Human Endogenous Metabolite; synthetic form of the thyroid hormone thyroxine (T4)

Thyroid stimulating hormone (TSH) levels are linked to deiodinases (DIOs), which catalyze the transformation of thyroxine (pro-hormone) to the active thyroid hormone. While DIO3 plays a role in inactivating the secretion, DIO1 and DIO2 catalyze the activation of thyroid hormone secretion. The negative feedback regulation of pituitary TSH secretion is largely dependent on the activities of DIO1 and DIO2[1]. Ionic channels, pumps, and regulatory contractile proteins are known to have their expression modulated by the hormones triiodothyronine (T3) and levothyroxine (T4). Additionally, it has been demonstrated that thyroid hormones affect the calcium flux and homeostasis that are in charge of excitation and contractility, with L-thyroxine and triiodothyronine influencing the pharmacological regulation and secretion of this process. Rats fed an iodine-free diet for 12 weeks showed a significant reduction in their levels of L-thyroxine and triiodothyronine compared to the control group fed a standard diet (p<0.001). L-thyroxine levels rise (p=0.02) in the group receiving low doses of the medication, but triiodothyronine levels essentially stay the same (p=0.19) as in the control group. Rats given large doses of L-thyroxine show a significant increase in circulating concentrations of both triiodothyronine and L-thyroxine relative to the hypothyroid group that was not treated (p<0.001 and p=0.004, respectively), as well as a significant increase in L-thyroxine levels relative to the control values (p=0.03)[2].

Thyroid-stimulating hormone (TSH) levels are correlated with the catalysis of thyroxine (prohormone) conversion to active thyroid hormone by deiodinase (DIO). Thyroid hormone secretion is activated by DIO1 and DIO2, whereas secretion is inactivated by DIO3. The regulation of pituitary TSH secretion by negative feedback is largely dependent on the actions of DIO1 and DIO2 [1]. The expression of ion channels, pumps, and regulating contractile proteins is regulated by the hormones triiodothyronine (T3) and L-thyroxine (T4). Moreover, it has been demonstrated that thyroid hormones affect calcium homeostasis and flux, which are in charge of excitation and contraction. Triiodothyronine and L-thyroxine are known to modify the pharmacological regulation and secretion of calcium. Triiodothyronine and L-thyroxine levels significantly decreased (p<0.001) in rats given an iodine-free diet for 12 weeks as compared to controls given a regular diet. Triiodothyronine levels were essentially comparable to those in the control group (p=0.19), but an increase in L-thyroxine was noted in the low-dose L-thyroxine treatment group (p=0.02). Rats treated with high-dose L-thyroxine showed significantly higher circulating concentrations of both triiodothyronine and L-thyroxine compared to the untreated hypothyroid group (p<0.001 and p=0.004, respectively), and L-thyroxine levels were significantly higher than the control value (p=0.03)[2].

Biochemical techniques[2] ELISA assays were performed using a standard rat Thyroxine (T4) and T3 ELISA kit according to the manufacturer's protocol. Western blot analysis was performed exactly as previously described.

Rats: The experiment uses 22 female Sprague-Dawley rats. There are four groups of non-pregnant rats: 1) No thyroid function, 2) hypothyroidism, 3) hypothyroidism treated with low doses of L-thyroxine (20 μg/kg/day), and 4) high doses of L-thyroxine (100 μg/kg/day). While the intervention rats (groups 2-4) are fed an iodine-free diet for 12 weeks to induce hypothyroidism, the control group (group 1) is fed a standard diet. This is followed by an additional 4 weeks of feeding to allow for L-thyroxine treatment and screening for hypothyroidism. You have unlimited access to food and water (iodine-free diet). Groups 3 and 4, which represent the hypothyroid group, receive intraperitoneal injections of 20 μg/kg and 100 μg/kg of L-thyroxine per day, respectively, every 24 hours. Within weeks 12 and 16 of starting the iodine-free or control diet, blood samples are taken for thyroid function screening. After treatment, a hysterectomy is performed under general anesthesia (isoflurane 2%), and the two uterine horns are kept in physiological Krebs' solution until isometric tension measurements are taken, which should take no longer than an hour.

