Testosterone is a hormone primarily responsible for male characteristics and is produced in the testicles of men, and to a much lesser extent, in the ovaries of women. It regulates sex drive and plays a crucial role in controlling bone mass, fat distribution, muscle mass, strength, as well as the production of red blood cells and sperm. Testosterone levels in men naturally decline after the age of 30, but lower than normal levels can occur at any age, causing symptoms such as low libido, erectile dysfunction, difficulty in gaining and maintaining muscle mass, and lack of energy. Women have much lower levels of testosterone than men, but it is equally important for regulating libido, muscle and fat distribution, and red blood cell formation. The normal reference range for testosterone levels varies slightly depending on the laboratory and population tested, but the British Society for Sexual Medicine recommends that low testosterone can be diagnosed when testosterone is consistently below the reference range, or when levels are below 12 nmol/L, especially in men who report symptoms of low testosterone or have low levels of free testosterone.

The majority of testosterone present in the bloodstream is bound to proteins, predominantly SHBG and albumin, with only a small amount (2-3%) existing in an unbound or free state, and thus available for use by cells. The test in question employs a formula to determine the level of free or unbound testosterone based on the total testosterone level, as well as levels of SHBG and albumin.

Oestradiol is a steroid hormone that is primarily produced in the ovaries of women and to a lesser extent in the testes of men. It is one of three types of estrogen and has important functions in regulating the female reproductive system, as well as promoting breast tissue growth and maintaining bone density. Oestradiol levels naturally fluctuate throughout the menstrual cycle, peaking at ovulation in pre-menopausal women. As women age, their oestradiol levels decrease and eventually stop altogether with menopause. Low levels of oestradiol can lead to symptoms commonly associated with menopause such as hot flushes, night sweats, and mood swings. Additionally, low levels of oestradiol can increase the risk of developing osteoporosis.

SHBG (sex hormone binding globulin) is a protein produced by the liver that binds with sex hormones such as testosterone, estrogen, and dihydrotestosterone (DHT) in the bloodstream. When hormones are bound to SHBG, they become inactive and are not available for use by cells. Measuring the level of SHBG in the blood can provide important information about the levels of free or unbound hormones, which are biologically active and available for use.

Albumin is a protein produced mainly by the liver that helps to maintain the osmotic pressure of the blood, keeping water inside the vessels. It also aids in the transport of nutrients, medications, and other substances throughout the body, and is important for tissue growth and repair. Additionally, albumin carries hormones in the blood, and measuring its levels can provide information about the amount of hormone available to the tissues.