Introduction: Thyrotoxicosis resistant to the usual treatment is rare, but potentially fatal. In such situations, the optimal next treatment is unclear. Iopanoic acid (IA) was historically used as an oral contrast agent; its capacity to treat thyrotoxicosis has been limited in recent years due to its restricted availability.
Methods: Retrospective case note review of patients treated with IA for resistant thyrotoxicosis at our institution over the past 10 years.
Results: 7 patients (3 GD, 4 AIT) received IA. Fall in FT4 and FT3 levels was seen in all (Table 1). There were no adverse effects. 6 patients underwent uncomplicated thyroidectomy. One patient (case 5) developed severe multifactorial myopathy (steroids, LMNA mutation, AIT) prior to iopanoic acid administration; he later died of pneumonia.
|Age, gender, outcome, cause tof hyroid disease||FT4 pmol/l* RR 10-19.8||FT3 pmol/l* RR 3.5-6.5|
|Case 1||42, F, GD||69, 45||28, 6.4|
|Case 2||36, F, GD||63, 16||n/a, 4.8|
|Case 3||37, F, GD||44, 25||n/a, 3.3|
|Case 4||55, M, AIT||95.6, 38||17.9, 3.3|
|Case 5||65, M, AIT||85, 72||9.1, 6.6|
|Case 6||64, M, AIT||64, 48||10.8, 2,.4|
|Case 7||36, M, AIT||86, 57||21.8, 2.9|
|*Before, and after, treatment with Iopanoic acid.|
Conclusion: Iopanoic acid was highly efficacious at rapidly inducing biochemical euthyroidism (normal FT3) in all patients, was well tolerated and enabled safe thyroidectomy. Notably, FT4 remained elevated due to the predominant mechanism of action of inhibition of T4 to T3 conversion. Iopanoic acid is a safe and effective method of restoring euthyroidism in resistant thyrotoxicosis, current availability is limited to laboratory grade; development of a pharmaceutical grade compound would be highly advantageous in urgent or resistant cases of thyrotoxicosis.