Levothyroxine Sodium

Thyroid hormones, including Levothyroxine Sodium for Injection, should not be used for the treatment of obesity or for weight loss. ( 5.3) 
Larger doses may produce serious or even life threatening manifestations of toxicity. ( 6)

An initial intravenous loading dose of Levothyroxine Sodium for Injection between 300 to 500 mcg, followed by once daily intravenous maintenance doses between 50 and 100 mcg, should be administered, as clinically indicated, until the patient can tolerate oral therapy.  The age, general physical condition, cardiac risk factors, and clinical severity of myxedema and duration of myxedema symptoms should be considered when determining the starting and maintenance dosages of Levothyroxine Sodium for Injection.

Levothyroxine Sodium for Injection produces a gradual increase in the circulating concentrations of the hormone with an approximate half-life of 9 to 10 days in hypothyroid patients.  Daily administration of Levothyroxine Sodium for Injection should be maintained until the patient is capable of tolerating an oral dose and is clinically stable.  For chronic treatment of hypothyroidism, an oral dosage form of levothyroxine should be used to maintain a euthyroid state.  Relative bioavailability between Levothyroxine Sodium for Injection and oral levothyroxine products has not been established.  Based on medical practice, the relative bioavailability between oral and intravenous administration of Levothyroxine Sodium for Injection is estimated to be from 48 to 74%.  Due to differences in absorption characteristics of patients and the oral levothyroxine product formulations, TSH and thyroid hormone levels should be measured a few weeks after initiating oral levothyroxine and dose adjusted accordingly.

Intravenous levothyroxine may be associated with cardiac toxicity-including arrhythmias, tachycardia, myocardial ischemia and infarction, or worsening of congestive heart failure and death-in the elderly and in those with underlying cardiovascular disease.  Therefore, cautious use, including doses in the lower end of the recommended range, may be warranted in these populations.

Reconstitute the lyophilized Levothyroxine Sodium for Injection by aseptically adding 5 mL of 0.9% Sodium Chloride Injection, USP only.  Shake vial to ensure complete mixing. The resultant solution will have a final concentration of approximately 20 mcg per mL, and 100 mcg per mL for the 100 mcg and 500 mcg vials, respectively.  Reconstituted drug product is preservative free and is stable for 4 hours.  Discard any unused portion.  DO NOT ADD LEVOTHYROXINE SODIUM FOR INJECTION TO OTHER IV FLUIDS.  Parenteral drug products should be inspected visually for particulate matter and discoloration prior to administration, whenever solution and container permit.

Excessive bolus dosing of Levothyroxine Sodium for Injection (greater than 500 mcg) are associated with cardiac complications, particularly in the elderly and in patients with an underlying cardiac condition.  Adverse events that can potentially be related to the administration of large doses of Levothyroxine Sodium for Injection include arrhythmias, tachycardia, myocardial ischemia and infarction, or worsening of congestive heart failure and death.  Cautious use, including doses in the lower end of the recommended range, may be warranted in these populations.  Close observation of the patient following the administration of Levothyroxine Sodium for Injection is advised.

Occasionally, chronic autoimmune thyroiditis, which can lead to myxedema coma, may occur in association with other autoimmune disorders such as adrenal insufficiency, pernicious anemia, and insulin‑dependent diabetes mellitus.  Patients should be treated with replacement glucocorticoids prior to initiation of treatment with Levothyroxine Sodium for Injection, until adrenal function has been adequately assessed.  Failure to do so may precipitate an acute adrenal crisis when thyroid hormone therapy is initiated, due to increased metabolic clearance of glucocorticoids by thyroid hormone.  With initiation of Levothyroxine Sodium for Injection, patients with myxedema coma should also be monitored for previously undiagnosed diabetes insipidus.

Thyroid hormones, including Levothyroxine Sodium for Injection, either alone or with other therapeutic agents, should not be used for the treatment of obesity or for weight loss.  In euthyroid patients, doses within the range of daily hormonal requirements are ineffective for weight reduction.  Larger doses may produce serious or even life threatening manifestations of toxicity, particularly when given in association with sympathomimetic amines such as those used for their anorectic effects [see Adverse Reactions ( 6) and Overdosage ( 10)].

