Senior researcher Professions in Healthcare and Manpower Planning
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Impact of a forced dose-equivalent levothyroxine brand switch on plasma TSH: a cohort study.
Flinterman, L.E., Kuiper, J.G., Korevaar, J.C., Dijk, L. van, Hek, K., Houben, E., Herings, R., Franken, A.A.M., Graaf, J.P. de, Horikx, A., Janssens, M., Meijer, R., Wijbenga, A., Puijenbroek, E. van, Wolffenbuttel, B.H.R., Links, T.P., Bisschop, P.H., Fliers, E. Impact of a forced dose-equivalent levothyroxine brand switch on plasma TSH: a cohort study. Thyroid: 2020, 30(6), 821-828
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Background
Patients with primary hypothyroidism are treated with levothyroxine in order to normalize their serum TSH. Finding the optimal dosage is a long-lasting process and a small change can have major impact. Currently, limited data are available on the impact of dose equivalent substitution between brands. This study aimed to determine the effect of the shortage of the levothyroxine brand Thyrax® in the Netherlands and the resulting dose-equivalent switch to another brand on plasma TSH concentrations in a large cohort of patients.
Methods
Observational cohort study. Two registries representative for the Dutch population containing prescription and laboratory test data: the Nivel Primary Care Database and the PHARMO database network. Patients using at least 25 μg Thyrax® daily for one year or longer were included. Two cohorts were formed: a switch cohort consisting of patients who switched from Thyrax® to an alternative brand, and a Thyrax® cohort including patients who continued to use Thyrax®. Patients in the switch cohort did switch from Thyrax® to a different brand of levothyroxine in 2016 and had two consecutive TSH measurements on the same dose of levothyroxine, one before and one six weeks after the switch. Patients in the Thyrax® cohort had two consecutive TSH measurements on the same dose of Thyrax® that were six weeks apart.
Results
In the Thyrax® cohort, 19% of euthyroid patients using ≤100 μg had a TSH outside the reference range at the subsequent measurement as compared to 24% in the switch cohort (p<0.0001). For patients using ˃100 μg Thyrax® these figures were 24% and 63%, respectively (p<0.0001). Furthermore, patients using ˃50 μg Thyrax® were 4-5 times more likely to become hyperthyroid after a dose-equivalent switch to a different brand compared to patients who stayed on Thyrax®.
Conclusion
In euthyroid patients continuing the levothyroxine product Thyrax® at the same dose, TSH was out of range in 19-24% at least 6 weeks later. A dose-equivalent switch from Thyrax® to other levothyroxine brands induced biochemical signs of overdosing in an even larger proportion (24-63%) of patients. The results indicate that a dose-equivalent levothyroxine brand switch may necessitate a dose adjustment in a large number of patients.
Patients with primary hypothyroidism are treated with levothyroxine in order to normalize their serum TSH. Finding the optimal dosage is a long-lasting process and a small change can have major impact. Currently, limited data are available on the impact of dose equivalent substitution between brands. This study aimed to determine the effect of the shortage of the levothyroxine brand Thyrax® in the Netherlands and the resulting dose-equivalent switch to another brand on plasma TSH concentrations in a large cohort of patients.
Methods
Observational cohort study. Two registries representative for the Dutch population containing prescription and laboratory test data: the Nivel Primary Care Database and the PHARMO database network. Patients using at least 25 μg Thyrax® daily for one year or longer were included. Two cohorts were formed: a switch cohort consisting of patients who switched from Thyrax® to an alternative brand, and a Thyrax® cohort including patients who continued to use Thyrax®. Patients in the switch cohort did switch from Thyrax® to a different brand of levothyroxine in 2016 and had two consecutive TSH measurements on the same dose of levothyroxine, one before and one six weeks after the switch. Patients in the Thyrax® cohort had two consecutive TSH measurements on the same dose of Thyrax® that were six weeks apart.
Results
In the Thyrax® cohort, 19% of euthyroid patients using ≤100 μg had a TSH outside the reference range at the subsequent measurement as compared to 24% in the switch cohort (p<0.0001). For patients using ˃100 μg Thyrax® these figures were 24% and 63%, respectively (p<0.0001). Furthermore, patients using ˃50 μg Thyrax® were 4-5 times more likely to become hyperthyroid after a dose-equivalent switch to a different brand compared to patients who stayed on Thyrax®.
Conclusion
In euthyroid patients continuing the levothyroxine product Thyrax® at the same dose, TSH was out of range in 19-24% at least 6 weeks later. A dose-equivalent switch from Thyrax® to other levothyroxine brands induced biochemical signs of overdosing in an even larger proportion (24-63%) of patients. The results indicate that a dose-equivalent levothyroxine brand switch may necessitate a dose adjustment in a large number of patients.
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