There are two types of AIT - type 1 and type 2. The basic principle is that in type 1 there is increased synthesis of thyroid hormone (thyrotoxicosis with hyperthyroidism) and in type 2 there is destructive thyroiditis leading to excess release of thyroid hormone without increased synthesis (thyrotoxicosis without hyperthyroidism).
AIT - Type 1
In type 1 AIT, the excess iodine load from amiodarone acts as a substrate for increased thyroid hormone synthesis. This is usually seen in patients with preexisting multinodular goitre or latent Graves’ disease. Essentially, the thyroid gland is primed to increase synthesis. In AIT type 1, a radioactive uptake scan is usually normal, there are no signs of inflammation (e.g. IL-6 normal) and treatment is with anti-thyroid drugs.
AIT - Type 2
In type 2 AIT, amiodarone is directly toxic to the thyroid gland leading to a release of excess stored thyroid hormone. Importantly, there is no increased synthesis of thyroid hormone and it usually occurs in patients without pre-existing thyroid disease. Like most thyroiditis-related diseases, the initial release of thyroid hormone and thyrotoxic state is followed by a hypothyroid phase due to the thyroid gland essentially ‘running out’ of stored hormone. In AIT type 2, a radioactive uptake scan is decreased (due to no increased hormone synthesis), and there are signs of inflammation (e.g. IL-6 raised). These patients may be treated with prednisolone and need to be monitored for hypothyroidism.