Addition of levothyroxine to antidiabetic or insulin therapy may result in increased antidiabetic agent or insulin requirements.  Careful monitoring of diabetic control is recommended, especially when thyroid therapy is started, changed, or discontinued.

Levothyroxine increases the response to oral anticoagulant therapy.  Therefore, a decrease in the dose of anticoagulant may be warranted with correction of the hypothyroid state or when the Levothyroxine Sodium for Injection dose is increased.  Prothrombin time should be closely monitored to permit appropriate and timely dosage adjustments.

The therapeutic effects of digitalis glycosides may be reduced by levothyroxine.  Serum digitalis glycoside levels may be decreased when a hypothyroid patient becomes euthyroid, necessitating an increase in the dose of digitalis glycosides.

Concurrent use of tricyclic (e.g., amitriptyline) or tetracyclic (e.g., maprotiline) antidepressants and levothyroxine may increase the therapeutic and toxic effects of both drugs, possibly due to increased receptor sensitivity to catecholamines.  Toxic effects may include increased risk of cardiac arrhythmias and CNS stimulation; onset of action of tricyclics may be accelerated. Administration of sertraline in patients stabilized on levothyroxine may result in increased levothyroxine requirements.

Concurrent use may produce marked hypertension and tachycardia; cautious administration to patients receiving thyroid hormone therapy is recommended.

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.

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 9,000.

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 nontreatment is associated with a high probability of significant morbidity or mortality to the maternal patient and the fetus.

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 nontreatment is associated with a high probability of significant morbidity or mortality to the maternal patient and the fetus.

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.

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.

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 )].

Thyroid hormones exert their physiologic actions through control of DNA transcription and protein synthesis.  Triiodothyronine (T ₃) and levothyroxine (T ₄) 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 T ₃, the majority of which (approximately 80%) is derived from T ₄ by deiodination in peripheral tissues.

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, T ₄ and T ₃, by the thyroid gland.  Circulating serum T ₃ and T ₄ levels exert a feedback effect on both TRH and TSH secretion.  When serum T ₃ and T ₄ 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, ethioamide, 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.

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 T ₄ partially explains the higher serum levels, slower metabolic clearance, and longer half life of T ₄ compared to T ₃.  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 – T ₄ is slowly eliminated.  The major pathway of thyroid hormone metabolism is through sequential deiodination.  Approximately eighty percent of circulating T ₃ is derived from peripheral T ₄ by monodeiodination.  The liver is the major site of degradation for both T ₄ and T ₃, with T ₄ deiodination also occurring at a number of additional sites, including the kidney and other tissues.  Approximately 80% of the daily dose of T ₄ is deiodinated to yield equal amounts of T ₃ and reverse T ₃ (r T ₃).  T ₃ and r T ₃ 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 T ₄ decreases with age.

Table 1: Pharmacokinetic Parameters of Thyroid Hormones in Euthyroid Patients

T ₄:  Levothyroxine

T ₃:  Liothyronine

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

² Includes TBG, TBPA, and TBA.

Drug Interactions

A listing of drug interaction with T ₄ 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 T ₄ and T ₃ Serum Transport Without Affecting free T ₄ Concentration (Euthyroidism)

Table 3:  Drugs That May Alter Hepatic Metabolism of T ₄ (Hypothyroidism)

Potential impact: Stimulation of hepatic microsomal drug-metabolizing enzyme activity may cause increased hepatic degradation of levothyroxine, resulting in increased levothyroxine requirements.

Table 4:  Drugs That May Decrease Conversion of T ₄ to T ₃

Potential impact: Administration of these enzyme inhibitors decreases the peripheral conversion of T ₄ to T ₃, leading to decreased T ₃ levels.  However, serum T ₄ levels are usually normal but may occasionally be slightly increased.

Animal studies have not been performed to evaluate the carcinogenic potential, mutagenic potential or effects on fertility of Levothyroxine Sodium for Injection.

No animal toxicology studies have been conducted with Levothyroxine Sodium for Injection.

Levothyroxine Sodium for Injection is available in two dosage strengths.

Protect from light and store dry product at 20° to 25°C (68° to 77°F) [see USP Controlled Room Temperature].  Reconstituted drug product is preservative free.  Discard any unused portion.

This container closure is not made with natural rubber latex.

PREMIERProRx ^® is a registered trademark of Premier Healthcare Alliance, L.P., used under license.