L-Thyroxine

Absorption

Absorption of orally administered T4 from the gastrointestinal tract ranges from 40% to 80% with the majority of the levothyroxine dose absorbed from the jejunum and upper ileum. T4 absorption is increased by fasting, and decreased in malabsorption syndromes and by certain foods such as soybeans, milk, and dietary fiber. Absorption may also decrease with age. In addition, many drugs affect T4 absorption including bile acide sequestrants, sucralfate, proton pump inhibitors, and minerals such as calcium (including in yogurt and milk products), magnesium, iron, and aluminum supplements. To prevent the formation of insoluble chelates, levothyroxine should generally be taken on an empty stomach at least 2 hours before a meal and separated by at least 4 hours from any interacting agents.

Toxicity

LD50=20 mg/kg (orally in rat). Hypermetabolic state indistinguishable from thyrotoxicosis of endogenous origin. Symptoms of thyrotoxicosis include weight loss, increased appetite, palpitations, nervousness, diarrhea, abdominal cramps, sweating, tachycardia, increased pulse and blood pressure, cardiac arrhythmias, tremors, insomnia, heat intolerance, fever, and menstrual irregularities.

Description

L-Thyroxine is a synthetic form of the thyroid hormone thyroxine. In vivo, L-thyroxine (0.9 and 2.7 μg) inhibits synthesis and release of thyrotropin induced by thyrotropin-releasing hormone from the anterior pituitary in mice. It also reverses decreases in levels of circulating thymic serum factor (FTS) and the number of T rosette-forming cells in an old age-induced mouse model of hypothyroidism. Formulations containing L-thyroxine have been used in the treatment of hypothyroidism.

Description

L-Thyroxine is an amino acid that is one of two hormones located in the thyroid gland; the other is triiodothyronine1. Under the name levothyroxine, it is also a manufactured drug used to treat hypothyroidism. The molecule’s enantiomer, D-thyroxine, has no pharmacological activity.
In 1915, chemist Edward. C. Kendall at the Mayo Foundation (Rochester, MN) isolated thyroxine from unspecified animals. In the following decade, Kendall wrote several more articles on the biochemistry and pharmacology of thyroxine, including the influence of the thyroid gland on oxidation in the animal organism. Kendall received the Nobel Prize for Physiology or Medicine in 1950 for his research on adrenal gland hormones, not thyroid hormones.
In 1926, Charles R. Harington?at University College London elucidated the structure of thyroxine. The next year, he and George Barger synthesized it in several steps beginning with p-hydroxyanisole and 3,4,5-triidonitrobenzene; later in the synthesis, they introduced the tyrosine moiety.
To this day, thyroxine, familiarly known by the trade name Synthroid (Abbvie, North Chicago, IL), is the second most prescribed drug in the United States. The active ingredient is the sodium salt, not the free acid.
1. CAS Reg. No. 6893-02-3.

Chemical properties

Crystalline Solid

Originator

Synthroid,Flint,US,1953

The Uses of L-Thyroxine

antihypercholesterimic, thyromimetic

The Uses of L-Thyroxine

One of the thyroid hormones involved in the maintenance of metabolic homeostasis. Synthesized and stored as amino acid residues of thyroglobulin, the major protein component of the thyroid follicular colloid. Synthesis and secretion are regulated by the pituitary hormone (TSH). Deiodinated in peripheral tissues to the active metabolite, liothyronine. The D-form has very little activity as a thyroi d hormone, but has been used to treat hyperlipidemia.

The Uses of L-Thyroxine

Thyroxine is one of the thyroid hormones involved in the maintenance of metabolic homeostasis. Synthesized and stored as amino acid residues of thyroglobulin, the major protein component of the thyroid follicular colloid. Synthesis and secretion are regulated by the pituitary hormone (TSH). Deiodinated in peripheral tissues to the active metabolite, liothyronine. The D-form has very little activity as a thyroid hormone, but has been used to treat hyperlipidemia.

Indications

Levothyroxine is indicated as replacement therapy in primary (thyroidal), secondary (pituitary) and tertiary (hypothalamic) congenital or acquired hypothyroidism. It is also indicated as an adjunct to surgery and radioiodine therapy in the management of thyrotropin-dependent well-differentiated thyroid cancer.

