Long-term effect of postoperative radioactive iodine therapy on parathyroid function in patients with differentiated thyroid cancer.

Objective: The study aimed to assess the impact of postoperative radioactive iodine (RAI) therapy on parathyroid function in patients who underwent total or subtotal thyroidectomy for differentiated thyroid cancer (DTC).

Methods: Data from 150 patients treated with RAI for DTC and 76 patients with low-risk DTC not receiving RAI were retrospectively analyzed. Clinical characteristics, preoperative and 1-month postoperative biochemical parameters, and adjusted calcium, phosphorus, parathyroid hormone (PTH), and 25-hydroxyvitamin D3 (25-OH-D) levels at 3 months, 1 year, 3 years, and 5 years post-RAI (or in the low-risk group) were recorded.

Results: A total of 226 DTC patients were included in the study (80.5% female, mean age 42.7 ± 13.2 years). Total thyroidectomy was performed in 97.3% (n = 220) of patients, with central lymph node dissection (CLND) in 41.6% (n = 94). No significant preoperative differences in PTH, aCa, P, Mg, or 25-hydroxyvitamin D3 levels were observed. However, patients receiving ≥ 3.7 GBq (or 100 mCi) RAI (n = 70) had lower calcium and PTH levels at the end of the first year following RAI treatment (p = 0.048, p = 0.032). The non-RAI group showed significantly higher calcium levels at one month postoperatively (p = 0.031) and lower rates of CLND and neck dissection. No significant differences in biochemical parameters were found at the five-year follow-up, except for one patient who developed normocalcemic hyperparathyroidism after RAI.

Conclusion: High-dose RAI therapy may lead to transient decreases in calcium and PTH levels in the early post-treatment period. However, long-term parathyroid function appears to remain unaffected in DTC patients, regardless of the RAI dose administered. Nonetheless, close monitoring of calcium and PTH levels is recommended, particularly in the early post-treatment period, to promptly manage any potential transient hypoparathyroidism.