Endocrine Abstracts

Objectives: Transient thyroid hypofunction during the initial 3 months following radioiodine (RAI) therapy is well recognised with a reported incidence of 10–15%. There are no clear diagnostic criteria, often leading to management uncertainty, inappropriate diagnosis of ‘cure’ and institution of life-long thyroxine therapy. The aim of our audit was to assess the incidence and identify predictors for transient euthyroidism or hypothyroidism following administration of a standard 400 MBq RAI therapy.

Methods: Retrospective audit of database maintained on all patients who have received RAI for management of hyperthyroidism over the past 6 years.

Results: Fifteen of the 161 (9%) patients developed transient hypothyroidism or euthyroidism at the stage of 6 weeks or 3 months post-RAI although this is likely to be an underestimate. Nine (5.5%) had spontaneous conversion from hypothyroidism to either euthyroidism or hyperthyroidism and six (3.7%) from euthyroidism to hyperthyroidism. Patients who developed transient hypothyroidism or euthyroidism and those who did not, were comparable in terms of age (mean 49 vs 52 years respectively) gender (% male −13 vs 21) and aetiology (% Graves’ disease −61 vs 60) (P=NS for all). There was no difference in the proportion of patients who developed transient hypothyroidism in our current cohort (fixed RAI dose of 400 MBq) as compared to our earlier cohort who received calculated dose RAI therapy (mean RAI dose 267 MBq). Patients who developed transient hypothyroidism or euthyroidism were more likely to have a large goitre (40 vs 19%), had a higher 24-h radioiodine uptake (mean 65 vs 55%) and a higher pre-RAI free T₄ level (60 vs 34 pmol/l) (P<0.05 for all).

Conclusion: A significant proportion of hyperthyroid patients develop transient euthyroidism or hypothyroidism within first 3 months post-RAI. Patients with positive predictors should be made aware of this and delaying thyroxine supplementation should be considered.