Levothyroxine

Introduction Overview

Generic Name

levothyroxine

Brand Name

Unithroid

Dosage Form

Oral (Capsule, Tablet, Solution)

Drug Class

Thyroid drugs

Pharmacokinetics

Absorption:
Levothyroxine is efficiently absorbed from the gastrointestinal tract, primarily the small intestine. After oral administration, its absorption rate varies from 40% to 80%, influenced by gastrointestinal pH, gastric emptying time, and presence of food or other substances. Fasting increases absorption, which is why Unithroid is recommended to be taken on an empty stomach, at least 30 to 60 minutes before breakfast. Certain substances such as calcium, magnesium-containing antacids, iron supplements, and high-fiber foods can significantly impair absorption by forming insoluble complexes with levothyroxine, leading to decreased bioavailability.

Distribution:
Once absorbed, levothyroxine extensively binds to plasma proteins including thyroxine-binding globulin (TBG), transthyretin, and albumin, with approximately 99.97% protein binding. This high plasma protein affinity results in a large volume of distribution, allowing hormone availability to tissues in a controlled manner. Levothyroxine crosses the placenta and is secreted in small amounts in breast milk, necessitating careful dose adjustments in pregnant and breastfeeding women to avoid fetal or neonatal thyrotoxicosis or hypothyroidism.

Metabolism:
Levothyroxine undergoes peripheral conversion primarily in the liver, kidneys, and skeletal muscle tissues to the active form triiodothyronine (T3) via deiodination. Additional metabolic pathways include conjugation through glucuronidation and sulfation, facilitating elimination. Hepatic metabolism can be altered by hepatic diseases or concurrent medications that induce or inhibit liver enzymes, impacting therapeutic efficacy. The half-life of levothyroxine ranges from 6 to 7 days in euthyroid patients, but is reduced in hyperthyroid states and prolonged in hypothyroid states.

Elimination:
The primary route of elimination of levothyroxine and its metabolites is via the kidneys. Biliary excretion also occurs but to a lesser extent. Renal impairment may modestly alter clearance, but clinical significance is generally minimal. Steady-state plasma concentrations are typically achieved after 4 to 6 weeks of consistent dosing, reflecting the drug's long half-life. Monitoring thyroid function tests is critical to guide dosing and ensure appropriate therapeutic levels.

Dosage Forms

Available Dosage Forms:
- Capsule, Liquid Filled
- Solution
- Tablet

Dosages for Adults and Adolescents:
- Capsules and Tablets for Hypothyroidism:
- Adults and adolescents (>12 years, growth and puberty complete): dosing based on body weight, typically 1.6–1.7 mcg/kg/day; lower doses may be required in elderly or cardiac patients.
- Adolescents (>12 years, growth incomplete): 2–3 mcg/kg/day with dose adjustments as needed.
- Capsules and Tablets for Children (Hypothyroidism):
- 6–12 years: 4–5 mcg/kg/day.
- Children under 6 years: generally not recommended to use capsules due to swallowing difficulties.
- Solution for Hypothyroidism:
- Adults and adolescents (>12 years): usually starting dose 1.7 mcg/kg/day, titrated accordingly.
- Children vary by age:
- 6–12 years: 4–5 mcg/kg/day.
- 1–5 years: 5–6 mcg/kg/day.
- 6–12 months: 6–8 mcg/kg/day.
- 3–6 months: 8–10 mcg/kg/day.
- 0–3 months: 10–15 mcg/kg/day.

Dosages for Thyroid Cancer:
- Adults: higher doses generally >2 mcg/kg/day as determined by physician.
- Children: dosing individualized by treating physician.

Specific Patient Populations:
- Pediatric: Dose adjustments depend on age and developmental status; close monitoring is essential.
- Geriatric: Lower start doses due to potential cardiac sensitivity; careful titration necessary.
- Pregnant Women: Dose often increased to maintain euthyroid status and support fetal development.
- Patients with swallowing difficulties: Tablets can be crushed and mixed with water for immediate administration but capsules are not suitable in children under 6 or patients unable to swallow capsules.
- Patients on interacting medications or consuming interfering foods (e.g., calcium, iron, soybean, fiber) require adjusted dosing schedules (usually separation by at least 4 hours).

