levothyroxine 175 mcg tablet generic synthroid, levoxyl, unithroid

Uses

Hypothyroidism

Levothyroxine sodium is used as replacement or supplemental therapy in congenital or acquired hypothyroidism of any etiology, except transient hypothyroidism during the recovery phase of subacute thyroiditis. Levothyroxine sodium is specifically indicated for use in the management of subclinical hypothyroidism and primary (thyroidal), secondary (pituitary), and tertiary (hypothalamic) hypothyroidism. Primary hypothyroidism may result from functional deficiency, primary atrophy, partial or complete absence of the thyroid gland, or from the effects of surgery, radiation, or antithyroid agents, with or without the presence of goiter.

Replacement therapy with levothyroxine sodium must be maintained continuously to control the symptoms of hypothyroidism. Levothyroxine sodium generally is the preferred thyroid agent for replacement therapy because its hormonal content is standardized and its effect is therefore predictable.

Levothyroxine sodium also is considered the drug of choice for the treatment of congenital hypothyroidism (cretinism). For a discussion on the use of levothyroxine in the treatment of congenital hypothyroidism,

Levothyroxine sodium IV injection is used in the treatment of myxedema coma. Levothyroxine sodium injection has been used in other conditions when rapid thyroid replacement is required; however, this is not an FDA-labeled use for the currently available injection.

Levothyroxine sodium may be used with antithyroid agents in the treatment of thyrotoxicosis to prevent goitrogenesis and hypothyroidism. While administration of levothyroxine occasionally may be useful to prevent antithyroid agent-induced hypothyroidism in the management of thyrotoxicosis during pregnancy, combination therapy generally is considered unnecessary since it may increase the requirement for antithyroid agents and therefore the risk of fetal hypothyroidism, which is not amenable to exogenous thyroid agent therapy.

Pituitary TSH Suppression

Levothyroxine sodium may be used to suppress the secretion of thyrotropin (thyroid-stimulating hormone, TSH) in the treatment or prevention of various types of euthyroid goiters, including thyroid nodules, subacute or chronic lymphocytic thyroiditis (Hashimoto's thyroiditis), and multinodular goiter. Levothyroxine sodium also is used as an adjunct to surgery and radioiodine therapy in the management of thyrotropin-dependent well-differentiated (papillary and follicular) thyroid cancer.

Dosage and Administration

Reconstitution and Administration

Levothyroxine sodium is administered orally or by IV injection. The drug also has been administered by IM injection; however, the IV route is preferred since absorption may be variable following IM administration. IM administration is not an FDA-labeled route of administration for the currently available levothyroxine sodium injection.

Oral Administration

Levothyroxine sodium usually is administered orally on an empty stomach, preferably one-half to one hour before breakfast or the first food of the day. Because Levoxyl tablets may rapidly swell and disintegrate following oral administration (resulting in choking, gagging, or difficulty in swallowing), the manufacturer states that Levoxyl tablets should be taken with a full glass of water.

In individuals who are unable to swallow the intact tablets (e.g., pediatric patients), the appropriate dose of levothyroxine tablets may be crushed and placed in a small amount (5-10 mL) of water; the resultant suspension should be administered by spoon or dropper immediately and should not be stored.

Foods that decrease absorption of levothyroxine (e.g., soybean infant formula, soybean flour, cotton seed meal) should not be used for administering levothyroxine. (See Pharmacokinetics: Absorption.) Oral levothyroxine sodium should be administered at least 4 hours apart from drugs that are known to interfere with its absorption (e.g., antacids, bile acid sequestrants, cation-exchange resins, ferrous sulfate, sucralfate, simethicone, calcium carbonate). .

IV Administration

Levothyroxine sodium also is administered by IV injection.

Levothyroxine sodium powder for injection should be reconstituted by adding 5 mL of 0.9% sodium chloride injection to a vial labeled as containing 100, 200, or 500 mcg of the drug, and shaking the vial to mix completely. The resultant solutions contain approximately 20, 40, or 100 mcg/mL, respectively, of levothyroxine sodium. Prior to administration, the reconstituted solution should be inspected visually for particulate matter and discoloration whenever solution and container permit. Reconstituted solutions of levothyroxine sodium should be used immediately and any unused portions discarded; the solutions should not be admixed with IV infusion solutions.(See Chemistry and Stability: Stability.)

