Levothyroxine is a synthetic form of the thyroid hormone thyroxine (T4), used primarily to treat hypothyroidism—a condition where the thyroid gland does not produce enough thyroid hormone. It is also used in the management of thyroid cancer (alongside surgery and radioactive iodine therapy) and to treat myxedema coma, a severe form of hypothyroidism.
Key Facts
Brand Names: Synthroid, Levoxyl, Euthyrox, Unithyroid, Levo-T, Tirosint, Ermeza, and others.
Mechanism: Replaces deficient thyroid hormone, increasing metabolic rate and reducing thyroid-stimulating hormone (TSH) levels. It is converted in the body to the more active form, triiodothyronine (T3).
Dosage & Administration: Typically taken once daily on an empty stomach, 30–60 minutes before breakfast. Avoid taking with caffeine, calcium, soy, or kelp, as they can interfere with absorption.
Onset of Action: Begins working immediately, but symptom improvement may take several weeks.
Monitoring: Dose is adjusted based on regular blood tests measuring TSH and free T4 levels. Steady-state levels are reached after about 6 weeks.
Common Side Effects
Caused by excessive dosing (mimicking hyperthyroidism):
Weight loss
Fast heart rate (palpitations)
Anxiety, insomnia, tremors
Sweating, heat intolerance
Headaches, nausea, diarrhea
Note: Side effects usually resolve when the dose is adjusted to the correct level.
Special Considerations
Pregnancy & Breastfeeding: Safe and recommended. Dose often needs to be increased during pregnancy; regular monitoring is essential.
Children: Safe and critical for normal growth and development.
Heart Conditions: Not recommended in people with recent heart attacks or unstable heart disease; starting doses should be low and gradual.
Long-Term Use: Usually lifelong. No evidence of reduced fertility, but regular monitoring prevents complications like osteoporosis from long-term over-replacement.
Important: Switching between brands may cause changes in symptoms due to differences in bioavailability. If switching, discuss with your doctor and monitor thyroid levels closely.
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Does levothyroxine cause hair loss?
Yes, levothyroxine, the main treatment for a sluggish thyroid gland, may cause hair loss in addition to other side effects. You may experience partial hair loss in the first few months of treatment, but this often resolves and goes back to normal in a short time.
Can I take other medications with levothyroxine?
Levothyroxine is known to interact with many other medications (called a drug-drug interaction), but there are different kinds of drug-drug interactions. Some interactions require that drugs not be taken at the same time, while other interactions require more frequent laboratory testing or monitoring of your disease symptoms.