Usage
- Medical Conditions: Thyroxine is primarily prescribed for hypothyroidism, a condition where the thyroid gland doesn’t produce enough thyroid hormone. It is also used to treat goiter (enlarged thyroid gland) and thyroid cancer. It is sometimes used to help diagnose certain thyroid disorders.
- Pharmacological Classification: Thyroid hormone.
- Mechanism of Action: Thyroxine (T4) is a synthetic form of the natural thyroid hormone. It acts as a prohormone, converting to the active hormone triiodothyronine (T3), which regulates metabolism in body tissues by binding to thyroid hormone receptors in the nucleus of cells.
Alternate Names
- Alternate Names: Levothyroxine Sodium, L-thyroxine.
- Brand Names: Synthroid, Levoxyl, Eltroxin, Unithroid, Tirosint.
How It Works
- Pharmacodynamics: Thyroxine increases the metabolic rate, influencing protein synthesis, carbohydrate and fat metabolism, and cardiovascular, renal, and gastrointestinal function.
- Pharmacokinetics:
- Absorption: Primarily absorbed in the jejunum and ileum. Absorption is affected by food intake, certain medications, and gastrointestinal diseases.
- Metabolism: T4 is converted to T3 in the liver and other tissues.
- Elimination: Primarily eliminated via hepatic metabolism and biliary excretion, with some renal excretion.
- Mode of Action: Binds to thyroid hormone receptors in the cell nucleus, regulating gene expression related to metabolism.
- Receptor Binding/Enzyme Inhibition: Binds to thyroid hormone receptors.
- Elimination Pathways: Hepatic metabolism, biliary excretion, renal excretion.
Dosage
Standard Dosage
Adults:
- Initial dose: Usually 50-100 mcg orally once daily.
- Dose titration: Increased by 12.5-25 mcg increments every 4-6 weeks based on TSH levels and clinical response.
- Usual maintenance dose: 100-200 mcg daily. Dosages > 200 mcg/day are seldom required.
Children:
- Dosage is weight-based and individualized according to age and clinical response.
- Neonates (0-3 months): 10-15 mcg/kg/day.
- Infants (3-12 months): Variable depending on age.
- Children (1-12 years): Variable depending on age and weight.
- Adolescents (>12 years): 2-3 mcg/kg/day.
Special Cases:
- Elderly Patients: Start with a lower dose (e.g., 12.5-25 mcg/day) due to increased sensitivity and potential cardiac risks.
- Patients with Renal Impairment: No specific dosage adjustment typically needed.
- Patients with Hepatic Dysfunction: No standard dosage adjustment, careful monitoring recommended.
- Patients with Comorbid Conditions (e.g., diabetes, cardiovascular disease): Start with a lower dose and titrate cautiously due to increased sensitivity.
Clinical Use Cases
The listed clinical use cases (Intubation, Surgical Procedures, Mechanical Ventilation, ICU Use, Emergency Situations) do not routinely utilize thyroxine. Thyroxine addresses hypothyroidism, a chronic condition usually managed with long-term hormone replacement. However, in the rare case of myxedema coma (severe hypothyroidism), intravenous thyroxine may be administered.
Dosage Adjustments
Dose adjustments are based on TSH levels, clinical response, age, weight, cardiovascular status, concomitant medications, and the presence of other medical conditions. Regular monitoring of TSH levels is essential for optimal management.
Side Effects
Common Side Effects:
Headache, hair loss, insomnia, nervousness, irritability, sweating, heat intolerance, changes in appetite, weight changes, menstrual irregularities, diarrhea.
Rare but Serious Side Effects:
Allergic reactions, chest pain, irregular heartbeat, heart failure, seizures.
Long-Term Effects:
Osteoporosis with excessive doses.
Adverse Drug Reactions (ADR):
Thyrotoxicosis (symptoms of hyperthyroidism) due to overtreatment, including palpitations, tachycardia, heat intolerance, nervousness, tremors, and weight loss.
Contraindications
- Untreated thyrotoxicosis
- Uncorrected adrenal insufficiency
- Hypersensitivity to thyroxine or its excipients.
- Recent myocardial infarction
- Acute myocarditis, pancarditis
Drug Interactions
- CYP450 Interactions: Not a substrate or inhibitor of major CYP450 enzymes.
