Inclusion Criteria:  confirmed diagnosis (arms "Cushing's Disease", "Acromegaly", "Diabetes Mellitus Type 1", "Primary Hyperparathyroidism") active phase of the disease (arms "Cushing's Disease", "Acromegaly") no history of surgical and specific pharmacological treatment (arm "Primary Hyperparathyroidism") HbA1c <8.0% (arm "Diabetes Mellitus Type 1") absence of the specified endocrine disorders (arm "Control group")  Exclusion Criteria:  factors associated with vitamin D level  intake of the following drugs during the 3 months preceding the study: vitamin D medications; glucocorticosteroids; antiretroviral drugs; antifungal drugs; cholestyramine, orlistat; antiepileptic drugs; antidepressants (fluoxetine); diuretics (spironolactone); antimicrobial agents (macrolides, tetracyclines, isoniazid, rifampin, primaquine); chemotherapy (cyclophosphamide, tamoxifen, paclitaxel, ifosfamide, irinotecan, etoposide, vinblastine); immunosuppressants (cyclosporin A, tacrolimus, sirolimus); H2 receptor antagonists. BMI >35 kg/m2 pregnancy granulomatous disease (sarcoidosis, tuberculosis, histoplasmosis, berylliosis, coccidiomycosis) disease with malabsorption syndrome (Crohn's disease, ulcerative colitis, celiac disease, post-bariatric surgery condition, decompensation of chronic pancreatitis) reduced renal function (eGFR <60 ml/min/1.73m2) laboratory signs of liver failure (hypoalbuminemia, hypoprothrombinemia)  hypercalcemia or risk factors for hypercalcemia  serum total calcium >3.0 mmol/L myeloma immobilization thiazide diuretics intake allergy to vitamin D drugs total 25(ОН)D >60 ng/ml (determined by chemiluminescent immunoanalysis)