From the Research
It's generally not recommended to take L-tyrosine supplements at the same time as levothyroxine, and these substances should be separated by at least 4 hours to avoid potential interference with levothyroxine absorption. Levothyroxine is best taken on an empty stomach, 30-60 minutes before breakfast, while L-tyrosine can be taken later in the day. The concern is that L-tyrosine, an amino acid that serves as a precursor for thyroid hormones, may interfere with the absorption of levothyroxine when taken simultaneously, potentially reducing the effectiveness of the prescribed thyroid medication 1. Additionally, since levothyroxine already provides the thyroid hormone the body needs, supplementing with L-tyrosine could theoretically lead to excessive thyroid hormone levels.
Some studies have investigated the effects of levothyroxine on thyroid health and exercise performance, but none have directly addressed the interaction between L-tyrosine and levothyroxine. For example, a study published in 2023 found that levothyroxine improved strength, mobility, and endurance performance in patients with subclinical hypothyroidism, but did not affect body composition or energy expenditure 2. Another study from 2009 found that levothyroxine replacement improved exercise tolerance in patients with subclinical hypothyroidism, but did not examine the effect of L-tyrosine supplementation 3.
It's essential to discuss the use of L-tyrosine supplements with a healthcare provider before taking them while on levothyroxine therapy, as they can provide personalized advice based on the specific thyroid condition and overall health status. The healthcare provider can help determine the best course of action and ensure that the use of L-tyrosine supplements does not interfere with the effectiveness of levothyroxine treatment. Furthermore, studies have shown that lifestyle changes, such as those that occur during Ramadan, can impact thyroid function tests in hypothyroid patients taking levothyroxine, highlighting the importance of careful management and monitoring of thyroid hormone levels 4.
In terms of specific guidance, the most recent and highest-quality study on levothyroxine interactions with food and dietary supplements found that the co-administration of levothyroxine with food depends on the drug formulation, and that certain substances, such as coffee, soy products, and fiber, can decrease levothyroxine absorption 1. However, this study did not specifically address the interaction between L-tyrosine and levothyroxine.
Overall, while there is limited direct evidence on the interaction between L-tyrosine and levothyroxine, it is generally recommended to separate these substances by at least 4 hours to avoid potential interference with levothyroxine absorption, and to discuss the use of L-tyrosine supplements with a healthcare provider before taking them while on levothyroxine therapy.