Levothyroxine
Encyclopedia
Levothyroxine, also L-thyroxine, synthetic T4
, or 3,5,3',5'-tetraiodo-L-thyronine, is a synthetic form of thyroxine
(thyroid hormone), used as a hormone replacement for patients with thyroid
problems. The natural hormone is chemically in the chiral
L-form, as is the pharmaceutical agent. Dextrothyroxine (D-thyroxine) briefly saw research as an anticholesterol agent but was pulled due to cardiac side-effects.
. It may also be used to treat goiter via its ability to lower thyroid-stimulating hormone
(TSH), a hormone that is considered goiter-inducing.
levels within the normal reference range. Long-term suppression of TSH values below normal values will frequently cause cardiac side-effects and contribute to decreases in bone mineral density (high TSH levels are also well known to contribute to osteoporosis
).
Patients prescribed too high a dose of levothyroxine may experience effects that mimic hyperthyroidism. Overdose can result in heart palpitations, abdominal pain, nausea, anxiousness, confusion, agitation, insomnia
, weight loss, and increased appetite. Allergic reactions to the drug are characterized by symptoms such as difficulty breathing, shortness of breath, or swelling of the face and tongue. Acute overdose may cause fever
, hypoglycemia
, heart failure, coma
and unrecognized adrenal insufficiency
.
Acute massive overdose may be life-threatening; treatment should be symptomatic and supportive. Massive overdose may require beta-blockers for increased sympathomimetic activity.
The effects of overdosing appear 6 hours to 11 days after ingestion.
and iron
supplements within 4 hours, as well as soy products within 3 hours of the medication, as these can reduce absorption of the drug. Grapefruit juice may delay the absorption of levothyroxine, but it is not believed to have a significant effect on bioavailability. Other substances that reduce absorption are aluminium
and magnesium
containing antacids, simethicone
or sucralfate
, cholestyramine
, colestipol
, Kayexalate. A study of eight women suggested that coffee may interfere with the intestinal absorption of levothyroxine, though at a level less than eating bran. Different substances cause other adverse effects that may be severe. Ketamine
may cause hypertension
and tachycardia
and tricyclic
and tetracyclic
antidepressants increase its toxicity. On the other hand lithium
can cause hyperthyroidism (but most often hypothyroidism) by affecting iodine
metabolism of the thyroid itself and thus inhibits synthetic levothyroxine as well.
versions.
Thyroxine
Thyroxine, or 3,5,3',5'-tetraiodothyronine , a form of thyroid hormones, is the major hormone secreted by the follicular cells of the thyroid gland.-Synthesis and regulation:...
, or 3,5,3',5'-tetraiodo-L-thyronine, is a synthetic form of thyroxine
Thyroxine
Thyroxine, or 3,5,3',5'-tetraiodothyronine , a form of thyroid hormones, is the major hormone secreted by the follicular cells of the thyroid gland.-Synthesis and regulation:...
(thyroid hormone), used as a hormone replacement for patients with thyroid
Thyroid
The thyroid gland or simply, the thyroid , in vertebrate anatomy, is one of the largest endocrine glands. The thyroid gland is found in the neck, below the thyroid cartilage...
problems. The natural hormone is chemically in the chiral
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....
L-form, as is the pharmaceutical agent. Dextrothyroxine (D-thyroxine) briefly saw research as an anticholesterol agent but was pulled due to cardiac side-effects.
Medical uses
Levothyroxine is typically used to treat hypothyroidismHypothyroidism
Hypothyroidism is a condition in which the thyroid gland does not make enough thyroid hormone.Iodine deficiency is the most common cause of hypothyroidism worldwide but it can be caused by other causes such as several conditions of the thyroid gland or, less commonly, the pituitary gland or...
. It may also be used to treat goiter via its ability to lower thyroid-stimulating hormone
Thyroid-stimulating hormone
Thyrotrophin-stimulating hormone is a peptide hormone synthesized and secreted by thyrotrope cells in the anterior pituitary gland, which regulates the endocrine function of the thyroid gland.- Physiology :...
(TSH), a hormone that is considered goiter-inducing.
Adverse effects
Dosing must be carefully controlled to achieve TSHThyroid-stimulating hormone
Thyrotrophin-stimulating hormone is a peptide hormone synthesized and secreted by thyrotrope cells in the anterior pituitary gland, which regulates the endocrine function of the thyroid gland.- Physiology :...
levels within the normal reference range. Long-term suppression of TSH values below normal values will frequently cause cardiac side-effects and contribute to decreases in bone mineral density (high TSH levels are also well known to contribute to osteoporosis
Osteoporosis
Osteoporosis is a disease of bones that leads to an increased risk of fracture. In osteoporosis the bone mineral density is reduced, bone microarchitecture is deteriorating, and the amount and variety of proteins in bone is altered...
).
Patients prescribed too high a dose of levothyroxine may experience effects that mimic hyperthyroidism. Overdose can result in heart palpitations, abdominal pain, nausea, anxiousness, confusion, agitation, insomnia
Insomnia
Insomnia is most often defined by an individual's report of sleeping difficulties. While the term is sometimes used in sleep literature to describe a disorder demonstrated by polysomnographic evidence of disturbed sleep, insomnia is often defined as a positive response to either of two questions:...
