Finasteride
medicationA prescription pill that slows male-pattern hair loss by lowering DHT.
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No. 059hormone
Levothyroxine
Levothyroxine is a hormone that is FDA-approved, prescription only.
Also known as Synthroid, Levoxyl, Unithroid, Euthyrox, Tirosint
Levothyroxine replaces the thyroid hormone T4 when the thyroid does not make enough. It is one of the most prescribed drugs in the US, usually taken for life, and adjusted using TSH blood tests.
Is Levothyroxine FDA-approved?
Approved by the FDA. A licensed clinician has to decide it is right for you and prescribe it. FDA-approved, prescription only, with brand and generic versions. The label carries a boxed warning that thyroid hormone must not be used for obesity or weight loss.
As of October 7, 2026.
Key takeaways
- Levothyroxine is synthetic T4 thyroid hormone, the American Thyroid Association's treatment of choice for an underactive thyroid.
- It is FDA-approved, prescription only, and was the second most prescribed drug in the US in recent data.
- Its boxed warning says it must not be used for weight loss.
- TSH is usually rechecked 6 to 8 weeks after starting or changing it, and most people take it for life.
How does Levothyroxine work?
The thyroid makes T4, which the body converts into the more active T3 to set the pace of metabolism, heart rate and energy use. When the thyroid fails, as in Hashimoto thyroiditis or after surgery, levothyroxine replaces T4 and the body converts it as usual.
The pituitary watches thyroid hormone levels and releases TSH when they are low. That is why TSH is the main test clinicians use to judge whether the amount of levothyroxine is right.
What interferes with levothyroxine?
The Synthroid label says to take it on an empty stomach and keep it at least 4 hours apart from calcium, iron, antacids, bile acid sequestrants and proton pump inhibitors. Soybean flour, walnuts, cottonseed meal and dietary fiber can lower absorption, and grapefruit juice may delay it. DailyMed: Synthroid (levothyroxine) prescribing information
The American Thyroid Association guideline notes coffee and fiber may impair absorption, and that taking it consistently matters more than the exact time of day. American Thyroid Association: Guidelines for the treatment of hypothyroidism (Thyroid 2014)
The label also advises stopping biotin supplements before thyroid blood tests, because biotin can distort results. DailyMed: Synthroid (levothyroxine) prescribing information
What does the evidence say about Levothyroxine?
Strong human evidenceThe American Thyroid Association calls levothyroxine the treatment of choice for hypothyroidism, backed by decades of clinical use and trials.
What to expect
What trials and prescribing information measured over time. Everyone responds differently.
- 4 to 6 weeks
The Synthroid label says peak therapeutic effect may not be reached for 4 to 6 weeks. The ATA says steady state is generally reached in about 6 weeks.
- 6 to 8 weeks
The label and the ATA advise rechecking TSH 6 to 8 weeks after starting or changing treatment.
- Long term
The ATA says most people take thyroid medicine for life, with periodic TSH checks.
What do clinicians say about Levothyroxine?
Endocrinologists treat levothyroxine as the standard for hypothyroidism, with the work lying in getting TSH into range and keeping absorption consistent.
The American Thyroid Association recommends levothyroxine as the preparation of choice for hypothyroidism and says it should remain the standard of care.
American Thyroid Association: Guidelines for the treatment of hypothyroidism (Thyroid 2014)The ATA guideline says a switch between identifiable levothyroxine products should be followed by a TSH recheck.
American Thyroid Association: Guidelines for the treatment of hypothyroidism (Thyroid 2014)The Synthroid label warns that over-treatment can bring on angina or arrhythmias, lists atrial fibrillation as a risk in older adults, and notes it can increase bone loss, especially after menopause.
DailyMed: Synthroid (levothyroxine) prescribing informationThe label states that pregnancy may increase levothyroxine requirements and advises checking TSH each trimester.
DailyMed: Synthroid (levothyroxine) prescribing information
Summaries of published guidelines, drug labels, regulators and reviews, each attributed to its source. DiscoverMedica does not employ clinicians.
Is Levothyroxine right for you, and who should avoid it?
Often considered for
- Adults and children with hypothyroidism, including Hashimoto thyroiditis or after thyroid surgery or radioactive iodine
- People with thyroid cancer, where it is also used to suppress TSH alongside surgery and radioactive iodine
Not a fit, or needs extra care
- Anyone hoping to lose weight with it: the boxed warning forbids that use
- People with untreated adrenal insufficiency, until it is treated
- Older adults and people with heart disease need a cautious start and close monitoring
Only a licensed clinician who knows your history can decide whether it is right for you.
What are the risks and side effects of Levothyroxine?
- Too much causes symptoms of an overactive thyroid: a racing heart, anxiety, sweating, weight loss and poor sleep.
- Over-treatment raises the risk of atrial fibrillation and bone loss, especially in older adults and after menopause.
- Calcium, iron, antacids, some acid reflux drugs, soy, walnuts and fiber can reduce absorption, so timing matters.
- Biotin supplements can distort thyroid blood tests; pregnancy usually changes how much is needed.
What should you ask a clinician about Levothyroxine?
- When should my TSH be rechecked, and what range are we aiming for?
