[Randomized, double-blind crossover study of bioavailability of levothyroxine].

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 100 and Eferox 100.

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 "crossover design". To monitor the efficacy of the different drugs, baseline serum thyrotropin (TSH) and free thyroxine (fT4) levels were measured with the help of immunoenzyme tests.

Results: Compared to Eferox, L-Thyroxin Henning led to continuously higher fT4 levels (p = 0.0004). The area under the concentration-time curve (AUC) of fT4 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 group versus only 56% in the Eferox group was noted after a period of 14 days. Clinical symptoms of hyperthyroidism were not observed in the volunteers under both substances.

Conclusion: In this study, L-Thyroxin Henning 100 showed a significantly higher bioavailability than Eferox 100 (p = 0.001). According to these findings, we do recommend regular measurements of serum TSH and fT4 levels when changing LT4 preparations of different brands, to cope with metabolic decompensation by using a new LT4 dosage.