From the Guidelines
Changing the brand of levothyroxine can potentially affect TSH levels in some patients, and it is crucial to monitor TSH levels closely after the change. When switching between different brands or from a brand name to a generic version (or vice versa), patients should have their TSH levels monitored more closely for several weeks after the change. This is because different formulations may have slight variations in bioavailability, even though they contain the same active ingredient. According to the study published in JAMA 1, the indication for thyroid hormone therapy should be reviewed when the serum TSH concentration is between 0.1 and 0.45 mIU/L in a levothyroxine-treated individual. Some key points to consider when changing the brand of levothyroxine include:
- Monitoring TSH levels 4-6 weeks after switching to ensure the dose remains appropriate
- Taking levothyroxine consistently, on an empty stomach, 30-60 minutes before breakfast, and separate from other medications that might interfere with absorption
- Staying with the same manufacturer's product once stable thyroid function has been achieved to minimize potential variations in bioavailability. If TSH levels change significantly after switching brands, dose adjustments may be needed, and the target TSH level should be reviewed by the treating endocrinologist or other physician, as recommended by the study 1.
From the Research
Change in Levothyroxine Brand and TSH Levels
- The change in levothyroxine brand from a previous one may affect TSH levels for a patient, as switching among generic levothyroxine products made by different manufacturers can occur at the pharmacy and may impact serum thyrotropin (TSH) levels 2.
- A study published in JAMA Internal Medicine found that switching among different generic levothyroxine products was not associated with clinically significant changes in TSH level, with a mean TSH level of 2.7 mIU/L among non-switchers and 2.7 mIU/L among switchers (P = 0.94) 2.
- However, another study published in Thyroid Research found that switching from an ethanol-containing liquid levothyroxine formulation to an ethanol-free formulation resulted in a significant increase in TSH levels and a decrease in free T4 levels, suggesting that the ethanol-free formulation may be less effective over time 3.
Factors Influencing Levothyroxine Dose and TSH Levels
- Several factors can influence the levothyroxine dose and TSH levels, including pharmaceutical, pathophysiological, and behavioral factors, such as ulcerative colitis, coeliac disease, bariatric surgery, and certain medications 4.
- A study published in Reviews in Endocrine & Metabolic Disorders developed a decision tree for physicians to follow when faced with an abnormally high TSH level in a patient reporting adequate compliance with the recommended levothyroxine dose, including reviewing medications, medical history, and serum free T4 levels 4.
- The diagnosis of subclinical hypothyroidism should be confirmed by repeat thyroid function tests, and treatment is not necessary unless the TSH exceeds 7.0-10 mIU/L, with TSH goals being age-dependent 5.
Clinical Significance of TSH Levels
- The clinical significance of TSH levels is crucial in the management of hypothyroidism, with a normal serum TSH level being essential for optimal levothyroxine replacement therapy 6.
- A study published in the European Journal of Endocrinology found that mortality of hypothyroid patients treated with levothyroxine is increased when the serum TSH exceeds or is reduced outside the normal reference range, highlighting the importance of achieving a normal TSH level 6.