- How should I time it around my other medicines and supplements?
- Should I stay on the same brand or generic product?
These save with the compound when you add it to My Journal, so they’re on the printout for your visit.
How do people get Levothyroxine through telehealth?
Hypothyroidism is diagnosed with blood tests, usually TSH and free T4, ordered by a primary care clinician or endocrinologist.
If prescribed, generic or brand levothyroxine is filled at any pharmacy and is covered by most insurance plans.
TSH is rechecked 6 to 8 weeks after starting or changing, then periodically once stable.
Labs you may be asked for
- TSH
- Free T4
- Thyroid antibodies (TPO) in some cases
What programs often include
- Clinician diagnosis and prescription
- Periodic TSH testing
Check your levels first
Some services let you order common lab tests yourself, so the results are ready for your visit. Your clinician decides which labs, if any, you need, and what the results mean.
- Function HealthAnnual membership for broad lab panels, drawn at Quest sites or at home.Visit
- Quest HealthOrder individual lab tests from Quest Diagnostics online, without a doctor’s visit.Visit
From each service’s own site, checked September 23, 2026.
How some providers do it
- Quest HealthNot a prescriber. Sells a TSH test you can order without a doctor visit and take to your own clinician.
- Amazon PharmacyFills generic levothyroxine from your own prescriber.
From each provider’s own site, checked October 7, 2026. Programs change, so confirm the details before you sign up.
Where can I get Levothyroxine?
7 providers list Levothyroxine on DiscoverMedica, lowest listed price first.
$15/mo + membershiplisted price†
Start at Hone Health see site
Start at Amazon Pharmacy see site
Start at Lemonaid Health see site
Start at Paloma Health see site
Start at Walmart Pharmacy † Prices as listed by the provider when we checked; they change.
How much does Levothyroxine cost?
The listed price is $15 a month, from 1 provider, as checked October 7, 2026.
Prices as listed by the provider when we checked; they change. Membership or consult fees may be separate; see each provider.
Levothyroxine: questions, answered
What are the side effects of levothyroxine?
At the right amount, most people have few side effects, because it replaces a hormone the body already makes. The Synthroid label says adverse reactions are mainly signs of too much thyroid hormone: a racing or irregular heartbeat, anxiety, poor sleep, sweating, heat intolerance, weight loss, tremor and diarrhea. Over-treatment over time can contribute to bone loss and atrial fibrillation.
Can levothyroxine help me lose weight?
No. The label carries a boxed warning that thyroid hormones must not be used to treat obesity or for weight loss. In people with normal thyroid function, amounts that would affect weight can cause serious or life-threatening toxicity. If you have hypothyroidism, treatment may ease related symptoms, but it is not a weight-loss drug.
Can I take levothyroxine with coffee or breakfast?
The Synthroid label advises taking it on an empty stomach before breakfast and keeping it at least 4 hours away from calcium, iron and antacids. The American Thyroid Association notes coffee and fiber may reduce absorption and stresses consistency. If your routine changes, tell your clinician, since a TSH recheck may be needed.
Is generic levothyroxine as good as Synthroid?
Generic levothyroxine is FDA-approved and widely used. The American Thyroid Association advises that after any switch between identifiable products, brand or generic, TSH should be rechecked, because small absorption differences can matter. Many people do well on generics; the key is to stay consistent and retest after a change.
How long does levothyroxine take to work?
Some people feel better within a few weeks, but the Synthroid label says peak effect may take 4 to 6 weeks. The American Thyroid Association says levels reach a steady state in about 6 weeks, which is why TSH is rechecked 6 to 8 weeks after starting or changing treatment.
Does levothyroxine need adjusting in pregnancy?
Often, yes. The Synthroid label says pregnancy may increase how much levothyroxine is needed and advises checking TSH in each trimester. Anyone with hypothyroidism who is planning a pregnancy or has just found out should contact their clinician promptly rather than waiting for a routine check.
Important safety information
Key risks
- Too much causes symptoms of an overactive thyroid: a racing heart, anxiety, sweating, weight loss and poor sleep.
- Over-treatment raises the risk of atrial fibrillation and bone loss, especially in older adults and after menopause.
- Calcium, iron, antacids, some acid reflux drugs, soy, walnuts and fiber can reduce absorption, so timing matters.
- Biotin supplements can distort thyroid blood tests; pregnancy usually changes how much is needed.
Talk to a clinician first, or avoid it, if
- Anyone hoping to lose weight with it: the boxed warning forbids that use
- People with untreated adrenal insufficiency, until it is treated
- Older adults and people with heart disease need a cautious start and close monitoring
This is not a complete list of risks. Read the full prescribing information and any Medication Guide, and talk to a licensed clinician or pharmacist about your own situation. If you have a severe reaction, call 911. You can report side effects to the FDA through MedWatch at 1-800-FDA-1088.
Sources
- DailyMed: Synthroid (levothyroxine) prescribing information
- American Thyroid Association: Guidelines for the treatment of hypothyroidism (Thyroid 2014)
- American Thyroid Association: Hypothyroidism (patient information)
Also checked for this guide
Guide reviewed October 7, 2026.
Educational information only, not medical advice. It does not replace a conversation with a licensed clinician.
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