Mechanism Of Action

Levothyroxine is a synthetic form of the endogenous thyroid hormone thyroxine (T4), the prohormone that circulates in plasma and is converted into the active form triiodothyronine (T3) within target tissues. It mimics the physiological functions of endogenous thyroid hormone by binding to nuclear thyroid hormone receptors (TRs), specifically TRα and TRβ, which are transcription factors regulating gene expression critical for growth, development, and metabolism.

Upon entering cells, levothyroxine is deiodinated to T3, which then translocates into the nucleus and binds thyroid hormone receptors bound to thyroid hormone response elements (TREs) on DNA. This binding modulates transcription of numerous genes involved in metabolic pathways, protein synthesis, and mitochondrial activity, resulting in increased basal metabolic rate, oxygen consumption, and thermogenesis.

At the cellular level, levothyroxine influences carbohydrate, lipid, and protein metabolism, enhances gluconeogenesis, and stimulates cardiovascular function by increasing heart rate and contractility. Additionally, levothyroxine affects neurological development in children, ensuring normal brain maturation.

Clinically, restoration of euthyroid state with levothyroxine corrects symptoms of hypothyroidism such as fatigue, weight gain, cold intolerance, and bradycardia. Its suppressive effect on thyroid-stimulating hormone (TSH) secretion also facilitates management of goiter and suppression of residual thyroid tissue or tumors in thyroid cancer. Levothyroxine shares molecular action similarities with other thyroid hormone preparations, but as a pure T4 compound, it relies on peripheral deiodination, differing from desiccated thyroid products containing T3 and T4.

Pharmacological Actions

Replacement therapy for deficient thyroid hormone in hypothyroidism.
Suppression of pituitary TSH secretion to reduce stimulation of thyroid tissue.
Normalization of metabolic activity by regulating gene expression influencing basal metabolic rate.
Enhancement of cardiovascular function via increased myocardial contractility and heart rate.
Promotion of neurological development and maturation, especially in pediatric patients.
Reduction of goiter size through negative feedback inhibition of TSH secretion.
Supportive therapy in management of thyroid cancer to suppress tumor progression.
Modulation of protein, fat, and carbohydrate metabolism to maintain homeostasis.

Therapeutic Or Clinical Uses

Treatment of hypothyroidism of any etiology including autoimmune thyroiditis (Hashimoto's disease).
Management and reduction of enlarged thyroid gland (goiter).
Hormone replacement after thyroidectomy or radioactive iodine therapy.
Suppression therapy in thyroid carcinoma to inhibit growth of residual malignant thyroid tissue.
Treatment of myxedema coma as part of emergency intervention.
Adjunctive therapy in pituitary TSH suppression.
Management of congenital hypothyroidism to prevent developmental delay.
Correction of hypothyroidism-induced infertility.
Support in euthyroid sick syndrome when indicated carefully.

Side Or Adverse Effects

Chest pain, discomfort, or tightness indicative of cardiac ischemia or arrhythmias.
Dyspnea or difficulty breathing suggesting cardiac or pulmonary involvement.
Fast, slow, irregular, pounding, or racing heartbeat or pulse due to thyrotoxicosis.
Irritability, nervousness, tremors, anxiety, or restlessness reflecting overstimulation.
Heat intolerance and excessive sweating from increased metabolic rate.
Menstrual irregularities and changes in menstrual bleeding patterns.
Hair loss commonly transient during early therapy.
Nausea, vomiting, diarrhea, or stomach cramps possibly related to gastrointestinal irritation.
Swelling of the eyes, lips, face, or tongue indicating potential allergic reactions.
Hives, skin rash, itching, or redness as hypersensitivity reactions.
Fainting or extreme fatigue signaling cardiovascular compromise.
Blurred or double vision, eye pain, or dizziness as central nervous system effects.
Severe headaches and seizures in rare cases.
Symptoms of overdose include confusion, disorientation, cold clammy skin, lightheadedness, sudden loss of coordination or consciousness.
Rare but serious myxedema coma due to severe hypothyroidism requiring immediate care.

