Randomized, double-blind cross-over study of levothyroxine bioavailability

square Objective: The synthetic thyroid hormone levothyroxine-sodium (LT4) is still the treatment of choice to replace thyroid hormone deficiency in hypothyroidism, and for adjuvant treatment of euthyroid goiter. A change of LT4 preparations during treatment may lead to major changes of thyroid hormone levels. In this study, we compared the bioavailability of two LT4 preparations, L-Thyroxin Henning((R)) 100 and Eferox((R)) 100. square Patients and Methods: In a double-blind trial, 60 euthyroid volunteers were, randomly assigned to two treatment groups. Over a period of 2 weeks, each group received 0.1 mg/d of the different preparations according to a "cross-over design". To monitor the efficacy of the different drugs, baseline serum thyrotropin (TSH) and free thyroxine (fT(4)) levels were measured with the help of immunoenzyme tests. square Results: Compared to Eferox((R)), L-Thyroxin Henning((R)) led to continuously higher fT(4) levels (p = 0.0004). The area under the concentration-time curve (AUC) of fT(4) also confirmed this highly significant difference. With respect to the influencing factors, a higher bioavailability in men compared to women (p = 0.004) was noted. Also, the increase of body weight was related to a lower bioavailability (p = 0.002). Regarding the baseline TSH serum levels, a reduction of 70% in the L-Thyroxin Henning((R)) group versus only 56% in the, Eferox((R)) group was noted after a period of 14 days. Clinical symptoms of hyperthyroidism were not observed in the volunteers under both substances. square Conclusion: In this study, L-Thyroxin Henning((R)) 100 showed a significantly higher bioavailability than Eferox((R)) 100 (p = 0.001). According to these findings, we do recommend regular measurements of serum TSH and fT(4) levels when changing LT4 preparations of different brands, to cope with metabolic decompensation by using a new LT4 dosage.