Background: Radioiodine (131I) therapy is widely used for treatment of non-toxic goiters. A limitation for this treatment is a low thyroid radioiodine uptake (RAIU), often encountered in these patients.
Aim: To estimate the impact of various factors on the thyroid RAIU.
Methods: We examined prospectively 170 patients (146 females; age range: 22-87 yrs) with nodular goiter (median 64 ml, range: 20-464 ml) selected for 131I therapy. Serum TSH was sub-normal in 42.4%. None were treated with anti-thyroid drugs. The thyroid RAIU was determined at 24h and 96 h. The goiter volume was measured by ultrasound (no.=127), or by magnetic resonance imaging (no.=43).
Results: The 24h and the 96 h RAIU were 34.2 ± 9.8(SD)% (range: 11.4-66.0%) and 34.0 ± 10.0% (range: 10.5-60.9%), respectively. Sixty-one patients had a 24h RAIU <30% and these individuals were older than patients with a 24h RAIU ≥ 30% (median 58 vs 51 yrs, p=0.02). These two subgroups did not differ significantly in other variables. Overall, the 24h RAIU was positively correlated to the serum (s) free T4-index (r=0.20, p=0.01), and negatively to age (r=-0.18, p=0.02), but not significantly related to serum TSH or thyroid volume. Age correlated positively with thyroid volume (r=0.31, p < 0.001). In a regression analysis, s-free T4-index and age remained as the only determinants of the 24h and the 96 h RAIU.
Conclusions: In patients with a symptomatic nodular goiter, serum T4 and age are the major determinants of the thyroid RAIU. A sub-normal serum TSH is not a marker of a compromised thyroid RAIU but reflects that the iodine is confined to a few 'hot spots'.