Levothyroxine
Synthetic thyroxine (T4), the standard replacement for an underactive thyroid, also used to suppress TSH in thyroid cancer.
Also called L-thyroxine (chemical name) or T4 (abbreviation).
Drug card
- Class
- Thyroid hormone
- How it works
- Binding nuclear thyroid receptors, which switch on genes for metabolism
- Used for
- Underactive thyroid (hypothyroidism), congenital or acquired
- TSH suppression in thyroid cancer, after surgery
- Known for
- Fast heart rate and palpitations
- Arrhythmias, including atrial fibrillation
- Bone loss with over-replacement
Study it as a flashcard, and it comes back before you forget it.
Mechanism of action
How does levothyroxine work?
A synthetic levo-isomer of thyroxine with the same action as the T4 the thyroid makes. Tissues deiodinate it to T3, which binds nuclear thyroid receptors and switches on genes for metabolism, heat production and growth. It needs gastric acid for good absorption and is best absorbed on an empty stomach.
Uses
What is levothyroxine used for?
- Underactive thyroid (hypothyroidism), congenital or acquired
- TSH suppression in thyroid cancer, after surgery
Side effects
Side effects of levothyroxine
- Fast heart rate and palpitationsKnown for
- Arrhythmias, including atrial fibrillationKnown forSerious
- Bone loss with over-replacementKnown for
- Angina, heart failure or heart attack in heart diseaseSerious
- Weight loss with increased appetite
- Tremor and muscle weakness
- Insomnia
- Nervousness, anxiety and irritability
- Diarrhea
- Hair loss
- Heat intolerance, sweating and flushing
- Seizures (rare, when starting)Serious
Known for marks what an exam expects you to know it for.
Watch for
Cautions and what sets it apart
- Boxed warning: not for obesity or weight loss; usual doses do not work for it and large doses can cause life-threatening toxicity.
- Calcium, iron, phosphate binders and bile acid sequestrants bind it, so doses are separated by 4 hours; PPIs and antacids can cut absorption.
- TSH tends to rise in pregnancy, so a higher dose is often needed and TSH is checked.
- Start low in older people and in heart disease, and correct adrenal insufficiency before starting.
Good to know: Adverse effects are mostly those of too high a dose. In primary hypothyroidism the dose is guided by TSH 6 to 8 weeks after each change; in pituitary causes TSH is not reliable.
Remember it
A way to remember it
T4 is the parcel, T3 the message: levothyroxine is T4, and tissues unwrap it into active T3.
About 80% of T3 comes from T4 in the tissues. Too much shows up as hyperthyroidism: a fast heart, weight loss, tremor and poor sleep, and over the years, bone loss.
Written for biofideX
Its class
Thyroid hormones
T4 and T3 pass into the cell nucleus, where they bind thyroid hormone receptors attached to DNA and activate gene transcription, changing which proteins the cell makes. About 80% of the active hormone, T3, is made from T4 by deiodination in the tissues. The effects include a higher metabolic rate and heat production, faster and stronger heartbeats, and normal growth and development.
Easily mixed up with thionamides.
Sources
Checked against
Written for biofideX in plain words, after checking these references:
For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.
Thyroid, steroids and bones
Levothyroxine is one of 9 drugs practiced together here: their classes, how they work, what they treat and what they're known for.
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