Dosage

The FDA states that all approved levothyroxine sodium preparations should be considered therapeutically inequivalent unless equivalence has been established and noted in FDA's Approved Drug Products with Therapeutic Equivalence Evaluations (Orange Book). Theoretically, such preparations can be used interchangeably, and in some cases, pharmacists may be able to substitute generic for proprietary (brand) preparations without notifying the prescriber. However, because of the narrow therapeutic index of the drug, the American Thyroid Association (ATA) and the American Association of Clinical Endocrinologists (AACE) state that levothyroxine sodium preparations generally should not be used interchangeably. If a patient switches levothyroxine sodium preparations (e.g., from brand to generic), pharmacists are encouraged to notify the patient and prescriber of the switch. In addition, serum thyrotropin (thyroid-stimulating hormone, TSH) concentration should be measured about 4-8 weeks after starting the new preparation and the levothyroxine dosage adjusted if needed.

Dosage of levothyroxine sodium must be carefully adjusted according to individual requirements and response. The age, body weight, and general physical condition of the patient and the severity and duration of hypothyroid symptoms determine the initial dosage and rate at which dosage may be increased to the eventual maintenance dosage. Under- or over-treatment, which may result in adverse effects on growth and development in pediatric patients, cardiovascular function, bone metabolism, reproductive function, cognitive function, emotional state, GI function, and glucose and lipid metabolism, should be avoided.

Dosage should be initiated at a lower level in geriatric patients, in patients with functional or ECG evidence of cardiovascular disease, and in patients with severe, long-standing hypothyroidism, since an abrupt increase in metabolic rate and demand for increased cardiac output associated with levothyroxine therapy may precipitate angina pectoris, myocardial infarction, congestive heart failure, arrhythmias, or sudden cardiac death in such patients. In patients with severe, long-standing hypothyroidism in whom pituitary and adrenal function may be secondarily decreased, rapid replacement therapy with levothyroxine sodium also may precipitate adrenal insufficiency; in addition, psychosis or agitation occasionally may develop during initiation of levothyroxine therapy, necessitating a lower initial dosage in these patients. Adjustment of thyroid replacement therapy should be determined mainly by the patient's clinical response and confirmed by appropriate laboratory tests.

The manufacturer of levothyroxine sodium for injection states that caution should be used when switching patients from oral to IV levothyroxine sodium since the relative bioavailability and an accurate dosing conversion between the oral and IV preparations have not been established. Relative bioavailability between oral and IV administration has been estimated to range from 48-74%.

Hypothyroidism

Adult Dosage

For the management of hypothyroidism in otherwise healthy individuals younger than 50 years of age and in those older than 50 years of age who have been recently treated for hyperthyroidism or who have been hypothyroid for only a short time (i.e., several months), the usual initial oral dosage (full replacement dosage) of levothyroxine sodium is 1.7 mcg/kg daily (e.g., 100-125 mcg daily for a 70-kg adult) given as a single dose. Older patients may require less than 1 mcg/kg daily. In one study, the usual maintenance dosage for geriatric patients was about 25% less than that for younger and heavier adults. Some manufacturers state that levothyroxine sodium dosages greater than 200 mcg daily are seldom required, and that failure to respond adequately to oral dosages of 300 mcg daily or greater is rare and should prompt reevaluation of the diagnosis, or suggest the presence of malabsorption, patient noncompliance, and/or drug interactions.

Patients should be evaluated initially about every 6-8 weeks to monitor the response to therapy. Once normalization of thyroid function and serum thyrotropin (thyroid-stimulating hormone, TSH) concentrations has been achieved, patients may be evaluated less frequently (i.e., every 6-12 months). However, if the dosage of levothyroxine sodium tablets is changed, serum TSH concentrations should be measured after 8-12 weeks or 4-8 weeks after switching from one levothyroxine sodium preparation to another.

For most patients older than 50 years of age or in patients younger than 50 years of age with underlying cardiovascular disease, the usual initial oral dosage of levothyroxine sodium is 25-50 mcg daily given as a single dose; dosage may be increased at intervals of 6-8 weeks. The usual initial dosage in geriatric patients with underlying cardiovascular disease is 12.5-25 mcg daily, with gradual dosage increments at intervals of 4-6 weeks. Dosage may be increased by increments of 12.5-25 mcg at recommended intervals until the patient becomes euthyroid and serum TSH concentrations return to normal.

