly on binding proteins. Two people with the same total testosterone can have different bioavailable concentrations when their SHBG or albumin levels differ. This is why the profile can be useful when total testosterone alone is difficult to interpret.

Testosterone varies throughout the day and may also vary from one day to another. In adult males, concentrations are commonly higher in the morning. Acute illness, insufficient sleep, obesity, substantial calorie restriction and certain medicines can lower the result temporarily. A low initial result may therefore require confirmation using another morning sample.

Diagnosis of male hypogonadism should not be based on one laboratory number. A doctor generally considers symptoms, repeated hormone results and possible causes. Additional tests may include LH, FSH, prolactin, thyroid tests, iron studies, pituitary investigations, semen analysis or imaging.

High testosterone in females may be associated with PCOS, medicines, supplements or ovarian and adrenal conditions. Rapidly progressing virilisation, such as deepening of the voice or marked new hair growth, requires prompt medical assessment. Laboratory results alone cannot identify the source.

Reference intervals vary substantially according to sex, age, pubertal stage, assay method and whether the reported value is total, free, calculated bioavailable or directly measured bioavailable testosterone. Patients should use only the reference interval printed on their Focus Diagnostics report.

Calculated results depend on the accuracy of all input measurements and the equation used. Very low or high concentrations of binding proteins, pregnancy, severe illness and certain uncommon binding abnormalities may reduce the reliability of the calculation. The doctor may request a specialised free-testosterone method when clinically necessary.

A low bioavailable testosterone result does not automatically mean testosterone treatment is required. Treatment decisions depend on symptoms, repeated results, underlying cause, fertility plans, contraindications and potential benefits and risks. Testosterone treatment can suppress sperm production and should not be started without specialist guidance.