Calcium homeostasis in human body is regulated by the parathyroid hormone (PTH) and vitamin D, which maintain the concentration of calcium in the blood, stimulate bone calcium reabsorption, calcium absorption in the intestine, and calcium excretion of the kidneys. Calcitonin is an antagonist of parathyroid hormone and it also participates in this regulation. Measurement of parathyroid hormone is important in differential diagnosis of hypercalciemia and hypocalciemia, and for evaluating the function of the parathyroid gland. To monitor the calcium status in the body, the concentration of total calcium and the concentration of free (ionized) calcium (iCa) are determined. The aim of this study was to determine the extent to which the operation of the thyroid gland disrupts calcium homeostasis in patients who undergo total thyroidectomy in relation to the patients who have other thyroid or paratyroid surgery. The study included 192 patients whose blood samples were collected prior to surgery and 24 to 72 hours after the operation. In serum samples were measured the concentrations of ionized calcium on the Rapidpoint 400 analyzer and parathyroid hormone on the cobas e 411 automated analyzer. Total thyroidectomy is the most common among surgical interventions on the thyroid gland, confirmed in a five month research period. The results show that there was no difference between the median concentration of PTH and iCa before the operation, depending on the type of surgery. A significant decrease in PTH and iCa concentrations was found after the procedure in all patients, irrespective of the type of procedure and sex (P <0.0001), and the decrease was significantly more pronounced in patients with removed thyroid gland (P ≤0.001). Women and men show the same changes and do not differ statistically. There is no difference in the lowering of iCa concentration between women and men, i.e. both sexes have significantly higher differences in concentrations when they are thyroid decompensated. A statistical significance of weaker power (P = 0.042) was observed in women, whose thyroid dehydration decreases PTH on average -24%, while other operations are only -4%. In the postoperative course all groups of patients are endangered by temporal dysfunction of the intestine, rapid decline in PTH hormone concentration and subsequent hypocalcemia development.