Objective: Radioiodine-131 is commonly used for treatment of hyperthyroidism but there are few available data on the effects of this treatment on male gonadal function. The untoward effects of (131)I have been mainly studied in male patients treated with high doses for thyroid cancer. In the present work we studied the absorbed radiation dose to the testes and testicular function in hyperthyroid men after (131)I treatment.

Patients and measurements: Nineteen male hyperthyroid patients were enrolled in the study before (131)I therapy. Seventeen of the patients had Graves' disease and two had toxic adenoma. The study was subdivided into two parts: a dosimetric and a clinical study. Six patients were enrolled for the dosimetric study and 13 for the clinical study. The beta dose delivered to the testes was evaluated by the Medical Internal Radiation Dose (MIRD) method. The gamma dose was measured by thermoluminescent dosimeters (TLDs) placed on the skin overlying the inferior poles of the testes for 3 weeks after therapy. The clinical evaluation included hormone determination, ultrasound (US) of the testes and sperm analysis. Patients were followed up for 12 months after (131)I therapy.

Results: In the dosimetric study, the beta dose absorbed in the testes was 12.5 +/- 8.8 mGy (range 29-15 mGy) and the gamma dose was 15.8 +/- 5.3 mGy (range 24-11 mGy). The total dose to the testes for administered activity unit was 39 +/- 14 microGy/MBq (range 27-86 microGy/MBq). In the clinical study, FSH did not change significantly after (131)I treatment for the majority of patients. Serum testosterone (T) and the T/LH ratio were significantly reduced 45 days after treatment and returned to basal levels after 12 months. Ten out of 15 hyperthyroid patients (67%) had low sperm motility before treatment. A significant increase in progressive motility was observed after (131)I therapy (Friedman test chi(2) = 12.65, P = 0.01). Conversely, there was no significant variation in sperm concentration and percentage of normal forms after (131)I.

Conclusions: After (131)I therapy, germinal epithelium and Leydig cell function undergo only marginal changes, which may have some significance in subjects with a pre-existing fertility impairment.