- Other Drug Interactions:
- Antacids
- Iron supplements
- Calcium supplements
- Bile acid sequestrants (e.g., cholestyramine)
- Sucralfate
- Proton pump inhibitors (e.g., omeprazole)
- Certain antibiotics (e.g., ciprofloxacin)
- Anticoagulants (e.g., warfarin)
- Antidiabetic medications (e.g., insulin)
- Estrogens
- Amiodarone
- Sertraline
- Chloroquine
- Propylthiouracil
- Glucocorticoids
- Beta blockers
- Cholestyramine
- Food and Lifestyle Factors: Soybeans, dietary fiber, grapefruit juice, and high-fiber foods can reduce thyroxine absorption.
Pregnancy and Breastfeeding
- Pregnancy Safety Category: A
- Fetal Risks: Untreated hypothyroidism during pregnancy can pose risks to both the mother and the fetus. Thyroxine is considered safe and necessary for managing hypothyroidism in pregnancy.
- Breastfeeding: Thyroxine is excreted in breast milk in small amounts, considered safe during breastfeeding. Monitoring infant’s growth and development is recommended.
Drug Profile Summary
- Mechanism of Action: Synthetic T4 that converts to T3, regulating metabolism by binding to nuclear thyroid hormone receptors.
- Side Effects: Headache, hair loss, insomnia, nervousness, palpitations, heat intolerance, changes in appetite and weight, menstrual irregularities, diarrhea. Serious side effects include cardiac arrhythmias, heart failure, allergic reactions.
- Contraindications: Untreated thyrotoxicosis, uncorrected adrenal insufficiency, hypersensitivity.
- Drug Interactions: Numerous drug interactions, including antacids, iron/calcium supplements, bile acid sequestrants, and certain medications.
- Pregnancy & Breastfeeding: Safe and often necessary during both.
- Dosage: Adult: Initial 50-100 mcg/day, titrated based on TSH. Children: weight and age-based.
- Monitoring Parameters: TSH, Free T4, and T3 levels, clinical response.
Popular Combinations
Thyroxine is usually used as monotherapy for hypothyroidism. It might be used in combination with other medications to manage coexisting conditions such as cardiovascular disease or diabetes, but there are no standard “popular combinations” specifically for thyroxine.
Precautions
- General Precautions: Monitor thyroid function tests, be cautious in elderly patients and those with cardiac disease.
- Specific Populations: Dosage adjustment might be needed in pregnant women, elderly individuals, and those with renal or hepatic dysfunction.
- Lifestyle Considerations: Take on an empty stomach, avoid interacting medications and foods.
FAQs (Frequently Asked Questions)
Q1: What is the recommended dosage for Thyroxine?
A: The recommended starting dose for adults is typically 50-100 mcg/day, adjusted according to TSH levels. Pediatric dosages are based on age and weight.
Q2: How should Thyroxine be taken?
A: Taken orally on an empty stomach, preferably 30-60 minutes before breakfast, with a full glass of water.
Q3: How long does it take for Thyroxine to work?
A: It might take several weeks or even months for symptoms to improve and thyroid function tests to normalize.
Q4: What are the common side effects of Thyroxine?
A: Common side effects include headache, hair loss, palpitations, heat intolerance, nervousness, changes in appetite and weight, menstrual changes, and diarrhea.
Q5: What should I do if I miss a dose?
A: Take the missed dose as soon as you remember, unless it’s close to the next dose. Do not double the dose.
Q6: Can Thyroxine be taken during pregnancy?
A: Yes, thyroxine is considered safe and is often necessary to manage hypothyroidism during pregnancy. Dosage adjustments may be needed.
Q7: What are the signs of Thyroxine overdose?
A: Signs of thyrotoxicosis, similar to symptoms of hyperthyroidism: palpitations, rapid heart rate, heat intolerance, nervousness, tremors, and weight loss. Seek immediate medical attention.
Q8: Does Thyroxine interact with other medications?
A: Yes, Thyroxine interacts with numerous medications including antacids, iron supplements, calcium supplements, certain antibiotics, and others. Inform your doctor about all medications you are taking.
Q9: Are there any dietary restrictions while taking Thyroxine?
A: Avoid taking thyroxine with food, especially foods high in fiber, calcium, or iron. Grapefruit juice can also interfere with absorption.
Q10: How often do I need to have my thyroid levels checked while on Thyroxine?
A: Initially, thyroid function tests (TSH, T4) are typically monitored every 4-6 weeks until the dose is stabilized. Once stable, annual monitoring is usually sufficient.
