The impact of the treatment of hyperthyroidism (HT) with I-131 on serum calcium (Ca) and parathyroid hormone (PTH)

Objectives HT is a common disease, usually treated in the US with the use of radioiodine (I-131), which can cause extensive regional damage. Incidents of both hyperparathyroidism and hypoparathyroidism have been reported following I-131, but no prospective study of this effect is reported in the literature.

Methods Twenty patients receiving treatment with I-131 at our institution were prospectively enrolled in this study. Serial phlebotomy was performed at 0, 4, 12, and 24 weeks after treatment. Serum Ca, ionized Ca, and PTH were tracked over time and analyzed to identify factors associated with any change.

Results Serum Ca and ionized Ca levels significantly decreased with the resolution of HT. PTH levels significantly increased. Factors which contributed to the change in Ca included the radioiodine uptake(RAIU) value (p<0.001), the initial Ca level (p=0.022) and the initial Free T4(FT4) (p=0.004). Factors which contributed to the decrease in PTH included the RAIU (p=0.008) and the initial FT4 (p=0.004). The I-131 dose delivered to the thyroid did not impact either Ca or PTH. Serum phosphate and calcidiol levels did not significantly change.

Conclusions Treatment of HT with I-131 is associated with a decrease in serum Ca levels. The magnitude of the decrease appears related to the severity of the thyrotoxicosis and hypercalcemia. The associated increase in PTH appears to argue against a significant radiation effect on the parathyroid glands and favors a primary etiology of resolving thyrotoxicosis to explain the decrease in calcium levels.