Source: FDA, National Drug Code (US) Revision Year: 2023
TIROSINT is contraindicated in patients with uncorrected adrenal insufficiency [see Warnings and Precautions (5.3)].
Overtreatment with levothyroxine may cause an increase in heart rate, cardiac wall thickness, and cardiac contractility and may precipitate angina or arrhythmias, particularly in patients with cardiovascular disease and in elderly patients. Initiate TIROSINT therapy in this population at lower doses than those recommended in younger individuals or in patients without cardiac disease [see Dosage and Administration (2.3) and Use in Specific Populations (8.5)].
Monitor for cardiac arrhythmias during surgical procedures in patients with coronary artery disease receiving suppressive TIROSINT therapy. Monitor patients receiving concomitant TIROSINT and sympathomimetic agents for signs and symptoms of coronary insufficiency . If cardiac symptoms develop or worsen, reduce the TIROSINT dose or withhold it for one week and restart at a lower dose.
Myxedema coma is a life-threatening emergency characterized by poor circulation and hypometabolism, and may result in unpredictable absorption of levothyroxine sodium from the gastrointestinal tract. Use of oral thyroid hormone drug products is not recommended to treat myxedema coma. Administer thyroid hormone products formulated for intravenous administration to treat myxedema coma.
Thyroid hormone increases metabolic clearance of glucocorticoids. Initiation of thyroid hormone therapy prior to initiating glucocorticoid therapy precipitate an acute adrenal crisis in patient with adrenal insufficiency. Treat patients with adrenal insufficiency with replacement glucocorticoids prior to initiating treatment with TIROSINT [see Contraindications (4)].
TIROSINT has a narrow therapeutic index. Over- or under-treatment with TIROSINT may have negative effects on growth and development, cardiovascular function, bone metabolism, reproductive function, cognitive function, emotional state, gastrointestinal function, and on glucose and lipid metabolism. Titrate the dose of TIROSINT carefully and monitor response to titration to avoid these effects [see Dosage and Administration (2.4)] . Monitor for the presence of drug or food interactions when using TIROSINT and adjust the dose as necessary [see Drug Interactions (7) and Clinical Pharmacology (12.3)].
Addition of levothyroxine therapy in patients with diabetes mellitus may worsen glycemic control and result in increased antidiabetic agent or insulin requirements. Carefully monitor glycemic control after starting, changing, or discontinuing thyroid hormone therapy [see Drug Interactions (7.2)] .
Increased bone resorption and decreased bone mineral density may occur as a result of levothyroxine over-replacement, particularly in post-menopausal women. The increased bone resorption may be associated with increased serum levels and urinary excretion of calcium and phosphorous, elevations in bone alkaline phosphatase, and suppressed serum parathyroid hormone levels. Administer the minimum dose of TIROSINT that achieves the desired clinical and biochemical response to mitigate against this risk.
Adverse reactions associated with TIROSINT therapy are primarily those of hyperthyroidism due to therapeutic overdosage [see Warnings and Precautions (5) and Overdosage (10)].
They include the following:
General: fatigue, increased appetite, weight loss, heat intolerance, fever, excessive sweating
Central nervous system: headache, hyperactivity, nervousness, anxiety, irritability, emotional lability, insomnia
Musculoskeletal: tremors, muscle weakness, muscle spasm
Cardiovascular: palpitations, tachycardia, arrhythmias, increased pulse and blood pressure, heart failure, angina, myocardial infarction, cardiac arrest Respiratory: dyspnea
Gastrointestinal (GI): diarrhea, vomiting, abdominal cramps, elevations in liver function tests
Dermatologic: hair loss, flushing, rash
Endocrine: decreased bone mineral density
Reproductive: menstrual irregularities, impaired fertility
Seizures have been reported rarely with the institution of levothyroxine therapy.
Pseudotumor cerebri and slipped capital femoral epiphysis have been reported in children receiving levothyroxine therapy. Overtreatment may result in craniosynostosis in infants and premature closure of the epiphyses in children with resultant compromised adult height.
Hypersensitivity reactions to inactive ingredients have occurred in patients treated with thyroid hormone products. These include urticaria, pruritus, skin rash, flushing, angioedema, various GI symptoms (abdominal pain, nausea, vomiting and diarrhea), fever, arthralgia, serum sickness and wheezing. Hypersensitivity to levothyroxine itself is not known to occur.
Many drugs can exert effects thyroid hormone pharmacokinetics (e.g., absorption, synthesis, secretion, catabolism, protein binding, and target tissue response) and may alter the therapeutic response to TIROSINT (see Tables 2 to 5 below).
