A total of 125 children and teenagers, aged 1 to 15 (including 74 patients with primary congenital hypothyroidism--CH--and 51 healthy children), were investigated in the city of Moscow for the purpose of elaborating new approaches towards the differentiated diagnostics of CH and of optimizing the assessment of the adequacy of its substitutive hormonal therapy (SHT). The concentrations of the thyrotropic hormone (TTH), free fractions of triiodothyronine and thyroxin (cT3 and cT4), thyreoglobulin (Tg) and of thyreoid peroxidase (hard-phase test-tube method, electro-chemiluminescent reaction, i.e. magnetic microparticles, based on the streptovidin-biotin technology; ruthenium mark at the "Elecsys 1010" analyzer with reagents manufactured by "F. Hoffman--La Rosh Ltd.", Switzerland.) were determined early in the morning on the empty stomach. On the basis of the above mentioned, the thyroid reserve index (TRI = Tg/TG), integral thyroid index (ITI = [cT3 + cT4]/TTG), and the integral of peripheral conversion (IPC = cT3/cT4) were calculated. The thyroid gland (TG) was visualized by the "Acuson-128 XP" ultrasonic device, "Acuson Corp.", USA, with a linear sensor of 10 Mhz, as well as by the "Toshiba-90B" gamma-chamber, Japan, with a low-energy collimator in 24 hours after an oral intake of sodium iodide marked by the iodine 123-isotope with an activity of 50 Mbc and after the cessation of SHT 3 to 5 weeks before. The complex diagnosis revealed 8 variations of primary CH; aplasia and TG dystopia and defects of the synthesis of the sodium-iodine symporter or Tg-concentration were found to belong to the most severe disease types according to hormonal parameters. As for Tg, in 52% of cases it was normal, undeterminable--23% and higher--25%. A simultaneously implemented ultrasonic scanning (USS) ensured a final diagnosis in subgroups, respectively, in all cases, in 25% and in 75% of patients. On the whole, after the two diagnostic stages (Tg + USS/ITR), the indications for isotope scanning left only for 23% of CH patients, i.e. a) if the Tg values could not be determined because the organ was not located in its typical place (17%); or b) if the Tg level was higher or if there was a goiter at the TG normal volume (6%). An in-depth hormonal analysis showed that, should CHT be monitored less than once in three months, it failed to secure a total compensation of the disease in above one third of CH children (39%). Therefore, a three-stage algorithm of nosological diagnostics is recommended for primary CH; it should comprise the determination of Tg level, and USS (for all patients) as well as TG scanning (according to rare indications). The result point at the necessity of hormonal monitoring of CH children and teenagers at least one in 2-3 months. The objectivity degree of CHT goes up as a number of estimated indices, i.e. ITI and IPC, are used alongside with absolute hormonal values.