Most of the risks of Synthroid (levothyroxine) are caused by taking a dose of Synthroid that is too high. An overdose of Synthroid may cause hyperthyroidism, or overactive thyroid. Symptoms of an overactive thyroid include: increased sweating, fever, flushing, inability to tolerate heat, increased appetite, weight loss, anxiety, nervousness, irritability, hyperactivity, difficulty sleeping, weakness, joint pain, leg cramps, broken bones, osteoporosis, emotional changes, hair loss, headaches, shakiness, seizures, vomiting, diarrhea, stomach cramps, menstrual changes, fertility difficulties, high blood pressure, difficulty breathing, pain while breathing and allergic reactions (including rash, itching and swelling). Heart problems may also occur. Such heart problems include: a fast or irregular heartbeat, palpitations, heart failure, chest pain or heart attack.
People with heart disease are more likely to experience side effects. If you have heart disease, your doctor will usually give you a low dose of Synthroid to begin, and will slowly increase your dose.
Osteoporosis, which causes a loss in bone density that can lead to broken bones, is another serious risk of Synthroid. You are at greater risk for osteoporosis if your dosage of Synthroid is too high.