The signs and symptoms of overdosage are those of hyperthyroidism. In addition, confusion and disorientation may occur. Cerebral embolism, shock, coma, and death have been reported. Seizures occurred in a 3-year-old child ingesting 3.6 mg of levothyroxine. Symptoms may not necessarily be evident or may not appear until several days after ingestion of levothyroxine sodium.
Reduce the THYRO-TABS dose or discontinue temporarily if signs or symptoms of overdosage occur. Initiate appropriate supportive treatment as dictated by the patient’s medical status.
For current information on the management of poisoning or overdosage, contact the National Poison Control Center at 1-800-222-1222 or www.poison.org.
THYRO-TABS is contraindicated in patients with uncorrected adrenal insufficiency.
Adverse reactions associated with THYRO-TABS therapy are primarily those of hyperthyroidism due to therapeutic overdosage. They include the following:
Seizures have been reported rarely with the institution of levothyroxine therapy.
Adverse Reactions In ChildrenPseudotumor 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 ReactionsHypersensitivity reactions to inactive ingredients have occurred in patients treated with thyroid hormone products. These include urticaria, pruritus, skin rash, flushing, angioedema, various gastrointestinal symptoms (abdominal pain, nausea, vomiting and diarrhea), fever, arthralgia, serum sickness, and wheezing. Hypersensitivity to levothyroxine itself is not known to occur.
Oral levothyroxine sodium is a synthetic T4 hormone that exerts the same physiologic effect as endogenous T4, thereby maintaining normal T4 levels when a deficiency is present.
Many drugs can exert effects on thyroid hormone pharmacokinetics and metabolism (e.g., absorption, synthesis, secretion, catabolism, protein binding, and target tissue response) and may alter the therapeutic response to THYRO-TABS (see Tables 2-5 below).
Table 2. Drugs That May Decrease T4 Absorption (Hypothyroidism)
| Potential impact: Concurrent use may reduce the efficacy of THYRO-TABS 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 THYRO-TABS at least 4 hours apart from these agents. |
| Orlistat | Monitor patients treated concomitantly with orlistat and THYRO-TABS 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 THYRO- TABS at least 4 hours prior to these drugs or monitor 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 THYRO-TABS 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 -Fenamtes | These drugs may cause protein-binding site displacement.Furosemide has been shown to inhibit the protein binding of T4 to TBG and albumin, causing an increase 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 drug-metabolizing enzyme activity may cause increased hepatic degradation of levothyroxine, resulting in increased THYRO-TABS 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’-diphospho-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 betaadrenergic antagonists may be impaired when a 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 drugs: Amiodarone | Amiodarone inhibits peripheral conversion of levothyroxine (T4) to triiodothyronine (T3) and may cause isolated biochemical changes (increase in serum free-T4, and decreased or normal free-T3) in clinically euthyroid patients. |
Included as part of the "PRECAUTIONS" Section
PRECAUTIONS Cardiac Adverse Reactions In The Elderly And In Patients With Underlying Cardiovascular DiseaseOver-treatment 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 THYRO-TABS therapy in this population at lower doses than those recommended in younger individuals or in patients without cardiac disease.
Monitor for cardiac arrhythmias during surgical procedures in patients with coronary artery disease receiving suppressive THYRO-TABS therapy. Monitor patients receiving concomitant THYROTABS and sympathomimetic agents for signs and symptoms of coronary insufficiency.
If cardiac symptoms develop or worsen, reduce the THYRO-TABS dose or withhold for one week and restart at a lower dose.
Myxedema ComaMyxedema 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.
Acute Adrenal Crisis In Patients With Concomitant Adrenal InsufficiencyThyroid hormone increases metabolic clearance of glucocorticoids. Initiation of thyroid hormone therapy prior to initiating glucocorticoid therapy may precipitate an acute adrenal crisis in patients with adrenal insufficiency. Treat patients with adrenal insufficiency with replacement glucocorticoids prior to initiating treatment with THYRO-TABS.
Prevention Of Hyperthyroidism Or Incomplete Treatment Of HypothyroidismTHYRO-TABS has a narrow therapeutic index. Over-or undertreatment with THYRO-TABS may have negative effects on growth and development, cardiovascular function, bone metabolism, reproductive function, cognitive function, emotional state, gastrointestinal function, and glucose and lipid metabolism. Titrate the dose of THYRO-TABS carefully and monitor response to titration to avoid these effects. Monitor for the presence of drug or food interactions when using THYRO-TABS and adjust the dose as necessary.
Worsening Of Diabetic ControlAddition 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 THYRO-TABS.
Decreased Bone Mineral Density Associated With Thyroid Hormone Over-ReplacementIncreased 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 THYRO-TABS that achieves the desired clinical and biochemical response to mitigate this risk.
Nonclinical Toxicology Carcinogenesis, Mutagenesis, Impairment Of FertilityStandard animal studies have not been performed to evaluate the carcinogenic potential, mutagenic potential or effects on fertility of levothyroxine.
Use In Specific Populations Pregnancy Risk SummaryExperience with levothyroxine use in pregnant women, including data from post-marketing 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 TSH levels may increase during pregnancy, TSH should be monitored and THYRO-TABS dosage adjusted during pregnancy [see Clinical Considerations]. There are no animal studies conducted with levothyroxine during pregnancy. THYRO-TABS 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.
Clinical ConsiderationsDisease-Associated Maternal andor Embryo/Fetal Risk
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.
Dose Adjustments During Pregnancy and the Postpartum Period
Pregnancy may increase THYRO-TABS requirements. Serum TSH levels should be monitored and the THYRO-TABS dosage adjusted during pregnancy. Since postpartum TSH levels are similar to preconception values, the THYRO-TABS dosage should return to the pre-pregnancy dose immediately after delivery.
DataHuman Data
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 post-marketing 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.
Lactation Risk SummaryLimited 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 THYRO-TABS and any potential adverse effects on the breastfed infant from THYRO-TABS or from the underlying maternal condition.
Pediatric UseThe initial dose of THYRO-TABS varies with age and body weight. Dosing adjustments are based on an assessment of the individual patient's clinical and laboratory parameters.
In children in whom a diagnosis of permanent hypothyroidism has not been established, discontinue THYRO-TABS administration for a trial period, but only after the child is at least 3 years of age. Obtain serum T4 and TSH levels at the end of the trial period, and use laboratory test results and clinical assessment to guide diagnosis and treatment, if warranted.
Congenital HypothyroidismRapid 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 THYRO-TABS therapy immediately upon diagnosis. Levothyroxine is generally continued for life in these patients.
Closely monitor infants during the first 2 weeks of THYRO-TABS therapy for cardiac overload, arrhythmias, and aspiration from avid suckling.
Closely monitor patients to avoid undertreatment or overtreatment. Undertreatment may have deleterious effects on intellectual development and linear growth. Overtreatment is associated with craniosynostosis in infants, may adversely affect the tempo of brain maturation, and may accelerate the bone age and result in premature epiphyseal closure and compromised adult stature.
Acquired Hypothyroidism In Pediatric PatientsClosely 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.
Geriatric UseBecause of the increased prevalence of cardiovascular disease among the elderly, initiate THYRO-TABS at less than the full replacement dose. Atrial arrhythmias can occur in elderly patients. Atrial fibrillation is the most common of the arrhythmias observed with levothyroxine overtreatment in the elderly.
