Levothyroxine Sodium (Page 2 of 3)

7.6 Sympathomimetics

Concurrent use may increase the effects of sympathomimetics or thyroid hormone. Thyroid hormones may increase the risk of coronary insufficiency when sympathomimetic agents are administered to patients with coronary artery disease.

7.7 Drug-Laboratory Test Interactions

Changes in thyroxine binding globulin (TBG) concentration must be considered when interpreting levothyroxine and triiodothyronine values, which necessitates measurement and evaluation of unbound (free) hormone and/or determination of the free levothyroxine index. Pregnancy, infectious hepatitis, estrogens, estrogen containing oral contraceptives, and acute intermittent porphyria increase TBG concentrations. Decreases in TBG concentrations are observed in nephrosis, severe hypoproteinemia, severe liver disease, acromegaly, and after androgen or corticosteroid therapy. Familial hyper or hypo thyroxine binding globulinemias have been described, with the incidence of TBG deficiency approximating 1 in 9000.


8.1 Pregnancy

Pregnancy Category A – There are no reported cases of Levothyroxine Sodium for Injection used to treat myxedema coma in patients who were pregnant or lactating. Studies in pregnant women treated with oral levothyroxine to maintain a euthyroid state have not shown an increased risk of fetal abnormalities. Therefore, pregnant patients who develop myxedema should be treated with Levothyroxine Sodium for Injection as the risk of non-treatment is associated with a high probability of significant morbidity or mortality to the maternal patient and the fetus.

8.2 Labor and Delivery

Patients in labor who develop myxedema have not been reported in the literature. However, patients should be treated with Levothyroxine Sodium for Injection as the risk of non-treatment is associated with a high probability of significant morbidity or mortality to the maternal patient and the fetus.

8.3 Nursing Mothers

Adequate replacement doses of thyroid hormones are required to maintain normal lactation. There are no reported cases of Levothyroxine Sodium for Injection used to treat myxedema coma in patients who are lactating. However, such patients should be treated with Levothyroxine Sodium for Injection as the risk of nontreatment is associated with a high probability of significant morbidity or mortality to the nursing patient.

8.4 Pediatric Use

Myxedema coma is a disease of the elderly. An approved, oral dosage form of levothyroxine should be used in the pediatric patient population for maintaining a euthyroid state in non-complicated hypothyroidism.

8.5 Geriatric Use and Patients with Underlying Cardiovascular Disease

See Section 2, Dosage and Administration, for full prescribing information in the geriatric patient population. Because of the increased prevalence of cardiovascular disease in the elderly, cautious use of Levothyroxine Sodium for Injection in the elderly and in patients with known cardiac risk factors is advised. Atrial fibrillation is a common side effect associated with levothyroxine treatment in the elderly [see Dosage and Administration (2) and Warnings and Precautions (5)].


In general, the signs and symptoms of overdosage with levothyroxine are those of hyperthyroidism [see Warnings and Precautions (5) and Adverse Reactions (6)]. In addition, confusion and disorientation may occur. Cerebral embolism, shock, coma, and death have been reported. Excessive doses of Levothyroxine Sodium for Injection (greater than 500 mcg) are associated with cardiac complications in patients with underlying cardiac disease.

Treatment of Overdosage

Levothyroxine Sodium for Injection should be reduced in dose or temporarily discontinued if signs or symptoms of overdosage occur. To obtain up-to-date information about the treatment of overdose, a good resource is the certified Regional Poison Control Center. In managing overdosage, consider the possibility of multiple drug overdoses, interaction among drugs, and unusual drug kinetics in the patient.

In the event of an overdose, appropriate supportive treatment should be initiated as dictated by the patient’s medical status.


Levothyroxine Sodium for Injection contains synthetic crystalline levothyroxine (L-thyroxine) sodium salt. Levothyroxine sodium has an empirical formula of C15 H10 I4 NNaO4 , a molecular weight of 798.85 g/mol (anhydrous), and the following structural formula:

Structural Formula
(click image for full-size original)

Levothyroxine Sodium for Injection is a sterile, preservative-free lyophilized powder consisting of the active ingredient, levothyroxine sodium, and the excipients dibasic sodium phosphate heptahydrate; mannitol; and sodium hydroxide, in single-dose amber glass vials. Levothyroxine Sodium for Injection is available at three dosage strengths: 100 mcg per vial, 200 mcg per vial, and 500 mcg per vial.


12.1 Mechanism of Action

Thyroid hormones exert their physiologic actions through control of DNA transcription and protein synthesis. Triiodothyronine (T3 ) and levothyroxine (T4 ) diffuse into the cell nucleus and bind to thyroid receptor proteins attached to DNA. This hormone nuclear receptor complex activates gene transcription and synthesis of messenger RNA and cytoplasmic proteins.

The physiological actions of thyroid hormones are produced predominantly by T3 , the majority of which (approximately 80%) is derived from T4 by deiodination in peripheral tissues.

12.2 Pharmacodynamics

Thyroid hormone synthesis and secretion is regulated by the hypothalamic pituitary-thyroid axis. Thyrotropin releasing hormone (TRH) released from the hypothalamus stimulates secretion of thyrotropin stimulating hormone (TSH) from the anterior pituitary. TSH, in turn, is the physiologic stimulus for the synthesis and secretion of thyroid hormones, T4 and T3 , by the thyroid gland. Circulating serum T3 and T4 levels exert a feedback effect on both TRH and TSH secretion. When serum T3 and T4 levels increase, TRH and TSH secretion decrease. When thyroid hormone levels decrease, TRH and TSH secretion increases. TSH is used for the diagnosis of hypothyroidism and evaluation of levothyroxine therapy adequacy with other laboratory and clinical data [see Dosage (2.1)]. There are drugs known to affect thyroid hormones and TSH by various mechanisms and those examples are diazepam, ethionamide, lovastatin, metoclopramide, 6-mercaptopurine, nitroprusside, perphenazine, and thiazide diuretics. Some drugs may cause a transient decrease in TSH secretion without hypothyroidism and those drugs (dose) are dopamine (greater than 1 mcg per kg per min), glucocorticoids (hydrocortisone greater than 100 mg per day or equivalent) and octreotide (greater than 100 mcg per day).

