Hypoparathyroidism after I-131 therapy with subsequent return of parathyroid function

Area of Science:

• Endocrinology
• Nuclear Medicine

Background:

• Radioactive iodine (I-131) therapy is a common treatment for hyperthyroidism.
• I-131 therapy can rarely lead to hypothyroidism and hypoparathyroidism due to damage to parathyroid glands.

Observation:

• A 22-year-old woman developed hypoparathyroidism 10 months after I-131 treatment for hyperthyroidism.
• Initial hypocalcemia was managed with Vitamin D2 and calcium, maintaining normocalcemia for 8 years.
• Hypercalcemia developed in 1979, leading to discontinuation of therapy, followed by spontaneous normocalcemia for 3 years.

Findings:

• Plasma parathyroid hormone (PTH) levels were measured retrospectively from 1970 and prospectively in 1982.
• In 1970, during hypocalcemia, bioactive PTH was low (0.26 pg/ml) while immunoreactive PTH levels varied.
• In 1982, during normocalcemia, both bioactive (5.18 pg/ml) and immunoreactive PTH levels normalized, indicating restored parathyroid function.

Implications:

• This case highlights the potential for delayed recovery of parathyroid function after I-131 therapy.
• Measurements of both bioactive and immunoreactive PTH are crucial for accurately assessing parathyroid status.
• The findings suggest that parathyroid function may spontaneously recover years after initial damage from I-131 treatment.