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
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.

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Route of Elimination
Thyroid hormones are primarily eliminated by the kidneys. A portion of the conjugated hormone reaches the colon unchanged and is eliminated in the feces. Approximately 20% of T₄ is eliminated in the stool. Urinary excretion of T₄ decreases with age.

Circulating thyroid hormones are greater than 99% bound to plasma proteins, including thyroxine-binding globulin (TBG), thyroxine-binding prealbumin (TBPA), and albumin (TBA), whose capacities and affinities vary for each hormone. The higher affinity of both TBG and TBPA for T4 partially explains the higher serum levels, slower metabolic clearance, and longer half-life of T4 compared to T3. Protein-bound thyroid hormones exist in reverse equilibrium with small amounts of free hormone. Only unbound hormone is metabolically active. Many drugs and physiologic conditions affect the binding of thyroid hormones to serum proteins. Thyroid hormones do not readily cross the placental barrier. NIH; DailyMed. Current Medication Information for Synthroid (Levothyroxine Sodium) Tablet (Updated: December 2015). Available from, as of April 4, 2016: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=665c1eab-2649-498b-8da8-b15b3b743a21

Levothyroxine Sodium for Injection is administered via the intravenous route. Following administration, the synthetic levothyroxine cannot be distinguished from the natural hormone that is secreted endogenously. NIH; DailyMed. Current Medication Information for Levothyroxine Sodium (Levothyroxine Sodium) Anhydrous Injection, Powder, Lyophilized, For Solution (Updated: March 2015). Available from, as of April 4, 2016: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=f88f44d8-2f18-4155-9d78-6323d19fbafe

Absorption of orally administered T4 from the gastrointestinal (GI) tract ranges from 40% to 80%. The majority of the levothyroxine dose is absorbed from the jejunum and upper ileum. The relative bioavailability of Synthroid tablets, compared to an equal nominal dose of oral levothyroxine sodium solution, is approximately 93%. T4 absorption is increased by fasting, and decreased in malabsorption syndromes and by certain foods such as soybean infant formula. Dietary fiber decreases bioavailability of T4. Absorption may also decrease with age. In addition, many drugs and foods affect T4 absorption. NIH; DailyMed. Current Medication Information for Synthroid (Levothyroxine Sodium) Tablet (Updated: December 2015). Available from, as of April 4, 2016: https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=665c1eab-2649-498b-8da8-b15b3b743a21

Levothyroxine is variably absorbed from the GI tract (range: 40-80%). In animals, levothyroxine is absorbed in the proximal and middle jejunum; the drug is not absorbed from the stomach or distal colon and little, if any, absorption occurs in the duodenum. Studies in humans indicate that levothyroxine is absorbed from the jejunum and ileum and some absorption also occurs in the duodenum. The degree of absorption of levothyroxine from the GI tract depends on the product formulation and type of intestinal contents, including plasma protein and soluble dietary factors that may bind thyroid hormone and make it unavailable for diffusion. In addition, concurrent oral administration of infant soybean formula, soybean flour, cotton seed meal, walnuts, foods containing large amounts of fiber, ferrous sulfate, antacids, sucralfate, calcium carbonate, cation-exchange resins (e.g., sodium polystyrene sulfonate), simethicone, or bile acid sequestrants may decrease absorption of levothyroxine. The extent of levothyroxine absorption is increased in the fasting state and decreased in malabsorption states (e.g., sprue); absorption also may decrease with age. American Society of Health-System Pharmacists 2015; Drug Information 2015. Bethesda, MD. 2015, p. 3230

For more Absorption, Distribution and Excretion (Complete) data for LEVOTHYROXINE (7 total), please visit the HSDB record page.

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Metabolism / Metabolites
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.

