Which common medication for thyroid problems can cause a fast heart rate as an adverse effect?

Levothyroxine, commonly known by its brand name Synthroid, is a medication that replaces or supplements thyroid hormone in individuals with hypothyroidism. As the thyroid hormone plays a crucial role in regulating metabolism and energy levels, an over-replacement or excessive dosage of Levothyroxine can lead to symptoms of hyperthyroidism. One of the hallmark symptoms of hyperthyroidism is a fast heart rate, or tachycardia, which can occur when there is too much thyroid hormone in the body.

The mechanism behind this is that thyroid hormone increases the metabolic rate and affects the heart's electrical system, making it beat faster. Therefore, when monitoring patients on Levothyroxine, it is essential to watch for signs of increased heart rate and other symptoms associated with excessive thyroid hormone levels, such as weight loss, anxiety, and heat intolerance.

Other options provided do not have direct links to thyroid function or metabolism. Metformin is an antidiabetic medication that primarily affects blood sugar regulation, Amitriptyline is an antidepressant that can impact mood and nerve pain, and Promethazine is an antihistamine used for allergy symptoms and nausea, none of which are associated with modifying heart rate in the context of thyroid function. Consequently, while Lev