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Levothyroxine: what it is used for and how to take it
Levothyroxine is a prescription thyroid hormone, the same one a healthy thyroid makes, used to treat hypothyroidism; Synthroid is one of its brands. Per the FDA label, it is taken once a day on an empty stomach, 30 to 60 minutes before breakfast. It is not a controlled substance and it is not for weight loss. The doctor adjusts the dose based on your TSH.
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Is this an emergency?
This is not an emergency service. Call 911 or go to the nearest emergency room if you have:
- Chest pain, a very fast or irregular heartbeat, or shortness of breath
- Marked tremor, heavy sweating, severe anxiety, or fever with a feeling of extreme heat
- Swelling of the face, lips or throat, or trouble breathing or swallowing
- Confusion, extreme sleepiness that will not lift, or a very low body temperature
- Confirmed or suspected pregnancy while taking levothyroxine: tell the doctor the same day and do not stop the pill on your own
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What is levothyroxine?
Levothyroxine (levothyroxine sodium) is the synthetic form of thyroxine (T4), the main thyroid hormone. It is a prescription medicine and not a controlled substance. It comes as a generic and under several brands, including Synthroid, Levoxyl, Unithroid, Euthyrox and Tirosint; the active ingredient is the same.
It replaces the hormone your body does not make in enough amount. It does not cure the cause of hypothyroidism; it controls it, which is why it is usually taken long term.
What is levothyroxine used for?
Per the FDA label, levothyroxine is approved to:
- Treat hypothyroidism (an underactive thyroid) of any cause, in adults and children.
- Support thyroid cancer treatment after surgery or radioactive iodine, at doses set by the specialist managing that case.
Levothyroxine: how and when to take it
On an empty stomach. Per the FDA label, take it once a day with a full glass of water, 30 to 60 minutes before breakfast. What matters is consistency: the same time and the same way every day. If you take it at night, the rule is that several hours have passed since your last meal; ask the doctor which fits you.
Separate other products. Calcium, iron, antacids and some supplements can keep you from absorbing the pill. The label suggests spacing them about 4 hours apart. Coffee and fiber can also change absorption, so keep your routine steady.
If you miss a dose, take it when you remember, unless it is almost time for the next one. Do not double up.
The doctor decides the dose. It depends on your weight, age, other conditions and your TSH. Do not change it or stop it on your own, even if you feel fine.
Levothyroxine and the TSH: when the test is repeated
TSH is the test that guides the dose. Per the label, after you start or change the dose the TSH is repeated at about 6 to 8 weeks; once the dose is stable, checks are usually every several months and then yearly.
If you already have results, the doctor can review them at a follow-up visit. If you need the test, she can give you a lab order for a lab near you. See how to read your results.
Levothyroxine is not for weight loss
The FDA label carries a boxed warning: levothyroxine should not be used to treat obesity or for weight loss. In people with a normal thyroid it does not help weight loss, and larger doses can cause serious or even life-threatening effects. If weight worries you, talk to the doctor about a full evaluation rather than taking thyroid hormone on your own.
Side effects and signs of too high a dose
When the dose is right, many people notice no side effects. If the dose is higher than your body needs, you get symptoms of too much thyroid hormone. Per the label and MedlinePlus:
- Fast heartbeat or palpitations
- Tremor or nervousness
- Excess sweating or heat intolerance
- Unintended weight loss, diarrhea or period changes
- Trouble sleeping or headache
- With high doses over a long time, it can also affect bone
Interactions and precautions
Tell the doctor and the pharmacist everything you take, including supplements. Per the label:
- Calcium, iron, antacids and some supplements: reduce absorption; space them from levothyroxine.
- Blood thinners such as warfarin: thyroid hormone can increase their effect; closer monitoring may be needed.
- Diabetes medicines: the dose you need can change as the thyroid is corrected.
- Heart disease and older adults: the doctor usually starts with a lower dose and raises it slowly.
- Pregnancy: the dose often needs to be adjusted; tell the doctor if you are pregnant or planning to be.
- Switching brands or generics: tell the doctor if your brand changes; sometimes the TSH is repeated.
When to go to the ER
If you have chest pain, a very fast heartbeat with shortness of breath, confusion, or an allergic reaction with swelling of the face or throat, call 911. See when to go to the ER.
Book an evaluation
Book an evaluation; the doctor decides whether a treatment is appropriate for you.
You see Dr. Diana Nieves Castro, MD, a Puerto Rico-licensed general practitioner, in Spanish or English. She can review your symptoms, your TSH results and the medicines you take, and decide whether to continue or adjust your treatment. Every day, any time (Puerto Rico time).
Already on levothyroxine? If you need to continue a medicine you already take (non-controlled), see prescription refills: the doctor reviews how you are doing and decides whether it is appropriate to renew or adjust it. For a new evaluation, see the online doctor visit.
The pharmacy charges for the medicine. We do not bill health plans; if you ask, we give you an itemized receipt (superbill). Visits are private and are not recorded. See the pricing and book your visit.
How we care for you
We evaluate, diagnose and treat; if needed, we give you a lab order for a lab near you.
We see adults and children of all ages. Patients under 21: a parent with patria potestad (parental authority) or a legal guardian is on camera for the whole visit and shows photo ID.
Real-time video visit. We send you a secure video link. It works on your phone, tablet or computer.
If appropriate, the doctor prescribes and we send your prescription to your pharmacy.
You must be physically in Puerto Rico during the visit (visitors, tourists and cruise passengers are welcome).
In an emergency, call 911.
Frequently asked questions
Can I get a levothyroxine refill by video?
Should levothyroxine be taken in the morning or at night?
Can I drink coffee with levothyroxine?
Does levothyroxine help you lose weight?
How often is the TSH checked?
What happens if I stop levothyroxine suddenly?
About this page
Author: The Health Initiative
Medical care provided by Dr. Diana Nieves Castro, MD (PR license 24027)
Disclosure: Founding Executive Director of The Health Initiative
Published: 2026-10-11 · Updated: 2026-10-11
Sources (3)
- DailyMed (NLM): Levothyroxine sodium tablets, FDA label (opens in a new tab) (accessed 2026-10-10)
- MedlinePlus: Levothyroxine (opens in a new tab) (accessed 2026-10-10)
- NIDDK: Hypothyroidism (underactive thyroid) (opens in a new tab) (accessed 2026-10-10)