Absorption
Oral T4 absorption in the gastrointestinal tract ranges from 40% to 80%, with most levothyroxine doses absorbed via the jejunum and upper ileum. Fasting increases T4 absorption, while malabsorption syndrome and certain foods (such as soy, milk, and dietary fiber) decrease it. Absorption may also decrease with age. Furthermore, many medications can affect T4 absorption, including bile acid sequestrants, sucralfate, proton pump inhibitors, and minerals such as calcium (including calcium in yogurt and dairy 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 meals and at least 4 hours apart from any interacting medications.

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Excretion
Thyroid hormones are primarily excreted through the kidneys. A portion of conjugated thyroid hormones reach the colon unchanged and are excreted in feces. Approximately 20% of T4 is excreted in feces. The amount of T4 excreted in urine decreases with age.
Over 99% of circulating thyroid hormones are bound to plasma proteins, including thyroxine-binding globulin (TBG), thyroxine-binding prealbumin (TBPA), and albumin (TBA). These proteins have varying binding affinity and affinity for each hormone. TBG and TBPA have a higher affinity for T4, which partly explains why T4 serum levels are higher than T3, its metabolic clearance is slower, and its half-life is longer. Protein-bound thyroid hormones are in an inverse equilibrium with a small amount of free hormones. Only the free hormones have metabolic activity. Many drugs and physiological conditions can affect the binding of thyroid hormones to serum proteins. Thyroid hormones do not readily cross the placental barrier. NIH; DailyMed. Latest medication information for levothyroxine sodium tablets (Synthroid) (updated December 2015).
As of April 4, 2016, information is available at: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=665c1eab-2649-498b-8da8-b15b3b743a21
Levothyroxine sodium for injection is administered intravenously. After administration, synthetic levothyroxine is indistinguishable from endogenously secreted natural hormone. National Institutes of Health; DailyMed. Latest information on the use of anhydrous levothyroxine sodium injection (lyophilized powder, for reconstitution) (updated March 2015). As of April 4, 2016, information is available at: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=f88f44d8-2f18-4155-9d78-6323d19fbafe
Oral T4 absorption in the gastrointestinal tract ranges from 40% to 80%. Most of the levothyroxine dose is absorbed in the jejunum and upper ileum. The relative bioavailability of levothyroxine tablets is approximately 93% compared to an equivalent dose of oral levothyroxine sodium solution. Fasting increases T4 absorption, while malabsorption syndrome and certain foods (such as soy infant formula) decrease T4 absorption. Dietary fiber decreases T4 bioavailability. Absorption may also decrease with age. Furthermore, many medications and foods can affect T4 absorption. National Institutes of Health; DailyMed. Latest medication information for Levothyroxine Sodium Tablets (Euthyrox) (updated December 2015). As of April 4, 2016, it is available at: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=665c1eab-2649-498b-8da8-b15b3b743a21
Individual differences exist in the gastrointestinal absorption rate of levothyroxine (range: 40-80%). In animal studies, levothyroxine is absorbed in the proximal and mid-jejunum; the drug is not absorbed in the stomach or distal colon, and absorption in the duodenum is minimal (if any). Human studies have shown that levothyroxine is absorbed in the jejunum and ileum, with some absorption also occurring in the duodenum. The extent of levothyroxine absorption in the gastrointestinal tract depends on the product formulation and the type of intestinal contents, including plasma proteins and soluble dietary factors, which may bind to thyroid hormones, preventing their diffusion. In addition, concurrent oral administration of infant formula, soy flour, cottonseed flour, walnuts, fiber-rich foods, ferrous sulfate, antacids, sucralfate, calcium carbonate, cation exchange resins (e.g., sodium polystyrene sulfonate), simethicone, or bile acid sequestrants may reduce levothyroxine absorption. Levothyroxine absorption increases in the fasting state and decreases in malabsorption states (e.g., celiac disease); absorption may also decrease with age. American Association of Health System Pharmacists, 2015; Drug Information, 2015, Bethesda, MD, 2015, p. 101. 3230
For more complete data on the absorption, distribution, and excretion of levothyroxine (7 types), please visit the HSDB record page.

