Reciprocal changes in parathyroid hormone and thyroid function after radioiodine treatment of hyperthyroidism

Hyperthyroidism is associated with negative calcium balance, normal to increased serum calcium concentrations, and decreased cortical bone mass. There is no agreement concerning serum PTH levels in such patients. In this study, we measured serum PTH concentrations using a newly developed sensitive 2-site immunoradiometric assay in 17 hyperthyroid patients before and after radioiodine therapy. The mean serum PTH and calcium concentrations were 28 +/- 15 (+/- SD) ng/L (normal range, 12-65 ng/L) and 2.4 +/- 0.5 mmol/L (normal range, 2.1-2.6 mmol/L) before therapy. After therapy serum PTH concentrations increased in 16 of the 17 patients. The increase in serum PTH was greater in the 9 patients who became hypothyroid rapidly (29 +/- 15 to 75 +/- 29 ng/L) compared with that in the 8 patients who became euthyroid gradually (26 +/- 16 to 45 +/- 24 ng/L). Serum PTH rose along with TSH as the patients became hypothyroid after radioiodine, and both serum PTH and TSH fell when L-T4 therapy was given. The reciprocal changes in serum PTH concentrations and thyroid function over time suggest a strong association of bone mineral metabolism and thyroid status.