Serum levels of calcium and parathyroid hormone after subtotal thyroid resection for Graves' disease.

The following serum constituents were measured in 19 patients operated on for Graves' disease: calcium, protein, parathyroid hormone (PTH), alkaline phosphatase, thyroxine, triiodothyronine, free thyroxine index, and thyroid-stimulating hormone. Blood was sampled systematically on the day before and 5 days, 1 month, 2, 3, 4 and 6 months after operation. All patients were pretreated with antithyroid drugs, and 2 months before surgery levothyroxine was added. The study showed that the thyroid function was impaired for 3 months postoperatively. Serum calcium level was lowered in the same period. Serum PTH increased with decreasing serum calcium, and vice versa. The results support the theory that the transient fall in serum calcium level is due to lowered PTH sensitivity in bone tissue caused by temporary impaired thyroid function after surgery. On the other hand, a direct action of lowered thyroid hormone on bone tissue is also a possible explanation.