Levothyoxine (T4)

Levothyroxine (T4)

25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg and 200 mcg

Levothyroxine (T4) is a synthetically prepared levo-isomer of thyroxine, a hormone secreted by the thyroid gland. Levothyroxine is used in the treatment of primary, secondary (pituitary), and tertiary (hypothalamic) hypothyroidism. Levothyroxine will potently suppress thyrotropin secretion in the management of goiter and chronic lymphocytic thyroiditis, and it can be used in combination with antithyroid agents to prevent the development of hypothyroidism or goitrogenesis during the treatment of thyrotoxicosis. In general, thyroid hormones influence the growth and maturation of tissues, increase energy expenditure, and affect the turnover of essentially all substrates. These effects are mediated through control of DNA transcription and, ultimately, protein synthesis. Thyroid hormones play an integral role in both anabolic and catabolic processes and are particularly important to the development of the central nervous system in newborns. They regulate cell differentiation and proliferation, and aid in the myelination of nerves and the development of axonal and dendritic processes in the nervous system. Thyroid hormones, along with somatotropin, are responsible for regulating growth, particularly of bones and teeth. Thyroid hormones also decrease cholesterol concentrations in the liver and the bloodstream, and have a direct cardiostimulatory action. Cardiac consumption is increased by the administration of thyroid hormone, resulting in an increased cardiac output. Administration of exogenous thyroid hormone to patients with hypothyroidism increases the metabolic rate by enhancing protein and carbohydrate metabolism, increasing gluconeogenesis, facilitating the mobilization of glycogen stores, and increasing protein synthesis. In response to reestablishing physiologic levels of thyroid hormone, thyroid-stimulating hormone (TSH) concentrations correct if the primary disorder is at the level of the thyroid. The release of T3 and T4 from the thyroid gland into the systemic circulation is regulated by TSH (thyrotropin), which is secreted by the anterior pituitary gland. Thyrotropin release is controlled by the secretion of thyroid-releasing hormone (TRH) from the hypothalamus and by a feedback mechanism dependent on the concentrations of circulating thyroid hormones.

Levothyroxine exhibits all the actions of endogenous thyroid hormone. Liothyronine (T3) is the principal hormone that exhibits these actions whereas levothyroxine (T4) is the major hormone secreted by the thyroid gland and is metabolically deiodinated to T3 in peripheral tissues. Serum concentrations of T4 and TSH are typically used as the primary monitoring parameters for determining thyroid function.

Oral aluminum hydroxide, magnesium salts, calcium salts, calcium carbonate, and antacids, containing any of these electrolyte salts have been reported to chelate oral levothyroxine within the GI tract when administered simultaneously, leading to decreased absorption. Some case reports have described clinical hypothyroidism resulting from coadministration of levothyroxine with oral calcium supplements and aluminum hydroxide. This interaction may also occur with liothyronine. To be prudent and to minimize this interaction, administer liothyronine at least 4 hours before or after antacids or other drugs containing aluminum, magnesium, or calcium. Polysaccharide-iron complex and other oral iron salts have been reported to chelate oral thyroid hormones within the GI tract when administered simultaneously, leading to decreased thyroid hormone absorption. Some case reports have described clinical hypothyroidism resulting from co-administration of thyroid hormones with oral iron supplements.14 To minimize the risk of interaction, oral thyroid hormones should be administered at least 4 hours before or after the ingestion of iron supplements. Cholestyramine can bind T3 and T4 in the gastrointestinal tract, impairing absorption of both hormones.8 Colestipol may have similar effects on absorption. Other cholesterol-lowering agents might also interfere with thyroid absorption.15 At least 4—6 hours should be allowed between the administration of thyroid hormones and either cholestyramine or colestipol. Administration of thyroid hormones with sucralfate16 may result in a decreased bioavailability of liothyronine. The exact mechanism of this interaction is not known, but the agents should be separated in administration.

Your health care provider needs to know if you have any of these conditions: angina; blood clotting problems; diabetes; dieting or on a weight loss program; fertility problems; heart disease; high levels of thyroid hormone; pituitary gland problem; previous heart attack; an unusual or allergic reaction to levothyroxine, thyroid hormones, other medicines, foods, dyes, or preservatives; pregnant or trying to get pregnant; breast-feeding. Levothyroxine capsules (Tirosint) are contraindicated for use in anyone unable to swallow a capsule, generally including young children < 6 years of age. Do not cut or crush the capsules. Oral levothyroxine is contraindicated in patients with an acute myocardial infarction. All levothyroxine dosage formulations are cardiostimulatory and should be used with great caution in patients with angina pectoris or other preexisting cardiac disease, including uncontrolled hypertension, cardiac arrhythmias, coronary artery disease, or a previous myocardial infarction.

Levothyroxine is classified in FDA pregnancy risk category A. Thyroid hormones undergo minimal placental transfer and human experience does not indicate adverse fetal effects; do not discontinue needed replacement during pregnancy. Also, hypothyroidism diagnosed during pregnancy should be promptly treated. Measure TSH during each trimester to gauge adequacy of thyroid replacement dosage since during pregnancy thyroid requirements may increase. Immediately after obstetric delivery, dosage should return to the pre-pregnancy dose, monitor a serum TSH 6—8 weeks postpartum to assess for needed adjustments.

Thyroid hormones, like levothyroxine, are generally compatible with breast-feeding; minimal amounts of thyroid hormones are excreted in breast milk. The American Academy of Pediatrics considers the use of levothyroxine to be usually compatible with breast-feeding due to a lack of reported adverse effects in nursing infants. It should be noted that in general, adequate thyroid replacement doses are needed to maintain normal lactation; levothyroxine is often the drug of choice to treat hypothyroidism during pregnancy and lactation. Comparisons of the levels of TSH and other thyroid function tests between breast-fed and bottle-fed infants have been published. Breast milk does not provide sufficient levothyroxine (T4) or liothyronine (T3) to prevent the effects of congenital hypothyroidism; therefore serum levels of TSH in breast-fed hypothyroid infants are markedly elevated.1314 Euthyroid babies who were breast-fed did not have differences in TSH levels when compared to euthyroid babies receiving formula feedings.

Some possible side effects include: changes in appetite; changes in menstrual periods; diarrhea; hair loss; headache; trouble sleeping; weight loss. This list may not describe all possible side effects. Call your doctor for medical advice about side effects. Clinicians should monitor for signs and symptoms of hypothyroidism that could require an upward adjustment of the thyroid hormone (e.g., levothyroxine) dosage. Signs or symptoms of underdosage or hypothyroidism include: constipation, cold intolerance, dry skin (xerosis) or hair, fatigue, impaired intellectual performance or other mental status changes (e.g., depression), deepening of voice, lethargy, weight gain, tongue enlargement, and, eventually, myxedema coma. Manifestations of excessive dosage or hyperthyroidism include anorexia, diaphoresis, diarrhea, dyspnea, elevated hepatic enzymes, emotional lability, fatigue, fever, flushing, headache, heat intolerance, hyperactivity, increased appetite, infertility, irritability, insomnia, menstrual irregularity (e.g., amenorrhea), muscle weakness, muscle cramps, nausea, vomiting, nervousness or anxiety, tremor, and weight loss. The clinician should be alert to constellations of symptoms that gradually worsen over time.

Store this medication at 68°F to 77°F (20°C to 25°C) and away from heat, moisture and light. Keep all medicine out of the reach of children. Throw away any unused medicine after the beyond use date. Do not flush unused medications or pour down a sink or drain.