Usefulness of free thyroxine to free triiodothyronine ratio for diagnostics of various types of hyperthyroidism

Abstract

Background: Different types of hyperthyroidism are treated differently. The correct diagnosis enables an adequate treatment. Clinical experience suggests that free thyroxine (fT4) to free triiodothyronine (fT3) ratio is different in different types of hyperthyroidism. Considering the paucity of literature data on the topic our aim was to evaluate the role of serum fT4 to fT3 (fT4/fT3) ratio in the diagnostics of various types of hyperthyroidism. Methods: In our retrospective clinical study we included 440 consecutive subjects, examined between February and August 2010, 350 females and 90 males aged between 15 and 97 years, among them 225 healthy subjects (HS), 80 patients with Graves’ disease (GD), 48 with toxic adenoma (TA), 61 patients with hyperthyroid Hashimoto’s thyroiditis (HHT), 17 with subacute thyroiditis (ST), and 9 patients with iodine-induced hyperthyroidism (IIH). Thyrotropin (TSH), fT4, fT3 and thyroid autoantibodies were measured. The fT4/fT3 ratio was calculated. Results: Mean fT4/fT3 ratio was significantly different for various disorders causing hyperthyroidism (p < 0.001). Compared with the mean fT4/fT3 ratio in HS (2.86 ± 0.52), the mean ratio was significantly higher in HHT and ST (3.27 ± 0.72 and 3.31 ± 0.54, respectively, p < 0.001 for both). In GD, the mean fT4/fT3 ratio was the lowest (2.55 ± 0.58) and in IIH the highest (5.13 ± 1.97). Both mean ratios significantly differed from the ratio in HS (p < 0.001 for both) and in other hyperthyroid patients (p < 0.001 for both). In patients with TA, the mean fT4/fT3 ratio was similar as in HS (2.85 ± 0.71) (p = 0.085). Conclusion: The fT4/fT3 ratio offers useful additional information for the diagnostics of thyroid disorders causing hyperthyroidism, especially in GD, where this ratio is significantly lower, and in Zdrav Vestn | Free tyhroid hormones ratio and hyperthyroidism 367 Izvirni članek/Original article IIH, where this ratio is significantly higher than in other types of hyperthyroidism.