Background

Levothyroxine is a synthetically produced form of thyroxine, a major endogenous hormone secreted by the thyroid gland. Also known as L-thyroxine or the brand name product Synthroid, levothyroxine is used primarily to treat hypothyroidism, a condition where the thyroid gland is no longer able to produce sufficient quantities of the thyroid hormones T4 (tetraiodothyronine or thyroxine) and T3 (triiodothyronine or Liothyronine), resulting in diminished down-stream effects of these hormones. Without sufficient quantities of circulating thyroid hormones, symptoms of hypothyroidism begin to develop such as fatigue, increased heart rate, depression, dry skin and hair, muscle cramps, constipation, weight gain, memory impairment, and poor tolerance to cold temperatures.
In response to Thyroid Stimulating Hormone (TSH) release by the pituitary gland, a normally functioning thyroid gland will produce and secrete T4, which is then converted through deiodination (by type I or type II 5′-deiodinases) into its active metabolite T3. While T4 is the major product secreted by the thyroid gland, T3 exerts the majority of the physiological effects of the thyroid hormones; T4 and T3 have a relative potency of ~1:4 (T4:T3). T4 and T3 act on nearly every cell of the body, but have a particularly strong effect on the cardiac system. As a result, many cardiac functions including heart rate, cardiac output, and systemic vascular resistance are closely linked to thyroid status.
Prior to the development of levothyroxine, Thyroid, porcine or desiccated thyroid, used to be the mainstay of treatment for hypothyroidism. However, this is no longer recommended for the majority of patients due to several clinical concerns including limited controlled trials supporting its use. Desiccated thyroid products contain a ratio of T4 to T3 of 4.2:1, which is significantly lower than the 14:1 ratio of secretion by the human thyroid gland. This higher proportion of T3 in desiccated thyroid products can lead to supraphysiologic levels of T3 which may put patients at risk of thyrotoxicosis if thyroid extract therapy is not adjusted according to the serum TSH.

What are the applications of Application

L-Thyroxine, free acid is L-Thyroxine was used to study the light-induced hormone conversion of T4 to T3 in regulation of photoperiodic response of gonads in Japanese quail. The product was used as a test compound for studying the impact of numerous hormones on the surface morphology of rat testicular cells in culture.

What are the applications of Application

Thyroxine-13C6 is a thyroid hormone involved in the maintenance of metabolic homeostasis

Definition

ChEBI: The L-enantiomer of thyroxine.

Indications

Effects of this drug depend heavily on dosage. In small doses, levothyroxine exhibits anabolic action. In medium doses, it stimulates growth and development of tissue, metabolism of protein, fats, and carbohydrates, increases functional activity of central nervous and cardiovascular systems, as well as kidneys and liver. In large doses, it slows the thyrotropic activity of the hypophysis and suppresses thyroid gland production. Levothyroxine is used for hypothyroidism, myxedema, thyrotoxicosis, erythyroid conditions, and cretinism.

Manufacturing Process

A 9.30 g portion of N-acetyl-L-diiodotyrosinamide was suspended in 100 ml of 0.05M boric acid (H3BO3) and 100 ml of 95% ethanol, and the solid was dissolved by adjusting the pH to 10.5 with 2N sodium hydroxide (NaOH). A 15% (by weight) portion of manganese sulfate monohydrate was added and the solution heated at 44°C under conditions of oxygenation while being agitated mechanically. After approximately 24 hours of incubation, the precipitated product was collected and separated from the catalyst, providing the amide of N-acetyl-L-thyroxine in 30.6% yield. On hydrolysis (removal of both amide functions), achieved by refluxing in glacial acetic acid-hydrochloric acid (approximately 2:1), L-thyroxine is obtained. It was isolated as the sodium salt, containing approximately 5 molecules of water of hydration.

brand name

Levo-T (Alara); Levolet (Vintage); Levothroid (Lloyd); Levoxyl (Jones); Novothyrox (Genpharm); Synthroid (Abbott); Unithroid (Stevens J).

Therapeutic Function

Thyroid hormone

Pharmacokinetics

Oral levothyroxine is a synthetic hormone that exerts the same physiologic effect as endogenous T4, thereby maintaining normal T4 levels when a deficiency is present.
Levothyroxine has a narrow therapeutic index and is titrated to maintain a euthyroid state with TSH (thyroid stimulating hormone) within a therapeutic range of 0.4–4.0 mIU/L. Over- or under-treatment with levothyroxine may have negative effects on growth and development, cardiovascular function, bone metabolism, reproductive function, cognitive function, emotional state, gastrointestinal function and glucose and lipid metabolism. The dose of levothyroxine should be titrated slowly and carefully and patients should be monitored for their response to titration to avoid these effects. TSH levels should be monitored at least yearly to avoid over-treating with levothyroxine which can result in hyperthyroidism (TSH <0.1mIU/L) and symptoms of increased heart rate, diarrhea, tremor, hypercalcemia, and weakness to name a few.
As many cardiac functions including heart rate, cardiac output, and systemic vascular resistance are closely linked to thyroid status, over-treatment with levothyroxine may result in increases in heart rate, cardiac wall thickness, and cardiac contractility and may precipitate angina or arrhythmias, particularly in patients with cardiovascular disease and in elderly patients. In populations with any cardiac concerns, levothyroxine should be initiated at lower doses than those recommended in younger individuals or in patients without cardiac disease. Patients receiving concomitant levothyroxine and sympathomimetic agents should be monitored for signs and symptoms of coronary insufficiency. If cardiac symptoms develop or worsen, reduce the levothyroxine dose or withhold for one week and restart at a lower dose.
Increased bone resorption and decreased bone mineral density may occur as a result of levothyroxine over-replacement, particularly in post-menopausal women. The increased bone resorption may be associated with increased serum levels and urinary excretion of calcium and phosphorous, elevations in bone alkaline phosphatase and suppressed serum parathyroid hormone levels. Administer the minimum dose of levothyroxine that achieves the desired clinical and biochemical response to mitigate this risk.
Addition of levothyroxine therapy in patients with diabetes mellitus may worsen glycemic control and result in increased antidiabetic agent or insulin requirements. Carefully monitor glycemic control after starting, changing or discontinuing levothyroxine.

