The frequency of dysthyroidism during treatment with amiodarone makes it necessary to watch the thyroid gland, especially since that organ may apparently be intact in such cases. Clinical observation is insufficient owing to the patients's age and to the beta-blocking effect of amiodarone. Biochemical monitoring is accurate, but amiodarone-induced abnormalities without dysthyroidism (e.g. reverse T3 levels syndrome) must be taken into account. The key to the diagnosis lies in the study of free thyroid hormones (T3 and T4), notably the ultrasensitive TSH. It is possible, although not yet demonstrated, that measuring TSH levels might be sufficient.