What it's for (Indications)
- Hypothyroidism (used either as a replacement or supplement to hormone thyroxine) and for pituitary TSH suppression (where the pituitary gland fails to produce sufficient TSH).
Dosage Information
| Type | Guideline |
|---|---|
| Standard | Levothyroxine sodium is a synthetic thyroid hormone administered orally, typically as a single daily dose, to replace deficient endogenous thyroid hormone. The dosage is highly individualized and determined primarily by serum thyroid-stimulating hormone (TSH) levels, clinical response, patient age, weight, and the presence of co-morbidities, particularly cardiovascular disease. For adults with primary hypothyroidism, a common initial full replacement dose is approximately 1.6 to 1.8 micrograms/kg/day, though lower starting doses (e.g., 12.5-50 mcg/day) are recommended for elderly patients, those with a history of cardiovascular disease, or long-standing severe hypothyroidism, to avoid precipitating cardiac events. Doses are titrated gradually, usually in increments of 12.5 to 25 mcg, every 4 to 6 weeks, until serum TSH normalizes and clinical symptoms resolve. It is crucial to administer levothyroxine consistently, ideally 30-60 minutes before breakfast with a full glass of water, on an empty stomach, and at least 4 hours apart from medications known to interfere with its absorption (e.g., iron supplements, calcium supplements, antacids, proton pump inhibitors). Pediatric dosing is weight-based and generally higher per kilogram than adults, especially in infants and neonates. During pregnancy, levothyroxine requirements often increase, necessitating close monitoring of TSH and dose adjustments. Lifelong therapy and periodic TSH monitoring (every 6-12 months once stable) are essential to maintain euthyroidism. |
Safety & Warnings
Common Side Effects
- Angina pain, cardiac arrhythmias, palpitation, skeletal muscle cramps, tachycardia, diarrhea, vomiting, tremors, excitability, insomnia, headaches, flushing, weight loss, muscle weakness, sweating, rash, pruritus, edema, restlessness, menstrual irregularity, pseudotumor cerebri (pressure development in the brain), heat intolerance, and transient alopecia.
Serious Warnings
- Black Box Warning: None
- Caution is advised in people with cardiovascular disorders, diabetes, a long history of hypothyroidism, adrenal insufficiency (where the adrenal gland is unable to produce sufficient steroid hormone), and in aged individuals.
- Pregnancy: Studies to date have not shown any harmful effects, however, it is recommended to consult your healthcare provider for advice.
- Lactation: This medication may be safe for lactating females; however, it is recommended to consult your healthcare provider for further advice.
How it Works (Mechanism of Action)
Levothyroxine (L-thyroxine) is a synthetic form of the naturally occurring thyroid hormone, thyroxine (T4), which is the primary hormone secreted by the thyroid gland. It functions as a prohormone, meaning it is biologically inactive until it undergoes deiodination in peripheral tissues, primarily the liver and kidneys, to form triiodothyronine (T3), the metabolically active form of thyroid hormone. T3 exerts its effects by binding to specific nuclear thyroid hormone receptors (TRs) located in the nucleus of target cells throughout the body. These receptors form heterodimers with retinoid X receptors (RXRs) and then bind to thyroid hormone response elements (TREs) in the promoter regions of target genes. Upon T3 binding to the TR-RXR complex, a conformational change occurs, leading to the dissociation of corepressor proteins and the recruitment of coactivator proteins. This process modulates gene transcription, resulting in the upregulation or downregulation of various proteins that regulate metabolic processes. Thyroid hormones are essential for normal growth, development, and metabolism, influencing protein, carbohydrate, and lipid metabolism, increasing basal metabolic rate, promoting cardiac function, and playing critical roles in neurological development and function. By replacing deficient endogenous thyroid hormone, exogenous levothyroxine restores physiological T3 and T4 levels, thereby reversing the signs and symptoms associated with hypothyroidism and maintaining systemic metabolic homeostasis.