Side effects of thyroxine are generally rare but can be serious. Thyroxine is a hormone replacement drug therapy that is typically well-tolerated by the body, and when side effects of thyroxine do occur, it is usually due to a dose that is too high. Hair loss can occur when first taking the drugs, but it is a temporary side effect. Uncommon, but serious, side effects that can happen include increased sweating, mood changes, shaking, and trouble sleeping. In very rare cases, one might experience chest pain, irregular heartbeat, seizures, or possibly an allergic reaction to thyroxine.

In general, most people will have little to no side effects of thyroxine, even during pregnancy. Some people will have hair loss during the first few months on the medication. This temporary side effect usually goes away once the body has had a chance to adjust. Also, women taking thyroxine drugs might find that they have irregular menstrual cycles.

When the body has too much thyroid hormone from an overdose of thyroxine, it can lead to hyperthyroidism. Some of the uncommon side effects include increased sweating, sensitivity to heat, diarrhea, and shaking. It can also cause mood changes such as nervousness, anxiety, and irritability. Additionally, it can result in headaches, shortness of breath, muscle cramps or weakness, and trouble sleeping. Further side effects can be unusual weight loss or gain, a change in appetite, and vomiting.

In very rare cases, the side effects of thyroxine can be severe. Chest pain, pounding or irregular heartbeat, seizures, and swelling of the hands, ankles, and feet are signs of severe side effects. If one has an allergic reaction to thyroxine, the side effects can be rash or flushing, extreme dizziness, and difficulty breathing. One might also experience itching and swelling, especially of the face, tongue, or throat. In the event of an overdose or allergic reaction, one should seek medical help immediately.

On the other hand, if one does not have a high enough dose of thyroxine, it can lead to side effects from inadequate levels of thyroid, also known as hypothyroidism. These include fatigue, muscle aches, dry skin, weight gain, and sensitivity to cold. A doctor will generally increase the dose of thyroxine to treat the condition.

Thyroxine is a type of medication used to increase thyroid hormone levels in people with an underactive thyroid. Having sufficient levels of thyroid hormones is necessary for mental and physical activity as well as maintaining efficient metabolism. It can take four weeks or more for the body to receive the full benefits of the drug and to adjust to the temporary side effects of thyroxine.