What is a potential side effect of Synthroid?

Synthroid, which is a brand name for levothyroxine, is a synthetic thyroid hormone used primarily to treat hypothyroidism. A potential side effect associated with Synthroid is an increased heart rate, which can manifest as tachycardia, with the heart rate sometimes rising to or exceeding 120 beats per minute. This occurs because the medication can cause an overstimulation of the cardiovascular system, particularly if the dosage is too high or if the patient is particularly sensitive to the effects of thyroid hormones.

This cardiovascular side effect stems from the relationship between thyroid hormones and metabolism. An excess of thyroid hormone can lead to a hypermetabolic state, which increases heart rate and could contribute to further complications like palpitations or arrhythmias.

In contrast, the other options do not typically align with the side effects of Synthroid. Respiratory depression is more commonly associated with sedatives or opioids, weight gain can occur in cases of under-treatment of hypothyroidism rather than from Synthroid itself, and hypoglycemia is generally linked with insulin or oral hypoglycemics rather than thyroid replacement therapy.