Calcium Homeostasis After Thyroid Surgery
Abstract: Calcium homeostasis after thyroid surgery was studied during the immediate postoperative period and after one year. One day after hemithyroidectomy ionised calcium decreased whereas basal PTH did not change. EDTA stimulated maximal and total PTH secretion and set-point decreased on postoperative day 1 and normalised on day 7. One year after hemithyroidectomy (n=10) total calcium and 1.25(OH)2D3 decreased. EDTA and calcium infusion tests showed that maximal and total PTH secretion were unaltered. However, ionised calcium at maximal PTH secretion was reduced, the set-point slightly decreased and the ionised calcium/PTH relation curve left shifted. One year after hemithyroidectomy (n=45) ionised calcium decreased from 1.25±0.05 to 1.22±0.04 mmol/L (p=0.001) despite an unaltered PTH level (2.8±1.0 vs. 3.1±1.5 pmol/L, p=0.50). The marker of bone resorption 1CTP decreased. A reduction in ionised calcium was seen only in groups with reduced FT4. An independent association was found between reduced ionised calcium and altered TSH and FT4 levels. The peripheral effects of PTH were studied by using a six hour hPTH-(1-34) infusion protocol. Induced changes in serum levels of ionised calcium and 1.25(OH)2D3 and in urinary excretion of phosphate and n-cAMP, were not altered by a slight reduction in thyroid hormones at 3 months after hemithyroidectomy. Intraoperative PTH, measured immediately after total or near-total thyroidectomy, showed an independent association with postoperatively reduced serum calcium levels. Intraoperative PTH was lower among patients who developed biochemical hypocalcemia (p<0.001). Low intraoperative PTH identified the patients who required calcium intravenously within 24 hours postoperatively and predicted biochemical hypocalcemia with a sensitivity and specificity of 90 % and 75 %, respectively. In summary, reduced PTH secretion is the main determinant of transient hypocalcemia after thyroid surgery. Low intraoperative PTH is a feasible predictor of postoperative hypocalcemia. Even minor thyroid surgery, i.e. hemithyroidectomy has sustained effects on calcium homeostasis due to the influence of reduced thyroid hormones on bone remodelling.
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