What is T4 (Levothyroxine)?
Levothyroxine (T4) is a synthetic thyroid hormone that replaces the body's own thyroxine production. It is the principal form of thyroid hormone secreted by the thyroid gland, bound to thyreoglobulin. After absorption, a portion of T4 is converted to T3 (triiodothyronine) in tissues, which is biologically more active.
Levothyroxine is the standard hormone for treating hypothyroidism and is also used to suppress TSH in thyroid cancer. This information is for research and educational purposes only. No medical advice.
How it works
Levothyroxine acts as a replacement for the natural thyroid hormone thyroxine. Its biological effects occur through:
- Binding to thyroid hormone receptors in nucleus and cytoplasm
- Increased metabolism and heat production
- Enhanced heart rate and cardiac output
- Improved mental function and cognitive ability
- Regulation of the nervous system through central and peripheral effects
- Partial conversion to T3 in peripheral tissues
Administration and dosing
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What the research shows
Clinical trials indicate that levothyroxine:
- Remains the standard treatment for hypothyroidism with worldwide acceptance
- Shows different dosing requirements in thyroid cancer patients (2.11 mcg/kg) versus primary hypothyroidism patients (1.63 mcg/kg)
- When combined with T3, lowers TSH levels and increases FT3
- Is effective in many patients at dosages of 75-150 mcg daily
- Has a 7-day half-life enabling once-daily dosing
- Is affected by genetic and individual differences in absorption and metabolism
Side effects
With overtreatment, may occur:
- Tremor and nervousness
- Palpitations and arrhythmias
- Sleep disturbances
- Weight loss
- Heat intolerance
- Bone density loss with prolonged over-suppression
With under-dosing:
- Fatigue and lethargy
- Weight gain
- Cold intolerance
- Depression and cognitive impairment
- Bradycardia
Storage
Levothyroxine tablets should be stored in a dry, cool place, protected from light. Storage temperature should be between 15-25 degrees Celsius. Original containers protect against moisture and should remain tightly sealed. Do not refrigerate. Levothyroxine is stable at room temperature when protected from moisture.