The follow-up of radioiodine-treated hyperthyroid patients: should thyroid function be monitored more frequently?

There is a lack of data regarding the timing and particularly the severity of hypothyroidism post radioiodine (RI).

To investigate the timing and severity of hypothyroidism in RI-treated hyperthyroid patients.

Retrospective examination of the records of 183 RI-treated hyperthyroid patients (79 autoimmune hyperthyroidism, 46 toxic multinodular goiter, and 58 hyperthyroidism of indeterminate etiology).

One hundred and fifty-nine patients requiring a single dose of RI (435 MBq), 107 (67%) developed hypothyroidism. Hypothyroidism detected in: 16% of patients at <8 weeks, 46% at 8 to <16 weeks, 24% at 16 to <24 weeks, 9% at 24 to <36 weeks, 3% at 36 to <52 weeks, and 2% at >52 weeks. One hundred and eighty-three patients had follow-up after one or more doses of RI and 124 (68%) patients developed hypothyroidism; of these, 44 (36%) had TSH>50 mU/l and 34 (27%) had free T4<5 pmol/l when hypothyroidism was first detected. Of those patients with a delayed outpatient visit (no.=77) and those with an outpatient visit within the recommended target interval (no.=47), median TSH was 23 (0.05-152) mU/l and 32 (0.05-150) mU/l, respectively (p=0.75) and median free T4 was 7.1 (1.3-16.7) pmol/l and 6.6 (1.3-15.4) pmol/l, respectively (p=0.21) at first detection of hypothyroidism.

The severity of hypothyroidism when first detected during follow-up is of concern and suggests that closer monitoring of thyroid function is required, particularly during the first 6 months post- RI therapy.