Pharmacokinetics

Because of its firmer binding to carrier proteins, synthetic crystalline L-T4 sodium salt (levothyroxine sodium, Synthtoid, Euthyrox) has a slower onset of action than crystalline T3 or a desiccated thyroid preparation. Its administration leads to a greater increase in serum T4 but a lesser increase in serum T3 than compared with Thyroid USP. The availability of 11 different tablet strengths, ranging from 25 to 300 μg, allows individual dosing

Synthesis

Levothyroxine, L-3-[4-(4-hydroxy-3,5-diiodophenoxy)-3,5-diiodophenyl] alanine (25.1.10), is synthesized in a multi-stage synthesis from 4-hydroxy-3-iodo- 5-nitrobenzaldehyde. Reacting this with benzenesulfochloride in pyridine gives the corresponding benzenesulfonate 25.1.1, the benzenesulfonyl group of which is easily replaced with a 4-methoxyphenyloxy- group upon reaction with 4-methoxyphenol. The resulting 3-iodo-4-(4-methoxyphenoxy)benzaldehyde (25.1.2) is reacted further with N-acetylglycine in the presence of sodium acetate in a Knoevenagel reaction, in which the resulting ylidene compound cyclizes to an oxazolone derivative 25.1.3. The oxazolone ring of this compound is opened upon reaction with sodium methoxide, forming the desired cinnamic acid derivative 25.1.4. The nitro group of this product is reduced to an amino group by hydrogen in the presence of a Raney nickel catalyst, forming the corresponding amine, and subsequent diazotation and replacement of the diazo group of which with iodine gives the methyl ester of |á-acetamido-3,5-diiodo-4-(4-methoxyphenoxy)crotonic acid (25.1.6). The resulting compound undergoes simultaneous reaction with hydrogen iodide and phosphorous in acetic acid, in which the double bond in the crotonic acid is reduced, and the methoxy protection is removed from the phenol ring. During this, a simultaneous hydrolysis of the acetyl group on the nitrogen atom also takes place, forming D,L-3,5-diiodothyronine (25.1.7). The amino group in this product is once again protected by the reaction with formic acid in the presence of acetic anhydride, which gives D,L-N-formyl-3,5-diiodothyronine. Separation of isomers in the resulting racemic mixture is accomplished using brucine, giving D-(+)-N-formyl-3, 5-diiodothyronine L-(+)-N-formyl-3,5-diiodothyronine (25.1.8). The protecting formyl group is hydrolyzed using hydrobromic acid, giving L-(+)-3,5-diiodothyronine (25.1.9), which undergoes direct iodination using iodine in the presence of potassium iodide in aqueous methylamine, to give the desired levothyroxine.

Metabolism

Approximately 70% of secreted T4 is deiodinated to equal amounts of T3 and reverse triiodothyronine (rT3), which is calorigenically inactive. T4 is slowly eliminated through its major metabolic pathway to T3 via sequential deiodination, where approximately 80% of circulating T3 is derived from peripheral T4. The liver is the major site of degradation for both T4 and T3, with T4 deiodination also occurring at a number of additional sites, including the kidney and other tissues.
Elimination of T4 and T3 involves hepatic conjugation to glucuronic and sulfuric acids. The hormones undergo enterohepatic circulation as conjugates are hydrolyzed in the intestine and reabsorbed. Conjugated compounds that reach the colon are hydrolyzed and eliminated as free compounds in the feces. Other minor T4 metabolites have been identified.

Purification Methods

Purification is the same as for the D-isomer above. Likely impurities are tyrosine, iodotyrosine, iodothyroxines and iodide. Dissolve it in dilute ammonia at room temperature, then crystallise it by adding di[] 546 +27.8o (c 5, EtOH). [Harrington et al. Biochem J 39 164 1945, Nahm & Siedel Chem Ber 96 1 1963, Reineke & Turner J Biol Chem 161 613 1945, Chalmers et al. J Chem Soc 3424 1949, Beilstein 14 II 378, 14 III 1566, 14 IV 2373.]