Blood pH, blood levels of Ca, P and protein were measured in 5 patients with thyroid diseases during and after the surgical maneuver of the gland and compared with those in a patient with a cervical cyst located between the anterior neck muscles who underwent a similar neck operation but without direct intervention into the thyroid. Immediately after the surgical handling of the thyroid, a statistically significant fall in the blood Ca level ensued that reached the lowest point within 1hour and recovered by 3hrs. Such a remarkable change was not observed with regard to the blood pH and the blood level of P and protein. The blood Ca level was maintained at the preoperative level, on the other hand, in the patient whose thyroid was not operated on. From these results a possibility was considered that the thyrocalcitonin leaked out of the damaged thyroid caused this hypocalcemia.