For the management of severe, long-standing hypothyroidism in adults, the usual initial oral dosage of levothyroxine sodium is 12.5-25 mcg daily given as a single dose. Although the manufacturers state that dosage may be increased by increments of 25 mcg at intervals of 2-4 weeks until serum TSH concentrations return to normal, some clinicians suggest that dosage may be adjusted at intervals of 4-8 weeks.

If treatment is considered necessary in patients with subclinical hypothyroidism, the manufacturers state that lower initial levothyroxine dosages (e.g., 1 mcg/kg daily) may be adequate to normalize TSH concentrations. If levothyroxine replacement therapy is not initiated, patients still should be monitored annually for changes in clinical status and thyroid laboratory parameters.

Although the average full replacement dosage of levothyroxine sodium is about 1.6-1.7 mcg/kg daily, some patients (e.g., younger pediatric patients, pregnant women) may require higher dosages.

Pediatric Dosage

Despite the smaller body size of pediatric patients, the dosage of levothyroxine sodium (on a weight basis) required to sustain a full rate of growth, development, and general thriving is higher in these patients than in adults. In general, levothyroxine sodium therapy should be initiated at full replacement dosages in pediatric patients as soon as possible after diagnosis of hypothyroidism to prevent deleterious effects on intellectual and physical growth and development; however, dosage should be initiated at a lower level in children with long-standing or severe hypothyroidism.

For the treatment of congenital hypothyroidism (cretinism) or acquired hypothyroidism in pediatric patients, levothyroxine sodium therapy usually is initiated at full replacement dosages; daily dose per body weight decreases with age. The following dosages have been recommended:

Age	Daily Dose
0-3 months	10-15 mcg/kg
3-6 months	25-50 mcg or 8-10 mcg/kg
6-12 months	50-75 mcg or 6-8 mcg/kg
1-5 years	75-100 mcg or 5-6 mcg/kg
6-12 years	100-150 mcg or 4-5 mcg/kg
Older than 12 years	> 150 mcg or 2-3 mcg/kg
Growth and puberty complete	1.6-1.7 mcg/kg

The usual initial oral dosage of levothyroxine sodium in otherwise healthy, full-term neonates is 10-15 mcg/kg daily given as a single dose. A lower initial dosage (e.g., 25 mcg daily) should be considered in neonates at risk of cardiac failure; dosage may be increased at intervals of 4-6 weeks as needed based on clinical and laboratory response to treatment. In neonates with very low (less than 5 mcg/dL) or undetectable serum thyroxine (T₄) concentrations, the usual initial dosage is 50 mcg daily.

The manufacturers state that hyperactivity in an older child may be minimized by initiating therapy at a dosage approximately one-fourth of the recommended full replacement dosage; the dosage may then be increased by an amount equal to one-fourth the full recommended replacement dosage at weekly intervals until the full recommended replacement dosage is reached.

For the treatment of severe, long-standing hypothyroidism in pediatric patients, the usual initial oral dosage of levothyroxine sodium is 25 mcg daily. Dosage may be increased in increments of 25 mcg at intervals of 2-4 weeks until the desired response is obtained.

Myxedema Coma

For the treatment of myxedema coma, levothyroxine sodium is given by IV injection; oral administration is not recommended because absorption of the drug from the GI tract is unpredictable in such patients.

Initial and maintenance dosages of IV levothyroxine sodium should be selected after taking into account the age, general physical condition, and cardiac risk factors of the patient, as well as the clinical severity and duration of myxedema symptoms. The manufacturer states that the initial adult IV loading dose for the treatment of myxedema coma is 300-500 mcg; however, some clinicians recommend an initial dose of 100-500 mcg. Lower IV maintenance dosages of 50-100 mcg once daily should be administered thereafter as clinically indicated until the patient's condition is stabilized and the drug can be given orally.

In the geriatric population and in patients with underlying cardiovascular disease, administration of IV levothyroxine sodium, especially loading doses exceeding 500 mcg, may precipitate severe adverse cardiovascular effects (e.g., arrhythmias, tachycardia, myocardial ischemia or infarction, worsening of heart failure, death). The manufacturer states that cautious use (e.g., dosages in the lower end of the recommended range) of IV levothyroxine sodium may be warranted in geriatric patients and in those with known cardiac risk factors.

Pituitary TSH Suppression

Some manufacturers caution that the target level for TSH suppression in the management of well-differentiated thyroid cancer and thyroid nodules has not been established. In addition, the efficacy of TSH suppression for benign nodular disease remains controversial. Therefore, dosage of levothyroxine sodium used for TSH suppression should be individualized based on patient characteristics and the nature of the disease.

