A method for the early diagnosis of fibrosis of liver, kidneys, and heart in experimental hyperthyroidism and hypothyroidism provides for assaying fibrosis markers in the blood of the laboratory animals. In the rats with the modeled hyperthyroidism and hypothyroidism, the peripheral blood is sampled and centrifuged at 1000 rpm for 5 min. The blood clot is washed with phosphate buffer and the centrifugation is repeated under the same conditions. The cell precipitate is suspended in pre-extraction buffer with protease inhibitors. The suspension is incubated on ice for 10 min. Then tubes are stirred and centrifuged at 12000 rpm for 1 min. Cytoplasmic supernatant is discarded. To the precipitate, the solution of dithiotreitol, protease inhibitor and extraction buffer are added. The mixture is incubated on ice for 15 min with periodical stirring, then centrifuged at 14000 rpm for 10 min at the temperature of 4 °C. The supernatant is transferred into the new tubes. Protein concentration is measured at 560 nm. Bovine serum albumin solutions (0, 0.25, 0.5, 1.0, 1.4 and 2 mg/mL) are used as reference. Then the activity of DNA-methyltransferase (DNMT) is assayed. The samples are placed into the wells according to the measured protein concentrations. The positive and negative controls are placed into the separate wells. For negative control, procaine amid hydrochloride as DNMT inhibitor is used at concentrations of 1 and 10 µg/mL. The wells are covered with parafilm and incubated for 90 min at the temperature of 37 °C. Following the incubation, the content of the wells is removed. The wells are washed thrice with washing buffer. To each well, antibodies are added in the volume of 50 µL. The wells are incubated for 60 min at the room temperature shaking the plate. Then the fluid from the wells is discarded. The wells are washed thrice with washing buffer. To each well, antibodies detecting the enzyme are added in the volume of 50 µL. The wells are incubated for 30 min at the ro
