Clinical Thyroidology® for the Public

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Switching levothyroxine brands frequently results in abnormal thyroid function tests

CTFP Volume 13 Issue 6

Hypothyroidism is a condition in which the thyroid gland is underactive and does not produce enough thyroid hormone. Hypothyroidism can be treated with levothyroxine, the main hormone secreted by the thyroid gland. There are different manufacturers that make levothyroxine. When levothyroxine is branded, for example Synthroid™, Levoxyl™ and Tirosint™ in the United States, Thyrax in Europe, every prescription filled is for that same brand. When levothyroxine is generic (non-branded), prescriptions may be filled with levothyroxine from different manufacturers. Because potency of levothyroxine between manufacturers is variable, there is a possibility that changing brands may affect thyroid function tests. However, there is limited data on the impact of changing brands on thyroid function.

In 2016, the manufacturer of the levothyroxine brand Thyrax experienced an interruption in production that required approximately 350,000 Dutch patients to change their levothyroxine supplier. The current study examined the effect of the shortage of the levothyroxine brand Thyrax in the Netherlands and subsequent switch to a different brand on thyroid stimulating hormone (TSH) levels.

Flinterman LE et al 2020 Impact of a forced dose-equivalent levothyroxine brand switch on plasma TSH: A cohort study. Thyroid. Epub 2020 Mar 18. PMID: 32188356 Summary of the study

Patient information in 2 databases representing more than 25% of the Dutch population were used for this study. Patients taking at least 25 mcg Thyrax daily for at least one year were included. Of these, 85% were women, and the average age was ~60 years. Two groups were included: a group of patients who switched from Thyrax to another brand and a group of patients who continued taking Thyrax. TSH levels were obtained at set intervals for patients in the same group receiving the same dose of levothyroxine. In the group of patients on <100 mcg levothyroxine who continued taking Thyrax daily and had a previously normal TSH, 19% had an abnormal TSH level in follow-up. In the group of patients on <100 mcg levothyroxine who were switched and had a previously normal TSH, 24% of the group had an abnormal TSH level in follow-up. For patients on >100 mcg of levothyroxine, there was a greater difference: 24% of the group taking Thyrax and 63% of those who switched brands had abnormal TSH levels. A low/suppressed TSH was more likely to occur in patients who switched brands than those who continued on Thyrax with no brand switch.

Changing to the same dose of a different brand levothyroxine was associated with abnormal TSH levels in more patients compared to those who continued on the brand Thyrax. These results suggest that switching levothyroxine brands may require a dose change in a larger number of patients.

— Priya Mahajan, MD


Hypothyroidism: a condition where the thyroid gland is underactive and doesn’t produce enough thyroid hormone. Treatment requires taking thyroid hormone pills.

Levothyroxine (T4): the major hormone produced by the thyroid gland and available in pill form as Synthroid™, Levoxyl™, Tirosint™ and generic preparations.

Thyroid hormone therapy: patients with hypothyroidism are most often treated with Levothyroxine in order to return their thyroid hormone levels to normal. Replacement therapy means the goal is a TSH in the normal range and is the usual therapy.

TSH: thyroid stimulating hormone — produced by the pituitary gland that regulates thyroid function; also the best screening test to determine if the thyroid is functioning normally.