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Metabolism/Metabolites
Approximately 70% of secreted T4 is deiodinated to produce an equal amount of T3 and inverse triiodothyronine (rT3), the latter of which does not produce calories. T4 is slowly metabolized to T3 via its primary metabolic pathway, which involves successive deiodination, with approximately 80% of circulating T3 originating from peripheral T4. The liver is the primary site of degradation for both T4 and T3, although deiodination of T4 also occurs in other sites, including the kidneys and other tissues. Elimination of T4 and T3 involves the liver's binding of T4 with glucuronic acid and sulfate. These hormones undergo enterohepatic circulation, where their conjugates are hydrolyzed and reabsorbed in the intestine. The conjugated compounds reaching the colon are hydrolyzed and excreted in feces as free compounds. Several other minor T4 metabolites have also been identified.
In rabbits and mice, L-tyrosine is generated. /From Table/ Goodwin, BL, Handbook of Intermediate Metabolism of Aromatic Compounds. New York: Wiley Press, 1976, p. T-14.
In humans, mice, dogs, and rabbits, 3,3',5-triiodo-L-thyroxine is generated. /From Table/ Goodwin, BL, Handbook of Intermediate Metabolism of Aromatic Compounds. New York: Wiley, 1976, p. T-14.
L-thyroxine-4'-β-D-glucuronide is produced in dogs, humans, and mice. L-thyroxine-4'-sulfate is produced in dogs. /Excerpt from Table/ Goodwin, BL, Handbook of Intermediate Metabolism of Aromatic Compounds. New York: Wiley, 1976, p. T-14.
3,3',5,5'-tetraiodothyropyruvate is produced in rats. L-thyroxine is produced in rats. /Excerpt from Table/ Goodwin, BL, Handbook of Intermediate Metabolism of Aromatic Compounds. New York: Wiley, 1976, p. T-14.
3,3'-diiodo-L-thyroxine is produced in dogs. 3,3',5,5'-tetraiodothyroacetic acid is produced in humans and rats. /Excerpt from Table/ Goodwin, BL, Handbook of Intermediate Metabolism of Aromatic Compounds. New York: Wiley Publishing, 1976, p. T-14. T-14

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Biological Half-Life: The T₄ half-life is 6 to 7 days. The T₃ half-life is 1 to 2 days.
Compared to humans, the elimination half-life of orally administered levothyroxine in dogs is relatively short. The serum half-life is approximately 12–16 hours. Plumb DC, Veterinary Medicine Handbook, 8th ed. (Pocket Edition), Ames, Iowa: Wiley-Blackwell Publishing, 2015, p. 842.
The plasma half-lives of levothyroxine and triiodothyronine are typically 6–7 days and approximately 1–2 days, respectively. The plasma half-lives of levothyroxine and triiodothyronine are shortened in patients with hyperthyroidism, while they are prolonged in patients with hypothyroidism.

Effects During Pregnancy and Lactation
◉ Overview of Medication Use During Lactation
Levothyroxine (T4) is a normal component of human breast milk. Data on exogenous levothyroxine supplementation during lactation is limited, but there is currently no evidence of adverse effects on infants. The American Thyroid Association recommends levothyroxine treatment for subclinical and clinical hypothyroidism in breastfeeding women planning to breastfeed. Adequate levothyroxine treatment during lactation may help hypothyroid breastfeeding mothers with insufficient milk production to restore normal milk production. For patients with Hashimoto's thyroiditis, the required dose of levothyroxine postpartum may be higher than before pregnancy.
◉ Effects on Breastfed Infants
There are no reports on the effects of maternal use of exogenous thyroid hormones on their infants. There have been reports that breastfeeding appears to reduce cretinism symptoms in hypothyroid infants, but the levels of thyroid hormones in breast milk are not ideal, so this result is controversial. The levels of thyroid hormones in the breast milk of mothers of extremely premature infants appear insufficient to affect the infant's thyroid function. The levels of thyroid hormones in breast milk are clearly insufficient to interfere with the diagnosis of hypothyroidism. In a telephone follow-up study, five breastfeeding mothers reported taking levothyroxine (dosage not specified). These mothers reported no adverse reactions in their infants. One mother who had undergone thyroidectomy took 100 mcg of levothyroxine daily, along with calcium carbonate and calcitriol. Her breastfed infant was reported to be "developing well" at 3 months of age. A woman with propionic acidemia took 50 mcg of levothyroxine sodium daily, along with biotin, carnitine, and multiple amino acids, while exclusively breastfeeding her infant for 2 months, followed by 10 months of mixed feeding. At that time, the infant's growth and development were normal. ◉ Effects on Lactation and Breast Milk Adequate serum thyroid hormone levels are necessary for normal lactation. Thyroid hormone supplementation can improve milk production caused by hypothyroidism. Supraphysiological doses are not expected to further improve lactation.