Table 2. Drugs That May Decrease T4 Absorption (Hypothyroidism):
| Potential impact: Concurrent use may reduce the efficacy of TIROSINT by binding and delaying or preventing absorption, potentially resulting in hypothyroidism | |
|---|---|
| Drug or Drug Class | Effect |
| Calcium Carbonate Ferrous Sulfate | Calcium carbonate may form an insoluble chelate with levothyroxine, and ferrous sulfate likely forms a ferric-thyroxine complex. Administer TIROSINT at least 4 hours apart from these agents. |
| Orlistat | Monitor patients treated concomitantly with orlistat and TIROSINT for changes in thyroid function. |
| Bile Acid Sequestrants -Colesevelam -Cholestyramine -Colestipol Ion Exchange Resins -Kayexalate -Sevelamer | Bile acid sequestrants and ion exchange resins are known to decrease levothyroxine absorption. Administer TIROSINT at least 4 hours prior to these drugs or monitor thyrotropin (TSH) levels. |
| Other drugs: Proton Pump Inhibitors Sucralfate Antacids -Aluminum & Magnesium Hydroxides -Simethicone | Gastric acidity is an essential requirement for adequate absorption of levothyroxine. Sucralfate, antacids and proton pump inhibitors may cause hypochlorhydria, affect intragastric pH, and reduce levothyroxine absorption. Monitor patients appropriately |
Table 3. Drugs That May Alter T4 and Triiodothyronine (T3) Serum Transport Without Affecting Free Thyroxine (FT4) Concentration (Euthyroidism):
| Drug or Drug Class | Effect |
|---|---|
| Clofibrate Estrogen-containing oral contraceptives Estrogens (oral) Heroin/Methadone 5-Fluorouracil Mitotane Tamoxifen | These drugs may increase serum thyroxine-binding globulin (TBG) concentration. |
| Androgens/Anabolic Steroids Asparaginase Glucocorticoids Slow-Release Nicotinic Acid | These drugs may decrease serum TBG concentration. |
| Potential impact (below) : Administration of these agents with TIROSINT results in an initial transient increase in FT4. Continued administration results in a decrease in serum T4 and normal FT4 and TSH concentrations. | |
| 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: Carbamazepine Furosemide (>80 mg IV) Heparin Hydantoins Non-Steroidal Anti-inflammatory Drugs -Fenamates | These drugs may cause proteinbinding site displacement . Furosemide has been shown to inhibit the protein binding of T4 to TBG and albumin, causing an increased free-T4 fraction in serum. Furosemide competes for T4-binding sites on TBG, prealbumin, and albumin, so that a single high dose can acutely lower the total T4 level. 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. Closely monitor thyroid hormone parameters. |
Table 4. Drugs That May Alter Hepatic Metabolism of T4 (Hypothyroidism):
| Potential impact: Stimulation of hepatic microsomal drugmetabolizing enzyme activity may cause increased hepatic degradation of levothyroxine, resulting in increased TIROSINT requirements. | |
|---|---|
| Drug or Drug Class | Effect |
| Phenobarbital Rifampin | Phenobarbital has been shown to reduce the response to thyroxine. Phenobarbital increases L-thyroxine metabolism by inducing uridine 5'-diphosphoglucuronosyltransferase (UGT) glucuronosyltransferase (UGT) and leads to a lower T4 serum levels. Changes in thyroid status may occur if barbiturates are added or withdrawn from patients being treated for hypothyroidism. Rifampin has been shown to accelerate the metabolism of levothyroxine. |
Table 5. 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, TSH levels remain normal, and patients are clinically euthyroid. 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 Table 3 above). |
| Other: Amiodarone | Amiodarone inhibits peripheral conversion of levothyroxine (T4) to triiodothyronine (T3) and may cause isolated biochemical changes (increase in serum free-T4, and decrease or normal free-T3) in clinically euthyroid patients. |
Addition of TIROSINT therapy in patients with diabetes mellitus may worsen glycemic control and result in increased antidiabetic agent or insulin requirements. Careful monitor glycemic control, especially when thyroid therapy is started, changed, or discontinued [see Warnings and Precautions (5.5)].
TIROSINT 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 TIROSINT dose is increased. Closely monitor coagulation tests to permit appropriate and timely dosage adjustments.
TIROSINT may reduce the therapeutic effects of digitalis glycosides. Serum digitalis glycoside levels may decrease 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 TIROSINT 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 central nervous system stimulation. TIROSINT may accelerate the onset of action of tricyclics. Administration of sertraline in patients stabilized on TIROSINT may result in increased TIROSINT requirements.
Concurrent use of ketamine and TIROSINT may produce marked hypertension and tachycardia. Closely monitor blood pressure and heart rate in these patients.
Concurrent use of sympathomimetics and TIROSINT 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.
Concurrent use of tyrosine-kinase inhibitors such as imatinib may cause hypothyroidism. Closely monitor TSH levels in such patients.
