Impact of Switching Between Different Brands of Levothyroxine on Maintaining Normal Thyrotropin Levels: A Prospective, Multicenter, Phase IV Clinical Trial

Background This study aimed to evaluate the impact of a switch from Thyronorm or Eltroxin tablets to Lethyrox tablets on serum thyroid-stimulating hormone (TSH) levels in participants with primary hypothyroidism who were stable on Thyronorm or Eltroxin. Methodology Patients aged 18 to 65 years diagnosed with primary hypothyroidism for at least a year, who were taking a stable dose of levothyroxine tablets (Thyronorm or Eltroxin) for at least the last eight weeks and had normal serum TSH levels (0.465 to 4.68 mIU/L), were included in this prospective, multicenter, phase IV clinical trial. For a period of 18 weeks, the patients were transferred to Lethyrox tablets at the same dosage they had previously been taking. The percentage of patients with normal blood TSH levels after six weeks of switching was the main study objective. Results A total of 150 patients were enrolled, with a mean (SD) age of 45 (11) years, including 104 (69.3%) females. The mean (SD) serum TSH at baseline was 2.49 (1.12) mIU/L. Overall, 142 patients qualified for the intent-to-treat analysis. Of these, 111 (78.2%) patients maintained normal serum TSH values after six weeks of switching to Lethyrox tablet. The serum TSH level was within the normal reference range for 104 (73.2%) patients and 108 (76.1%) patients after 12 and 18 weeks of switching, respectively. A dose change was not required in 129 (90.84%) patients after switching to Lethyrox tablet at weeks 6 and 12, respectively. The mean (SD) TSH values at 6, 12, and 18 weeks were 3.55 (7.90), 3.71 (4.83), and 3.00 (2.03) mIU/L, respectively. Conclusions In the majority of individuals with primary hypothyroidism, switching brands of levothyroxine tablets was safe as it did not result in clinically significant changes in serum TSH levels.