Stop using levothyroxine and get emergency medical help if you have any of these signs of an allergic reaction: hives; difficulty breathing; swelling of your face, lips, tongue, or throat. Call your doctor at once if you have any of these serious side effects: headache; sleep problems (insomnia); nervous or irritable feeling; fever, hot flashes, sweating; changes in your menstrual periods; appetite changes, weight changes; Less serious side effects may include mild hair loss. This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Cardiovascular side effects have included symptoms of palpitations, hypertension, tachycardia, and angina which may be exacerbated in patients with underlying cardiovascular disorders. Ischemic heart disease and significant effects on cardiac function including an increased incidence of premature ventricular beats, an increased left ventricular mass index, and enhanced left ventricular systolic function have been reported in clinical trials.

Cardiac function was evaluated in twenty patients requiring TSH suppression for either thyroid goiter or following thyroidectomy and radioactive iodine therapy for thyroid cancer and in twenty age- and sex-matched controls. TSH suppression was associated with an increased incidence of premature ventricular beats, an increased left ventricular mass index, and enhanced left ventricular systolic function. The clinical significance of these changes remains to be determined. A 38-year-old female experienced with severe hypothyroidism experienced myxedema coma and cardiac ischemia coincident with levothyroxine therapy. After 3 months of levothyroxine therapy (initial dose: 12.5 mcg/d; maintenance dose: 125 mcg/d), all abnormal laboratory values associated with hypothyroidism returned to normal. However, three weeks after initiating treatment, the patient reported intermittent chest pains during the course of treatment, and a coronary artery angiogram revealed diffuse stenosis of all 3 branches. She underwent coronary artery bypass grafting, with subsequent improvement in coronary perfusion.

Levothyroxine is contraindicated in patients with untreated thyrotoxicosis of any etiology and in patients with acute myocardial infarction. Levothyroxine is also contraindicated in patients with uncorrected adrenal insufficiency. Increased requirements for adrenocortical hormone by tissue could precipitate an acute adrenal crisis. In patients with nontoxic diffuse goiter or nodular thyroid disease, particularly elderly patients or those with underlying cardiovascular disease, levothyroxine is contraindicated if the serum TSH level is already suppressed due to the risk of precipitating over thyrotoxicosis. If the serum TSH level is not suppressed, levothyroxine should be used cautiously in conjunction with careful monitoring of thyroid function for evidence of hyperthyroidism and clinical monitoring for potential associated adverse cardiovascular signs and symptoms of hyperthyroidism. Unless associated with hypothyroidism, thyroid hormone is not appropriate treatment of male or female infertility.

In patients with secondary or tertiary hypothyroidism, additional hypothalamic/pituitary hormone deficiencies should be considered, and, if assessed, treated. Occasionally, chronic autoimmune thyroiditis may occur in association with other autoimmune disorders such as pernicious anemia, adrenal insufficiency, and insulin-dependent diabetes mellitus. Patients with concomitant adrenal insufficiency should be treated with replacement glucocorticoids prior to initiation of therapy with levothyroxine. Failure to do so may precipitate an acute adrenal crisis when thyroid hormone treatment is initiated, due to increased metabolic clearance of glucocorticoids by thyroid hormone. Patients with diabetes mellitus may require upward adjustments of their antidiabetic therapeutic regimens when administered levothyroxine. In patients with primary (thyroidal) hypothyroidism, serum TSH levels (using a sensitive assay) alone may be used to monitor treatment. Frequency of TSH monitoring during levothyroxine dosage titration depends on the clinical situation, but it is generally recommended at 6 to 8 week intervals until normalization. For patients who have recently initiated levothyroxine and whose serum TSH has normalized or in patients who have had their dosage or brand of levothyroxine changed, the serum TSH concentration should be assessed after 8 to 12 weeks. When the optimum replacement dosage has been attained, clinical (physical examination) and biochemical monitoring may be performed every 6 to 12 months, depending on the clinical situation, and whenever there is a change in patient status. It is recommended that a physical examination and a serum TSH measurement be performed at least annually in patients receiving levothyroxine therapy.

Serum digitalis glycoside levels may be reduced in hyperthyroidism or when the hypothyroid patient is converted to the euthyroid state. Therapeutic effect of digitalis glycosides may be reduced.

Excessive use of thyroid hormones with growth hormones may accelerate epiphyseal closure. However, untreated hypothyroidism may interfere with growth response to growth hormone.

Cardiovascular side effects have included symptoms of palpitations, hypertension, tachycardia, and angina which may be exacerbated in patients with underlying cardiovascular disorders. Ischemic heart disease and significant effects on cardiac function including an increased incidence of premature ventricular beats, an increased left ventricular mass index, and enhanced left ventricular systolic function have been reported in clinical trials.

Cardiac function was evaluated in twenty patients requiring TSH suppression for either thyroid goiter or following thyroidectomy and radioactive iodine therapy for thyroid cancer and in twenty age- and sex-matched controls. TSH suppression was associated with an increased incidence of premature ventricular beats, an increased left ventricular mass index, and enhanced left ventricular systolic function. The clinical significance of these changes remains to be determined. A 38-year-old female experienced with severe hypothyroidism experienced myxedema coma and cardiac ischemia coincident with levothyroxine therapy. After 3 months of levothyroxine therapy (initial dose: 12.5 mcg/d; maintenance dose: 125 mcg/d), all abnormal laboratory values associated with hypothyroidism returned to normal. However, three weeks after initiating treatment, the patient reported intermittent chest pains during the course of treatment, and a coronary artery angiogram revealed diffuse stenosis of all 3 branches. She underwent coronary artery bypass grafting, with subsequent improvement in coronary perfusion.

Levothyroxine is usually well tolerated. Side effects associated with levothyroxine therapy typically resulted from therapeutic overdosage and included the signs and symptoms of hyperthyroidism. Weight loss, increased appetite, insomnia, anxiety, heat intolerance, diarrhea or increase in bowel frequency, palpitations, hypertension, tachycardia, angina, and menstrual irregularities may be reported. Given the long half-life of levothyroxine, such effects may not be present for several weeks after therapy initiation or dosage increases.