After thyroidectomy, either total or partial, hypocalcaemia can occur in some cases temporary and in other permanent. For explaining transient hypocalcaemia are reported some pathogenetic mechanism can assume particular aspects in hyperthyroid subjects. Permanent hypocalcaemia is substantially due to anatomical lesions to the parathyroid glands. On the other hand, in subjects with hypothyroidism postoperative, a reduced concentration of circulating calcium can coexist with normal values or even elevated of parathormone. The condition of euthyroidism is basic for the resumption of the calcium homoeostasis.