Yields l-tyrosine in rabbit, in rat /From table/ Goodwin, B.L. Handbook of Intermediary Metabolism of Aromatic Compounds. New York: Wiley, 1976., p. T-14

Yields 3,3',5-triiodo-L-thyronine in man, rat, dog, rabbit. /From table/ Goodwin, B.L. Handbook of Intermediary Metabolism of Aromatic Compounds. New York: Wiley, 1976., p. T-14

Yields l-thyroxine-4'-beta-d-glucuronide in dog, in man, in rat. Yields l-thyroxine-4'-sulfate in dog. /From table/ Goodwin, B.L. Handbook of Intermediary Metabolism of Aromatic Compounds. New York: Wiley, 1976., p. T-14

Yields 3,3',5,5'-tetraiodothyropyruvic acid in rat. Yields l-thyronine in rat. /From table/ Goodwin, B.L. Handbook of Intermediary Metabolism of Aromatic Compounds. New York: Wiley, 1976., p. T-14

Yields 3,3'-diiodo-l-thyronine in dog. Yields 3,3',5,5'-tetraiodothyroacetic acid in man, in rat. /From table/ Goodwin, B.L. Handbook of Intermediary Metabolism of Aromatic Compounds. New York: Wiley, 1976., p. T-14

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Biological Half-Life
T₄ half-life is 6 to 7 days. T₃ half-life is 1 to 2 days.

In dogs orally administered levothyroxine has relatively ... short elimination half life when compared to humans. ... The serum half life is approximately 12-16 hours. Plumb D.C. Veterinary Drug Handbook. 8th ed. (pocket). Ames, IA: Wiley-Blackwell, 2015., p. 842

The usual plasma half-lives of thyroxine and triiodothyronine are 6-7 days and approximately 1-2 days, respectively. The plasma half-lives of thyroxine and triiodothyronine are decreased in patients with hyperthyroidism and increased in those with hypothyroidism.

Effects During Pregnancy and Lactation
◉ Summary of Use during Lactation
Levothyroxine (T4) is a normal component of human milk. Limited data on exogenous replacement doses of levothyroxine during breastfeeding indicate no adverse effects in infants. The American Thyroid Association recommends that subclinical and overt hypothyroidism should be treated with levothyroxine in lactating women seeking to breastfeed. Adequate levothyroxine treatment during lactation may normalize milk production in hypothyroid lactating mothers with low milk supply. Levothyroxine dosage requirement may be increased in the postpartum period compared to prepregnancy requirements in patients with Hashimoto's thyroiditis.
◉ Effects in Breastfed Infants
Effects of exogenous thyroid hormone administration to mothers on their infant have not been reported. One case of apparent mitigation of cretinism in hypothyroid infants by breastfeeding has been reported, but the amounts of thyroid hormones in milk are not optimal, and this result has been disputed. The thyroid hormone content of human milk from the mothers of very preterm infants appears not to be sufficient to affect the infants' thyroid status. The amounts of thyroid hormones in milk are apparently not sufficient to interfere with diagnosis of hypothyroidism.
In a telephone follow-up study, 5 nursing mothers reported taking levothyroxine (dosage unspecified). The mothers reported no adverse reactions in their infants.
One mother who had undergone a thyroidectomy was taking levothyroxine 100 mcg daily as well as calcium carbonate and calcitriol. Her breastfed infant was reportedly "thriving" at 3 months of age.
A woman with propionic acidemia took levothyroxine 50 mcg daily as well as biotin, carnitine, and various amino acids while exclusively breastfeeding her infant for 2 months and nonexclusively for 10 months. At that time, the infant had normal growth and development.
◉ Effects on Lactation and Breastmilk
Adequate thyroid hormone serum levels are required for normal lactation. Replacing deficient thyroid levels should improve milk production caused by hypothyroidism. Supraphysiologic doses would not be 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 levoisomer of thyroxine (T4) that is similar to the endogenous hormone produced by the thyroid gland. In peripheral tissues, levothyroxine is deiodinated by 5'-deiodinase to form triiodothyronine (T3). T3 enters the cell and binds to nuclear thyroid hormone receptors; the activated hormone-receptor complex in turn triggers gene expression and produces proteins required in the regulation of cellular respiration; thermogenesis; cellular growth and differentiation; and the metabolism of proteins, carbohydrates and lipids. T3 also exhibits cardiostimulatory effects.
The major hormone derived from the thyroid gland. Thyroxine is synthesized via the iodination of tyrosines (MONOIODOTYROSINE) and the coupling of iodotyrosines (DIIODOTYROSINE) in the THYROGLOBULIN. Thyroxine is released from thyroglobulin by proteolysis and secreted into the blood. Thyroxine is peripherally deiodinated to form TRIIODOTHYRONINE which exerts a broad spectrum of stimulatory effects on cell 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.)