Thyroid function after postoperative radiation therapy in patients with breast cancer.

Thyroid stimulating hormone (S-TSH) and thyroxine (S-T4) were measured in sera from 80 patients with breast cancer using radioimmunoassay. The patients had been irradiated after mastectomy 7.2 +/- 1.0 years earlier with 45 Gy in 3 to 4 weeks to parasternal, ipsilateral supraclavicular and axillary areas. Five patients (6%) had hypothyroidism with low S-T4 and elevated S-TSH levels. In addition, 12 patients (15%) had elevated (greater than 5 mU/l) S-TSH levels and normal S-T4 values. Shielding of the thyroid during irradiation and long-term follow-up of thyroid function with repeated hormone assays are recommended.