Free thyroid hormons ration in patients with amiodaron-induced thyrotoxicosis type 1 and type 2

Differentiation between amiodarone-induced thyrotoxicosis type 1 (AmIT1) and type 2 (AmIT2) is a diagnostic challenge. The current diagnostic tests are often unable to differentiate these two types of thyrotoxic syndrome. Several studies had shown that the serum T4 level and T3/T4 ratio are significantly different in patients with destructive thyroiditis, and those with Graves` disease. These studies showed that the serum T4 concentration is significantly higher, and the T3/T4 ratio is significantly lower in patients with destructive forms of thyroiditis compared to their values  in Graves’ disease. Since AmIT1 is known to develop in patients with latent Graves` disease, and AmIT2 is a destructive thyroiditis, the purpose of our study was to evaluate the serum FТ4 level and FT4/FT3 ratio in AmIT1 and AmIT2 as an additional diagnostic test for differentiating these types of thyrotoxicosis. 45 patients with thyrotoxicosis (33 with AmIT1 and 12 AmIT2) were included in the study. The diagnosis of thyrotoxicosis type (AmIT1 or AmIT2) was established on the basis of clinical data, color flow Doppler sonography (CFDS), the presence of TSH receptor autoantibody in patients with AmIT1, as well as the effect of treatment. There was no difference in FT3 levels in patients with AmIT1 and AmIT2, while the FT4 values were significantly higher in patients with AmIT2 (36,2±19,1 mmol/L) than in those with AmIT1 (17,8±3,7 mmol/L, p= 0,002). Also, a difference in the FT4/FT3 ratio was found between AmIT1 (2,7±0,8) and AmIT2 (6,1±5,7, p=0,048). Moreover, 75% of patients with AmIT1 had FT4/FT3ratio 3,65. The FT4/FT3 ratio can be used as an additional test in the differential diagnosis AmIT1 and AmIT2.