Endocrine Abstracts (2004) 7 P236

Hypothyroidism is less frequent following radioactive iodine treatment for toxic nodular goitre than Graves' disease

HI Adamali1, J Gibney1 & TJ McKenna1,2

1Department of Endocrinology, St. Vincent's University Hospital, Dublin, Ireland; 2The Conway Institute of Biomolecular and Biomedical Resarch, University College Dublin, Ireland.

Since the thyroid is diffusely active in Graves' disease (GD) but exhibits focal activity in toxic nodular goitre (TNG), we tested the hypothesis that hypothyroidism will occur more frequently following radioactive iodine treatment (RAI), in patients with GD compared to those with TNG. To do this we reviewed the outcome of treatment in 86 patients, age 49.7 plus/minus 1.5 years (mean plus/minus SEM), with GD, and 77 patients, age 62.7 plus/minus 1.3 years, with TNG, followed over 3.2 plus/minus 0.2 and 2.8 plus/minus 0.2 years post-RAI, respectively.

Hypothyroidism, defined clinically and with an elevated TSH value (greater than 5 milliunits per litre), was diagnosed in 72 percent with GD at 8.1 plus/minus 1.2 months, and 23 percent with TNG at 9.6 plus/minus 2.5 months post-RAI (p<0.001 between groups). The average dose of RAI was significantly higher in TNG (490.9 plus/minus 6.9 megabequerels) compared to GD (284.9 plus/minus 6.2 megabequerels, p<0.001). The average number of treatments administered was significantly higher in GD (1.4 plus/minus 0.1) compared to TNG (1.2 plus/minus 0.1, p<0.05)

We also examined the possibility that increased pre-treatment TSH values in patients with TNG might be associated with subsequent development of hypothyroidism. Pre-treatment values of TSH tended to be higher (p=0.088) in patients who became hypothyroid (1.29 plus/minus 0.54 mU/l), compared to those who remained euthyroid (0.60 plus/minus 0.15 mU/l).

The data indicate that hypothyroidism is three times more likely to occur following RAI in patients with GD than in patients with TNG, and provides preliminary evidence that the TSH level pre-RAI positively predicts the occurrence of hypothyroidism post-RAI in patients with TNG. If this finding is confirmed, then patients with TNG should undergo treatment with RAI in the setting of suppressed TSH.

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