CLINICAL STUDY Increased sensitivity to the inhibitory effect of excess iodide on thyroid function in patients with b-thalassemia major and iron overload and the subsequent development of hypothyroidism

Objective: Patients with b-thalassemia frequently develop primary hypothyroidism and other endocrine disorders due to iron overload. We studied whether administration of excess iodide to patients with apparently normal thyroid function could uncover an underlying thyroid disease. Design and methods: Twenty-®ve patients, 10 prepubertal (mean age 11 6 3 years) and 15 adults (mean age 23 6 5 years) with normal thyroid hormone and TSH levels, a normal response of TSH to TRH and negative thyroid peroxidase antibodies received 20 mg iodide three times daily for three weeks, and thyroid hormone and TSH levels were measured weekly during, and for three weeks after, iodide administration and every 3 months thereafter for the next 5 years. Results: During iodide administration there was a signi®cant decrease in thyroid hormone concentrations which remained within normal levels, and a signi®cant increase in TSH concentrations which in 14 out of 25 (56%) patients reached the hypothyroid level. Baseline TSH values were higher in those patients who developed subclinical hypothyroidism (2.31 6 0.71 mU/l vs 1.34 6 0.64 mU/l, Pˆ0.0016). Subclinical hypothyroidism developed in 70% of prepubertal and in 47% of adult patients. Serum ferritin was elevated in all patients. Nine of the fourteen patients (64.3%) who developed subclinical hypothyroidism during iodide administration developed hypothyroidism during the 5-year follow-up compared with only one of the eleven patients with a normal response to iodide (Pˆ0.004). Conclusions: Patients with b-thalassemia should not be exposed to excess iodide due to increased sensitivity to its inhibitory effects on thyroid function. The susceptible individuals frequently develop permanent hypothyroidism in the following years. European Journal of Endocrinology 143 319±325