Levothyroxine: what it treats, how to take it, side effects
Last updated September 3, 2026.
Levothyroxine is replacement thyroid hormone for a thyroid that no longer makes enough. You may know it as Synthroid, Levoxyl, or generic. It treats hypothyroidism and is also used with surgery and radioactive iodine therapy for thyroid cancer.
What does it treat?
Hypothyroidism. Without thyroid hormone, the body runs slow: MedlinePlus lists poor growth, slow speech, lack of energy, excessive tiredness, constipation, weight gain, hair loss, dry thick skin, increased sensitivity to cold, joint and muscle pain, heavy or irregular menstrual periods, and depression. Taken correctly, levothyroxine reverses these symptoms - but it controls the condition rather than curing it.
How do you take it?
Once a day, on an empty stomach, 30 minutes to 1 hour before breakfast, with a full glass of water - the label says the water prevents the tablet getting stuck in your throat. Consistency is everything: your doctor starts low and adjusts gradually based on blood tests, and it may take several weeks before you notice a change. The absorption traps are specific: calcium carbonate and iron supplements need a 4-hour gap before or after, and bile-acid drugs like cholestyramine, colesevelam, colestipol, sevelamer, and sodium polystyrene sulfonate need 4 hours after. Soybeans, walnuts, dietary fiber, and grapefruit can also change how it works - talk to your doctor about them. For infants or anyone who cannot swallow a tablet, crush it in 1 to 2 teaspoons of water and give it right away - never with food or soy formula. Missed dose: take it when you remember unless the next one is close; never double up.
Side effects to know
- Common: weight gain or loss, headache, vomiting, diarrhea, changes in appetite, fever, menstrual changes, sensitivity to heat, hair loss, joint pain, and leg cramps.
- Serious: shortness of breath, wheezing, hives, itching, rash, flushing, stomach pain, nausea, or swelling of hands, feet, ankles, or lower legs mean calling your doctor immediately. And the page's warning section: chest pain, rapid or irregular heartbeat, uncontrollable shaking, nervousness, anxiety, irritability, insomnia, or excessive sweating - signs your dose may be overshooting - also mean an immediate call.
Who should not take it?
MedlinePlus's warning is blunt: levothyroxine should never be used alone or with other treatments to treat obesity or cause weight loss, and in large doses - especially with amphetamines - it can cause serious or life-threatening problems. If you have adrenal insufficiency, your doctor may rule it out. Flag recent radiation therapy, diabetes, hardening of the arteries, bleeding problems or anemia, porphyria, osteoporosis, pituitary disorders, swallowing difficulties, and kidney, heart, or liver disease. OTC products that interact include proton pump inhibitors like omeprazole, antacids, simethicone, aspirin and other salicylates, niacin, and NSAIDs. If you become pregnant while taking it, call your doctor - pregnancy changes the dose, and your care team will adjust.
When is it an emergency?
Chest pain, a rapid or irregular heartbeat, or severe shortness of breath mean calling your doctor immediately or getting emergency care. So does an allergic reaction with hives, wheezing, or swelling. For an overdose, call Poison Control at 1-800-222-1222. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
Why do I have to take levothyroxine on an empty stomach?
Because food and drink compete with it. The label says take it once daily, on an empty stomach, one-half to one hour before breakfast with a full glass of water. Even soybeans, walnuts, dietary fiber, and grapefruit can change how much hormone you absorb - MedlinePlus says to talk to your doctor about them. Same time, same routine, every day is what keeps your blood levels steady.
How long does levothyroxine take to work?
Weeks, not days. MedlinePlus says it may take several weeks before you notice a change in your symptoms, and your doctor will start you on a low dose and increase it gradually based on periodic blood tests. The blood tests are how the dose gets tuned - do not skip them.
Can I take my vitamins with levothyroxine?
Not at the same time. MedlinePlus is specific: calcium carbonate and ferrous sulfate (iron) need at least 4 hours before or after levothyroxine, and a list of other binders - cholestyramine, colesevelam, colestipol, sevelamer, sodium polystyrene sulfonate - need 4 hours after. Antacids, proton pump inhibitors, and simethicone also interact. Morning thyroid pill, midday vitamins is the classic fix.
Can I use levothyroxine to lose weight?
No. MedlinePlus's warning section says levothyroxine should not be used alone or with other treatments to treat obesity or cause weight loss, and that large doses - especially combined with amphetamines - can cause serious or life-threatening problems. It replaces a hormone you are missing; it is not a weight tool.
Can I drink alcohol while taking levothyroxine?
Neither the FDA label nor MedlinePlus lists a specific alcohol interaction for levothyroxine. Moderate drinking away from your morning dose is generally compatible - confirm with your doctor, especially while your dose is still being adjusted.
What happens if my levothyroxine dose is too high?
You start looking like your thyroid is overactive. MedlinePlus's warning section lists the signals: chest pain, rapid or irregular heartbeat, uncontrollable shaking, nervousness, anxiety, irritability, insomnia, and excessive sweating. Any of those means calling your doctor immediately - the fix is a dose adjustment, not toughing it out.