Precautions

Regular monitoring of thyroid function tests to avoid under or over-treatment.
Dose adjustments required in elderly patients to reduce risk of cardiac adverse events.
Cautious use in patients with pre-existing cardiovascular disease including arrhythmias, ischemic heart disease, or angina.
Adjustment needed during pregnancy as requirements may increase to support fetal development.
Avoid abrupt withdrawal; taper dosage gradually if discontinuation is necessary.
Monitor diabetic patients closely as levothyroxine may alter glycemic control.
Be aware of potential interactions with medications and certain foods that impair absorption.
Avoid using levothyroxine for weight loss or obesity treatment; high doses may cause toxicity.
Patients with adrenal insufficiency should have their adrenal status evaluated and corrected before initiation.
Use cautiously in patients with osteoporosis due to potential for increased bone resorption.
Consider liver and kidney function as severe impairment may affect metabolism and clearance.
May cause temporary hair loss in initial months, typically self-limiting.
Monitor for symptoms of overtreatment including palpitations, insomnia, and nervousness.
Proper counseling on medication timing and interactions with food, supplements, and other drugs is essential.

Warnings

Do not use levothyroxine to treat obesity or weight loss; can cause life-threatening toxicity.
Risk of myxedema coma, a life-threatening severe hypothyroid state, requires immediate medical attention.
Potential for cardiac arrhythmias, ischemic events particularly in elderly or those with cardiac disease.
Avoid sudden discontinuation which may precipitate thyroid crisis or relapse of hypothyroid symptoms.
Over-replacement can lead to symptoms of hyperthyroidism and associated complications.
Monitor closely during pregnancy; insufficient dosing can result in adverse fetal outcomes.
Interference with absorption by many common medications and dietary factors necessitates strict adherence to administration guidelines.
Some patients may experience allergic reactions including rash and angioedema.
Not recommended for patients with untreated thyrotoxicosis or acute myocardial infarction.
Lab tests must be regularly monitored to detect potential alterations in thyroid hormone levels.
Discontinue or adjust treatment prior to surgery or diagnostic tests as recommended by healthcare provider.

Contraindication

Untreated thyrotoxicosis or overt hyperthyroid states.
Acute myocardial infarction to avoid precipitation of cardiac ischemia.
Hypersensitivity or allergic reactions to levothyroxine or any component of Unithroid formulations.
Adrenal insufficiency not controlled by corticosteroid therapy prior to levothyroxine initiation.
Patients who cannot swallow capsules should avoid capsule form; tablets or solutions preferred.
Use with caution in patients with untreated adrenal gland insufficiency.
Not indicated for weight loss or obesity management in euthyroid patients.
Severe untreated cardiac disease where initiation may impose cardiac stress.

Drug Interactions

Furosemide may alter serum potassium levels affecting thyroid hormone balance.
Iopromide and iodinated contrast agents can interfere with thyroid function testing and therapy.
Midodrine co-administration requires careful cardiovascular monitoring.
Antacids containing aluminum or magnesium reduce levothyroxine absorption; separate dosing by 4 hours.
Calcium carbonate and iron supplements bind levothyroxine reducing absorption; administer at least 4 hours apart.
Cholestyramine, colestipol, and colesevelam impair enterohepatic circulation affecting hormone levels.
Proton pump inhibitors like omeprazole, lansoprazole, and rabeprazole may reduce absorption via increasing gastric pH.
Phenytoin and fosphenytoin induce hepatic enzymes increasing levothyroxine metabolism, necessitating dose adjustment.
Rifampin induces hepatic metabolism decreasing levothyroxine serum concentrations.
Ritonavir and lopinavir influence cytochrome P450 affecting thyroid hormone levels.
Simvastatin may interact altering lipid profiles and thyroid hormone effects.
Certain estrogens and contraceptives increase thyroid-binding globulin, leading to dose adjustments.
Iron-containing multivitamins and supplements decrease absorption; separate by several hours.
Soybean and dietary fiber intake may reduce bioavailability; timing of administration important.
Sevelamer and lanthanum carbonate may bind levothyroxine in the gut reducing its efficacy.

Preparations Of The Drug

Oral Liquid Filled Capsules containing levothyroxine sodium in dosages suitable for individualized dosing.
Oral Tablets with varying mcg strength for flexible dose titration; tablets can be crushed and mixed with water if necessary.
Oral Solution provided in single unit dose ampules designed for precise dosing and improved bioavailability.
Unithroid specifically formulated as pure levothyroxine sodium without additives that interfere with absorption.
Packaged in light-resistant and moisture-protective containers to maintain drug stability.
Available in multiple dosage strengths to tailor therapy from neonatal to adult patients.
Formulations intended to be taken on empty stomach to optimize absorption and minimize variability.
Capsule formulation not recommended for young children or those with difficulty swallowing.
Solutions ideal for pediatric and enteral administration requiring dose flexibility.
Manufactured under stringent quality control to ensure potency and safety.