Consumption of certain foods may affect TIROSINT absorption thereby necessitating adjustments in dosing [see Dosage and Administration (2.1)] . Soybean flour (infant formula), cottonseed meal, walnuts, and dietary fiber may bind and decrease the absorption of TIROSINT from the GI tract. Grapefruit juice may delay the absorption of levothyroxine and reduce its bioavailability.
Consider changes in TBG concentration when interpreting T4 and T3 values. Measure and evaluate unbound (free) hormone and/or determine the free T4 index (FT4I) in this circumstance. Pregnancy, infectious hepatitis, estrogens, estrogen-containing oral contraceptives, and acute intermittent porphyria increase TBG concentrations. Nephrosis, severe hypoproteinemia, severe liver disease, acromegaly, androgens and corticosteroids decrease TBG concentration. Familial hyper- or hypo-thyroxine binding globulinemias have been described, with the incidence of TBG deficiency approximating 1 in 9000.
Experience with levothyroxine use in pregnant women, including data from postmarketing studies, have not reported increased rates of major birth defects or miscarriages [see Data]. There are risks to the mother and fetus associated with untreated hypothyroidism in pregnancy. Since thyroid-stimulating hormone (TSH) levels may increase during pregnancy, TSH should be monitored and TIROSINT dosage adjusted during pregnancy [see Clinical Considerations] . There are no animal studies conducted with levothyroxine during pregnancy. TIROSINT should not be discontinued during pregnancy and hypothyroidism diagnosed during pregnancy should be promptly treated.
The estimated background risk of major birth defects and miscarriage for the indicated population is unknown. In the U.S. general population, the estimated background risk of major birth defects and miscarriage in clinically recognized pregnancies is 2 to 4% and 15 to 20%, respectively.
Maternal hypothyroidism during pregnancy is associated with a higher rate of complications, including spontaneous abortion, gestational hypertension, pre-eclampsia, stillbirth, and premature delivery. Untreated maternal hypothyroidism may have an adverse effect on fetal neurocognitive development.
Pregnancy may increase TIROSINT requirements. Serum TSH level should be monitored and the TIROSINT dosage adjusted during pregnancy. Since postpartum TSH levels are similar to preconception values, the TIROSINT dosage should return to the prepregnancy dose immediately after delivery [see Dosage and Administration (2.3)].
Levothyroxine is approved for use as a replacement therapy for hypothyroidism. There is a long experience of levothyroxine use in pregnant women, including data from postmarketing studies that have not reported increased rates of fetal malformations, miscarriages or other adverse maternal or fetal outcomes associated with levothyroxine use in pregnant women.
Limited published studies report that levothyroxine is present in human milk. However, there is insufficient information to determine the effects of levothyroxine on the breastfed infant and no available information on the effects of levothyroxine on milk production. Adequate levothyroxine treatment during lactation may normalize milk production in hypothyroid lactating mothers. The developmental and health benefits of breastfeeding should be considered along with the mother’s clinical need for TIROSINT and any potential adverse effects on the breastfed infant from TIROSINT or from the underlying maternal condition
TIROSINT is indicated for use in pediatric patients 6 years and older. The initial dose of TIROSINT varies with age and body weight. Dosing adjustments are based on an assessment of the individual patient’s clinical and laboratory parameters [see Dosage and Administration (2.3, 2.4)].
In children in whom a diagnosis of permanent hypothyroidism has not been established, discontinue TIROSINT administration for a trial period. Obtain serum T4 and TSH levels at the end of the trial period, and use laboratory test results and clinical assessments to guide diagnosis and treatment, if warranted.
Rapid restoration of normal serum T4 concentrations is essential for preventing the adverse effects of congenital hypothyroidism on intellectual development as well as on overall physical growth and maturation. Therefore, initiate levothyroxine therapy immediately upon diagnosis. Levothyroxine is generally continued for life in these patients.
Closely monitor children during the first two weeks of TIROSINT therapy for cardiac overload and arrhythmias.
Closely monitor patients to avoid undertreatment and overtreatment. Undertreatment may have deleterious effects on intellectual development and linear growth. Overtreatment may adversely affect the tempo of brain maturation and accelerate the bone age with resultant premature closure of the epiphyses and compromised adult stature.
Closely monitor patients to avoid undertreatment and overtreatment. Undertreatment may result in poor school performance due to impaired concentration and slowed mentation and in reduced adult height. Overtreatment may accelerate the bone age and result in premature epiphyseal closure and compromised adult stature.
Treated children may manifest a period of catch-up growth, which may be adequate in some cases to normalize adult height. In children with severe or prolonged hypothyroidism, catch-up growth may not be adequate to normalize adult height.
Because of the increased prevalence of cardiovascular disease among the elderly, initiate TIROSINT therapy at less than the full replacement dose [ see Warnings and Precautions (5.1) and Dosage and Administration (2.3)]. Atrial arrhythmias can occur in elderly patients. Atrial fibrillation is the most common of the arrhythmias observed with levothyroxine overtreatment in the elderly .
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