, weight loss, and increased appetite. Allergic reactions to the drug are characterized by symptoms such as difficulty breathing, shortness of breath, or swelling of the face and tongue. Acute overdose may cause fever
Fever
Fever is a common medical sign characterized by an elevation of temperature above the normal range of due to an increase in the body temperature regulatory set-point. This increase in set-point triggers increased muscle tone and shivering.As a person's temperature increases, there is, in...
, hypoglycemia
Hypoglycemia
Hypoglycemia or hypoglycæmia is the medical term for a state produced by a lower than normal level of blood glucose. The term literally means "under-sweet blood"...
, heart failure, coma
Coma
In medicine, a coma is a state of unconsciousness, lasting more than 6 hours in which a person cannot be awakened, fails to respond normally to painful stimuli, light or sound, lacks a normal sleep-wake cycle and does not initiate voluntary actions. A person in a state of coma is described as...
and unrecognized adrenal insufficiency
Adrenal insufficiency
Adrenal insufficiency is a condition in which the adrenal glands, located above the kidneys, do not produce adequate amounts of steroid hormones , primarily cortisol, but may also include impaired aldosterone production which regulates sodium, potassium and water retention...
.
Acute massive overdose may be life-threatening; treatment should be symptomatic and supportive. Massive overdose may require beta-blockers for increased sympathomimetic activity.
The effects of overdosing appear 6 hours to 11 days after ingestion.
Interactions
There are also foods and other substances that can interfere with absorption of thyroxine replacement. People ought to avoid taking calciumCalcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
and iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
supplements within 4 hours, as well as soy products within 3 hours of the medication, as these can reduce absorption of the drug. Grapefruit juice may delay the absorption of levothyroxine, but it is not believed to have a significant effect on bioavailability. Other substances that reduce absorption are aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
and magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
containing antacids, simethicone
Simethicone
Simethicone is an oral anti-foaming agent used to reduce bloating, discomfort and pain caused by excess gas in the stomach or intestinal tract. It is a mixture of polydimethylsiloxane and silica gel.-Chemical action and pharmaceutical effects:...
or sucralfate
Sucralfate
Sucralfate is a cytoprotective agent, an oral gastrointestinal medication primarily indicated for the treatment of active duodenal ulcers. Brand names include Sucramal in Italy; Carafate in U.S.A.; Pepsigard, Sucral, Sucrafil, Hapifate in India; Sutra or Musin in parts of South-East Asia; Sulcrate...
, cholestyramine
Cholestyramine
Cholestyramine or colestyramine is a bile acid sequestrant, which binds bile in the gastrointestinal tract to prevent its reabsorption. It is a strong ion exchange resin, which means that it can exchange its chloride anions with anionic bile acids in the gastrointestinal tract and bind them...
, colestipol
Colestipol
Colestipol is a bile acid sequestrant.It works in the gut trapping bile salts, and preventing them from being reabsorbed.This leads todecreased enterohepatic recirculation of bile salts,increased synthesis of new bile salts by the liver,...
, Kayexalate. A study of eight women suggested that coffee may interfere with the intestinal absorption of levothyroxine, though at a level less than eating bran. Different substances cause other adverse effects that may be severe. Ketamine
Ketamine
Ketamine is a drug used in human and veterinary medicine. Its hydrochloride salt is sold as Ketanest, Ketaset, and Ketalar. Pharmacologically, ketamine is classified as an NMDA receptor antagonist...
may cause hypertension
Hypertension
Hypertension or high blood pressure is a cardiac chronic medical condition in which the systemic arterial blood pressure is elevated. What that means is that the heart is having to work harder than it should to pump the blood around the body. Blood pressure involves two measurements, systolic and...
and tachycardia
Tachycardia
Tachycardia comes from the Greek words tachys and kardia . Tachycardia typically refers to a heart rate that exceeds the normal range for a resting heart rate...
and tricyclic
Tricyclic
Tricyclics are chemical compounds that contain three interconnected rings of atoms.Many compounds have a tricyclic structure, but in pharmacology, the term has traditionally been reserved to describe heterocyclic drugs...
and tetracyclic
Tetracyclic antidepressant
Tetracyclic antidepressants are a class of drugs used primarily as antidepressants that were first introduced in the 1970s. They are named after their chemical structure which contains four rings of atoms and are closely related to the tricyclic antidepressants which contain three rings of...
antidepressants increase its toxicity. On the other hand lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
can cause hyperthyroidism (but most often hypothyroidism) by affecting iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
metabolism of the thyroid itself and thus inhibits synthetic levothyroxine as well.
Dosage
Dosages vary according to the age groups and the individual condition of the patient, body weight and compliance to the medication and diet. Monitoring of the patient's condition and adjustment of the dosage is periodical and necessary. Levothyroxine is taken on an empty stomach approximately half an hour to an hour before meals.Brand names
Common brand names include Thyrax, Euthyrox, Levaxin, L-thyroxine, Eltroxin and Thyrax Duotab in Europe; Thyrox in South Asia; Eutirox, Tirosint, Levoxyl and Synthroid in North America. There are also numerous genericGeneric drug
A generic drug is a drug defined as "a drug product that is comparable to brand/reference listed drug product in dosage form, strength, route of administration, quality and performance characteristics, and intended use." It has also been defined as a term referring to any drug marketed under its...
versions.