For the management of thyrotropin-dependent well-differentiated (papillary and follicular) thyroid cancer, an oral levothyroxine sodium replacement dosage of greater than 2 mcg/kg daily given as a single dose has been recommended to suppress TSH concentrations to less than 0.1 mU/L. In patients with high-risk tumors, the target level for TSH suppression may be less than 0.01 mU/L.

For the management of benign nodules and nontoxic multinodular goiter, TSH concentrations generally are suppressed to a higher target (e.g., 0.1-0.5 mU/L for nodules and 0.5-1 mU/L for multinodular goiter) than that used for the treatment of thyroid cancer.

Cautions

Levothyroxine sodium shares the toxic potentials of other thyroid agents and the usual precautions of thyroid agent therapy should be observed. Adverse reactions to levothyroxine sodium result from overdosage and are manifested principally as signs and symptoms of hyperthyroidism (e.g., chest pain, palpitations, cardiac arrhythmias, difficulty in sleeping). Hyperthyroidism is a risk factor for osteoporosis. Evidence from several studies in premenopausal women receiving levothyroxine sodium for replacement or suppressive therapy suggests that subclinical hyperthyroidism is associated with bone loss. Therefore, to minimize the risk of osteoporosis, dosage of levothyroxine sodium should be titrated to the lowest possible effective level.(See Dosage and Administration: Dosage.) In addition, hypothyroidism manifested by severe depression, fatigue, weight gain, constipation, cold intolerance, edema, and difficulty in concentration may occur in patients receiving levothyroxine sodium preparations with inadequate potency.

Choking, gagging, dysphagia, or lodging of a tablet in the throat has been reported with Levoxyl, particularly when the tablets were not administered with water. Therefore, the manufacturer states that Levoxyl tablets should be taken with a full glass of water.

Patients with a history of lactose intolerance may be sensitive to Levothroid, Synthroid, and Unithroid tablets, since lactose is used in the manufacture of the tablets.

Pharmacokinetics

Absorption

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.

The absorption of levothyroxine is variable following IM administration (not an FDA-labeled route of administration for the currently available levothyroxine sodium injection).

In the past, results of studies evaluating the bioequivalence and interchangeability of various commercially available oral preparations of levothyroxine have been conflicting. Results of several early studies indicated that various commercially available levothyroxine sodium tablets (i.e., Levothroid, Synthroid [formulation available prior to 1982], several nonproprietary [generic] preparations) were not bioequivalent based on measurements of thyroxine content in the tablets and of thyroid function in patients receiving the preparations. Potency of oral levothyroxine sodium preparations manufactured in the US after 1985 reportedly was more uniform since USP required all manufacturers of the drug to monitor levothyroxine content and ensure tablet potency. Several reports published after 1984 indicated that the drug content of various levothyroxine preparations (Synthroid, Levothroid, Levoxine, and certain nonproprietary preparations) was within FDA specifications (i.e., no less than 90% and no more than 110% of labeled potency). However, the FDA concluded in 1997 that stability problems with oral levothyroxine sodium preparations commercially available at that time continued to result in unpredictable drug potency and announced that orally administered levothyroxine sodium products are considered new drugs; manufacturers wishing to market levothyroxine preparations were required to submit a new drug application (NDA) to the FDA.(See Chemistry and Stability: Stability.) Results of one single-blind, randomized, 4-way cross-over study in women with hypothyroidism who were clinically and chemically euthyroid and who received levothyroxine sodium 100 or 150 mcg daily for a minimum of 3 months prior to study entry suggested that the commercially available levothyroxine sodium tablets tested in this study (i.e., Levoxyl [formerly Levoxine], Synthroid, 2 nonproprietary preparations) were bioequivalent and interchangeable in the majority of patients receiving such preparations, based on measurements of levothyroxine sodium content in the tablets and of patient thyroid function. However, the FDA states that all approved levothyroxine sodium preparations should be considered therapeutically inequivalent unless equivalency has been established and noted in FDA's Approved Drug Products with Therapeutic Equivalency Evaluations (Orange Book).(See Dosage and Administration: Dosage.)

Distribution

Because thyroxine is more highly and firmly protein bound than triiodothyronine, levothyroxine has a much slower onset of pharmacologic action and a longer duration of action than liothyronine.

Elimination

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.