Levothyroxine is a synthetic version of the thyroid hormone thyroxine (T4), mimicking its natural metabolic effects. It is converted to the active hormone triiodothyronine (T3), regulating metabolism via DNA transcription and protein synthesis.
INDICATIONS
Used for:
Treating hypothyroidism (low thyroid hormone levels)
Enhancing metabolism in bodybuilding (in conjunction with growth hormone for muscle retention and fat loss)
DOSAGE AND ADMINISTRATION
Timing: Take on an empty stomach, 30-60 minutes before meals with water.
Starting Dose: Begin with low doses, tailored to patient-specific factors (age, weight, medical condition).
Maintenance Dose: Typically individualized; extreme doses (300-400 mcg/day) are rare.
Cycle: Use for 5-7 weeks max, with at least 4-8 weeks off to restore thyroid function.
PHARMACOKINETICS
Absorption: 40-80% (primarily in the jejunum and ileum); fasting enhances absorption.
Elimination Half-Life:
Normal thyroid function: 6-7 days
Hypothyroidism: 9-10 days
Hyperthyroidism: 3-4 days
Excretion: 80% via kidneys, 20% in feces.
CONTRAINDICATIONS
Absolute: Hypersensitivity to levothyroxine or excipients, thyrotoxicosis, uncorrected adrenal insufficiency.
Relative: Cardiovascular disorders (e.g., recent myocardial infarction), elderly patients, and those with long-standing hypothyroidism (gradual introduction recommended).
SIDE EFFECTS
Common (Excessive Doses): Weight loss, sweating, anxiety, tremors, rapid heart rate, insomnia.
Severe: Allergic reactions, fever, heart failure, adrenal crisis, or coma in cases of massive overdose.
Delayed Overdose Symptoms: May appear 6 hours to 11 days post-ingestion.
DRUG INTERACTIONS
Reduced Absorption: Calcium, iron, soy products, aluminum/magnesium antacids, cholestyramine, sucralfate.
Increased Toxicity: Ketamine, tricyclic antidepressants, tetracyclic antidepressants.
Caution with: Lithium (affects iodine metabolism), beta-blockers (may mitigate adrenergic effects).
PRECAUTIONS
Special Populations: Use cautiously in elderly patients, those with cardiovascular disorders, diabetes, or adrenal insufficiency.
Bone Health: Avoid subclinical hyperthyroidism to minimize osteoporosis risk.
Gradual Introduction: For long-standing hypothyroidism or patients over 50.
STORAGE
Store between 15–25°C in a dry, cool environment, protected from light and out of reach of children.
