Objective: To evaluate the effects of radioactive iodine (RAI) treatment for differentiated thyroid cancer (DTC) on testis function.

Design: A prospective longitudinal single-centre study was performed. A comprehensive andrological evaluation including hormonal assessment, semen analysis and scrotal ultrasound was undertaken in male patients undergoing RAI treatment for DTC.

Methods: Hormonal assessment of FSH, LH, testosterone (T), sperm concentration and motility and testis volume were determined in 20 patients in basal conditions, 6 and 12 months after RAI. Results were analysed in the whole group of patients and then separately in those who received one single ablative treatment ('Single' group, n = 10) and those who received multiple treatments ('Multiple' group, n = 10).

Results: In basal conditions, 3 of 20 (15%) patients had a reduced sperm count and belonged to the 'Multiple' group. After RAI, an increase of FSH (8·8 ± 1·2 UI/l vs 5·2 ± 1·2, P < 0·005) and a decrease in sperm concentration (28·8 ± 7·7 millions/ml vs 54·5 ± 7·1, P < 0·005) and testis volume (15·2 ± 3·1 vs 13·7 ± 0·8 ml, P < 0·005) occurred at 6 months in the whole group. One year after RAI, seven patients had oligozoospermia (five from the 'Multiple' group and two from the 'Single' group). Permanent impairment of one or more testis function parameters was observed in patients who underwent multiple RAI treatments: 50% for sperm count, 40% for FSH levels and testis volume and, respectively, in 20 and 10% of those who received one single RAI treatment.

Conclusions: The single ablative RAI treatment in cancer patients is better tolerated respect multiple RAI treatments regard testis function. Multiple treatments for recurrent or metastatic disease may cause a permanent impairment of one or more parameters related to the reproductive potential of male patients.