Thyroid hormones regulate multiple metabolic processes and play an essential role in normal growth and development, and normal maturation of the central nervous system and bone. The metabolic actions of thyroid hormones include augmentation of cellular respiration and thermogenesis, as well as metabolism of proteins, carbohydrates and lipids. The protein anabolic effects of thyroid hormones are essential to normal growth and development.

12.3 Pharmacokinetics

Absorption – 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.

Distribution – 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 [see Warnings and Precautions (5) and Drug Interactions (7)]. Thyroid hormones do not readily cross the placental barrier [see Warnings and Precautions (5) and Use in Specific Populations (8)].

Metabolism – T4 is slowly eliminated. The major pathway of thyroid hormone metabolism is through sequential deiodination. Approximately eighty percent of circulating T3 is derived from peripheral T4 by monodeiodination. 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. Approximately 80% of the daily dose of T4 is deiodinated to yield equal amounts of T3 and reverse T3 (r T3 ). T3 and r T3 are further deiodinated to diiodothyronine. Thyroid hormones are also metabolized via conjugation with glucuronides and sulfates and excreted directly into the bile and gut where they undergo enterohepatic recirculation.

Elimination – Thyroid hormones are primarily eliminated by the kidneys. A portion of the conjugated hormone reaches the colon unchanged, where it is hydrolyzed and eliminated in feces as the free hormones. Urinary excretion of T4 decreases with age.

Table 1: Pharmacokinetic Parameters of Thyroid Hormones in Euthyroid Patients
Hormone Ratio in Thyroglobulin Biologic Potency Half-Life (Days) Protein Binding (%) 2

T4 : Levothyroxine

T3 : Liothyronine

1 3 to 4 days in hyperthyroidism, 9 to 10 days in hypothyroidism.

2 Includes TBG, TBPA, and TBA.

T4 10 to 20 1 6 to 81 99.96
T3 1 4 ≤ 2 99.5

Drug Interactions

A listing of drug interaction with T4 is provided in the following tables, although it may not be comprehensive due to the introduction of new drugs that interact with the thyroidal axis or the discovery of previously unknown interactions. The prescriber should be aware of this fact and should consult appropriate reference sources (e.g., package inserts of newly approved drugs, medical literature) for additional information if a drug-drug interaction with levothyroxine is suspected.

Table 2: Drugs That May Alter T4 and T3 Serum Transport Without Affecting free T4 Concentration (Euthyroidism)
Drugs That May Increase Serum TBG Concentration Drugs That May Decrease Serum TBG Concentration
ClofibrateEstrogen-containing oral contraceptivesEstrogens (oral)Heroin/Methadone5-FluorouracilMitotaneTamoxifen Androgens/Anabolic SteroidsAsparaginaseGlucocorticoidsSlow-Release Nicotinic Acid
Drugs That May Cause Protein-Binding Site Displacement
Potential impact: Administration of these agents with levothyroxine results in an initial transient increase in FT4 . Continued administration results in a decrease in serum T4 and normal FT4 and TSH concentrations and, therefore, patients are clinically euthyroid.
Salicylates (>2 g/day) Salicylates inhibit binding of T4 and T3 to TBG and transthyretin. An initial increase in serum FT4 is followed by return of FT4 to normal levels with sustained therapeutic serum salicylate concentrations, although total-T4 levels may decrease by as much as 30%.
Other drugs:Furosemide (>80 mg IV)HeparinHydantoinsNon-Steroidal Anti-inflammatory Drugs- Fenamates- Phenylbutazone
Table 3: Drugs That May Alter Hepatic Metabolism of T4 (Hypothyroidism)
Potential impact: Stimulation of hepatic microsomal drug-metabolizing enzyme activity may cause increased hepatic degradation of levothyroxine, resulting in increased levothyroxine requirements.
Drug or Drug Class
CarbamazepineHydantoins Phenytoin and carbamazepine reduce serum protein binding of levothyroxine, and total- and free- T4 may be reduced by 20% to 40%, but most patients have normal serum TSH levels and are clinically euthyroid.
Other drugs:PhenobarbitalRifampin
Table 4: Drugs That May Decrease Conversion of T4 to T3
Potential impact: Administration of these enzyme inhibitors decreases the peripheral conversion of T4 to T3 , leading to decreased T3 levels. However, serum T4 levels are usually normal but may occasionally be slightly increased.
Drug or Drug Class Effect
Beta-adrenergic antagonists(e.g., Propranolol >160 mg/day) In patients treated with large doses of propranolol (>160 mg/day), T3 and T4 levels change slightly, TSH levels remain normal, and patients are clinically euthyroid. It should be noted that actions of particular beta-adrenergic antagonists may be impaired when the hypothyroid patient is converted to the euthyroid state.
Glucocorticoids(e.g., Dexamethasone ≥4 mg/day) Short-term administration of large doses of glucocorticoids may decrease serum T3 concentrations by 30% with minimal change in serum T4 levels. However, long-term glucocorticoid therapy may result in slightly decreased T3 and T4 levels due to decreased TBG production (see above).
Other drug: Amiodarone

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