Professional and Linguistic Assessments Board (PLAB) Practice Exam

Which substances are known to interact with levothyroxine?

1. Aspirin and ibuprofen

2. Iron and calcium

3. Albuterol and metformin

4. Warfarin and heparin

The correct answer is: Iron and calcium

Levothyroxine, a synthetic form of the thyroid hormone thyroxine, is commonly used in the treatment of hypothyroidism. It is important to understand that the absorption and effectiveness of levothyroxine can be significantly influenced by various substances. Iron and calcium are well-documented substances known to interact with levothyroxine. The presence of iron can bind to levothyroxine in the gastrointestinal tract, which can reduce its absorption and therapeutic efficacy. Similarly, calcium supplements can also interfere with the absorption of levothyroxine when taken concurrently. This interaction is particularly relevant because patients on levothyroxine often need to be advised on when to take these supplements in relation to their thyroid medication to avoid decreased effectiveness. In contrast, the other options listed do not significantly impact the absorption or metabolism of levothyroxine. Aspirin and ibuprofen, which are non-steroidal anti-inflammatory drugs, do not typically affect thyroid hormone absorption. Albuterol and metformin are used for asthma and diabetes management, respectively, and do not have a relevant interaction with levothyroxine. Warfarin and heparin are anticoagulants and, while there may be some considerations regarding thyroid function tests and the effect of thyroid hormones on clot