The laboratory determination of the hormone testosterone in the blood, the main androgen most important in men, in many diseases has not only an important diagnostic and prognostic value, but also therapeutic and prophylactic.

The hormone is synthesized from cholesterol and acetic acid. In men, it is produced in the testicles by Leydig cells (95-98%) and, in small quantities, in the cortex of the adrenal glands (3-5%). Also, testosterone is partially formed as a result of peripheral metabolism. Its almost complete destruction and transformation into 17-ketosteroids occurs in the liver and partially in the tissues.

Testosterone fractions and their normal values

In the blood, the hormone circulates in three forms, or fractions:

  1. Strongly bound to SHBG (sex steroid binding globulin) and constituting 54-70%.
  2. Protein-bound albumin is an unstable bond — about 25–43%.
  3. Free testosterone , which is the most biologically available and active form - from 1 to 4%.

The second and third forms are biologically active. Only they are able to penetrate cells and bind directly to specific receptors or transform into active 5α-dihydrotestosterone . A strong bond with the SHBG does not allow testosterone to penetrate the cells, and therefore it is not able to manifest its biological activity.

Total Testosterone =   T associated with SHGP + T associated with albumin + T free.

Total testosterone is the sum of all hormone fractions, including free testosterone.

In most laboratory and diagnostic centers, only total testosterone is determined, which is normally in men 12-33 nmol / l . In some cases this is enough. However, only such a study is uninformative and limits the ability of the doctor to conduct early and differential diagnosis in cases of, for example, the presence of obesity, metabolic syndrome , type 2 diabetes in patients.

In this regard, the additional definition is optimal:

  • biologically active fractions, average norms of which are 3.5-12 nmol / l;
  • free testosterone - 4.25-42 pg / ml.

See: “Calculator for calculating free and biologically active testosterone” .

Age and norms of testosterone

All of the above indicators of the norm of the hormone are average. They do not take into account fluctuations in different age periods. The concentration of total testosterone in men begins to decline from 50-55 years of age by 0.8-1.6% annually and by the age of 80 averages 60%, compared with 20-year-old men. The content of the biologically active hormone in the blood is maximal at the age of 25-35, after which it decreases annually by 2-3%.

Fig. 1 - Age dynamics of secretion of total testosterone in men.

Thus, the desirable minimum content of the hormone in the blood (taking into account age-related changes) in men of young age are average indicators of the norm. For men over 50 years old - not below the minimum limit of the above norm.

Factors affecting blood testosterone levels

Laboratory indicators of normal testosterone levels are very variable, even in individuals of the same age. They are influenced by the specifics of the diagnostic equipment of laboratories, laboratory errors associated with the lack of standardization in commercial establishments, and other reasons that should be taken into account when evaluating analyzes.

Fig. 2 - Testosterone circadian and one-year rhythms:
graph 1 - testosterone circadian rhythm versus time of day;
graph 2 - testosterone circadian rhythm versus season.

The main factors affecting the content of both total and biologically active testosterone are:

  • Time of day and region of residence. Daily changes in total testosterone reach 45%, free - 68%, biologically available - 57%. The content of the hormone in the blood from 4 to 8 am maximum, the minimum - from 4 to 8 pm (see. Fig. 2). Therefore, the optimal time for blood sampling for research is morning. The daily rhythm of testosterone can differ in individuals working at different times of the day (shift work), which requires the correction of the time of blood sampling for analysis (after awakening) and its evaluation. The fluctuations in testosterone levels during the day are smoothed out as the age increases, which also needs to be taken into account when evaluating test results.
  • Season. In the autumn, the level of the hormone in the blood is the highest, the lowest - in the summer (see Fig. 2). Seasonal fluctuations of total testosterone reach 19%, free - 31%.
  • The nature of food: meal time and its composition. A significant amount of protein and fat in food contributes to the reduction of SHBG (see "Products that increase the level of testosterone" ).
  • Stress states and constant high physical exertion. Frequent and prolonged psycho-emotional and heavy physical exertion suppress the production of sex hormone. At the same time, immediately after exercise, the level of the hormone in the blood rises.
  • Acute or chronic diseases (arterial hypertension, coronary heart disease, diabetes, liver disease, etc.), as well as chemotherapy or radiation therapy. Chronic pathology leads to a decrease in total testosterone levels or only its free fraction.
  • A number of drugs and narcotic drugs (see "Drugs that reduce potency" ).

In the case of lower (compared with the age norm) values ​​of the hormone in the test results, it is necessary to conduct another 1-2 repeated studies of testosterone levels. Further, other, already targeted diagnostic studies are conducted to identify the causes of deviation of the hormonal level. For example, measurement of blood glucose, total cholesterol and low density lipoprotein, ECG, blood pressure monitoring, coronarocardiography, measurement of bone mineral density, etc.

Indications for the study of free testosterone

Determining the level of free testosterone in the serum of men is recommended for:

  • disorders of sexual development and testosterone synthesis, as well as in the early puberty of boys;
  • Reifenstein syndrome;
  • pathologies of the hypothalamic-pituitary system;
  • cryptorchidism and myotonic dystrophy;
  • dysfunction of the adrenal cortex;
  • suspected tumor of the adrenal glands or testicles;
  • pathology of the genital glands after traumatized testicles, viral parotitis;
  • chronic alcoholism;
  • taking anti-androgenic drugs and gonadoliberins;
  • old age.