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Background:A new liquid levothyroxine (LT4) dissolved in glycerol and water has recently been developed by a Greek pharmaceutical company (Uni-Pharma, Athens, Greece). Objectives:To evaluate the therapeutic equivalence of this new liquid LT4 preparation versus the already existing tablet formulation of the same manufacturer, in order to obtain approval by the Greek National Organization for Medicines. Methods:This was a prospective, randomized, cross-over phase III study. The study included 50 patients (9 men and 41 non-pregnant women, with a mean age of 42.5 ± 12.5 years), with documented overt primary hypothyroidism. All subjects were well controlled on substitution therapy with various LT4 formulations. None of the patients had known LT4 malabsorption. The patients were randomized into 2 groups (A and B). The individuals of group A initially received T4® tablets for 10 ± 2 weeks and subsequently switched to T4® drops (100 μg/mL solution) at the same dose for another 10 ± 2 weeks. In group B, the reverse procedure was followed. Total T3 (T3), free T4 (fT4), and TSH were measured in all participants at enrollment and at the end of each 10 ± 2-week trial period. Results:Out of the 50 recruited patients, 6 were lost to follow-up and 5 were excluded due to non-compliance with the study protocol. In the 39 patients who completed the study, the serum TSH levels after 10 ± 2 weeks of treatment either with T4® tablets or with T4® drops did not differ (1.759 ± 1.104 vs. 2.076 ± 1.334 mIU/L, mean ± SD). Conclusions:In hypothyroid patients, the new liquid LT4 preparation (T4® drops) is therapeutically equivalent to the tablet form (T4® tablets).