[1]. Association between genetic polymorphism and levothyroxine bioavailability in hypothyroid patients. Endocr J. 2018 Mar 28;65(3):317-323.

[2]. Levothyroxine treatment generates an abnormal uterine contractility patterns in an in vitro animalmodel. J Clin Transl Endocrinol. 2015 Sep 9;2(4):144-149.

Levothyroxine sodium is the sodium salt of levothyroxine, a synthetic levorotatory isomer of thyroxine (T4), structurally similar to endogenous hormones produced by the thyroid gland. In peripheral tissues, levothyroxine is deiodinated by 5'-deiodinase to form triiodothyronine (T3). T3 enters cells and binds to thyroid hormone receptors in the nucleus; the activated hormone-receptor complex then triggers gene expression, producing proteins required for cellular respiration, thermoproduction, cell growth and differentiation, and protein, carbohydrate, and lipid metabolism. T3 also has a cardiac excitatory effect. The main hormone of the thyroid gland is thyroxine. Thyroxine is synthesized through the iodination of tyrosine (monoiodotyrosine) and the coupling of iodotyrosine (diiodotyrosine) in thyroglobulin. Thyroxine is released from thyroglobulin via proteolytic hydrolysis and secreted into the bloodstream. Thyroxine is deiodinated in peripheral tissues to form triiodothyronine, which has a broad stimulatory effect on cellular metabolism.

C15H20I4NNAO9

888.926400000001

888.721

6106-07-6

Thyroxine sulfate;77074-49-8;L-Thyroxine;51-48-9;L-Thyroxine sodium;55-03-8

23665037

White to light yellow solid

2.381

207-210 (dec.)(lit.)

3.601

7

10

5

30

426

1

[O-]C([C@H](CC1=CC(I)=C(C(I)=C1)OC2=CC(I)=C(O)C(I)=C2)N)=O.[Na+].O.O.O.O.O

JMHCCAYJTTWMCX-QWPJCUCISA-M

InChI=1S/C15H11I4NO4.Na.5H2O/c16-8-4-7(5-9(17)13(8)21)24-14-10(18)1-6(2-11(14)19)3-12(20)15(22)23;;;;;;/h1-2,4-5,12,21H,3,20H2,(H,22,23);;5*1H2/q;+1;;;;;/p-1/t12-;;;;;;/m0....../s1

sodium;(2S)-2-amino-3-[4-(4-hydroxy-3,5-diiodophenoxy)-3,5-diiodophenyl]propanoate;pentahydrate

L-Thyroxine sodium salt pentahydrate; L-Thyroxine sodium salt pentahydrate; 6106-07-6; L-Thyroxine sodium pentahydrate; Sodium L-thyroxine pentahydrate; levothyroxine sodium pentahydrate; Levothyroxine sodium; Sodium levothyroxine; eltroxin; Sodium levothyroxine pentahydrate

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Note: Please store this product in a sealed and protected environment, avoid exposure to moisture.

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

DMSO: ~100 mg/mL (~112.5 mM)

Solubility in Formulation 1: ≥ 2.5 mg/mL (2.81 mM) (saturation unknown) in 10% DMSO + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 to the above solution and mix evenly; then add 450 μL normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 2: 2.5 mg/mL (2.81 mM) in 10% DMSO + 90% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), suspension solution; with ultrasonication.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of 20% SBE-β-CD physiological saline solution and mix evenly.
Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.

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 (Please use freshly prepared in vivo formulations for optimal results.)