Prescription medication · Thyroid Hormone
levothyroxine
Also sold as Synthroid, Levoxyl, Tirosint. This medicine treats hypothyroidism, a condition where the thyroid gland doesn't make enough thyroid hormone.
Data updated 2026-05-15
- 304,465
- FDA reportsHeavily reported
- 22
- InteractionsSeveral interactions
- In shortage
- FDA status
What the data shows
levothyroxine (Synthroid) is a prescription Thyroid Hormone, in the top 5% of tracked drugs by FAERS report volume (304,465 FDA reports), with 22 documented drug interactions.
Reporting volume reflects how widely a drug is used and studied, not how dangerous it is, a FAERS report documents a temporal association, never proof of cause.
levothyroxine (Synthroid) is a prescription Thyroid Hormone. This medicine treats hypothyroidism, a condition where the thyroid gland doesn't make enough thyroid hormone.
Levothyroxine is a medicine that replaces a hormone normally made by your thyroid gland. It is used when the thyroid doesn't make enough hormone on its own.
Verify with FDA → · CMS NADAC pricing →
Drug Shortage Alert
levothyroxine is currently listed as to be discontinued by the FDA. Affected manufacturer: EMD Serono, Inc..
View all drug shortages →Drug Pricing (NADAC)
Brand Price
$0.92/unit
Generic Price
$3.07/unit
Generic Available
Yes (11 manufacturers)
Pricing data from NADAC (CMS), effective December 18, 2024. Compare all drug costs →
What it does
This medicine treats hypothyroidism, a condition where the thyroid gland doesn't make enough thyroid hormone.
Common side effects
Fatigue, Increased appetite, Weight loss
Key warnings
Thyroid hormones, including levothyroxine, should not be used for weight loss or to treat obesity.
The sections below are summarized in plain English from levothyroxine's FDA-approved prescribing information. They describe what the official label says, and are not personal medical advice.
How It Works
Levothyroxine provides a synthetic form of thyroxine (T4), which is the main hormone produced by the thyroid gland. Your body converts T4 into triiodothyronine (T3), the active form of the hormone. By providing T4, levothyroxine helps restore normal thyroid hormone levels in your body.
How to Take It
Take levothyroxine once a day on an empty stomach. It is best to take it 30 minutes to 1 hour before breakfast with a full glass of water. Wait at least 4 hours before or after taking medicines that can affect how your body absorbs levothyroxine. Certain foods can also affect absorption, so talk to your doctor if you regularly eat these foods within an hour of taking levothyroxine.
This is a plain-language summary of levothyroxine's FDA labeling, not individualized dosing advice. Ask a pharmacist or prescriber before changing how you take this medication.
Pregnancy & Breastfeeding
Tell your doctor if you are pregnant or plan to become pregnant. Your levothyroxine dose may need to be adjusted during pregnancy. Levothyroxine passes into breast milk, but it is not expected to harm the baby.
This is a plain-language summary of levothyroxine's FDA labeling, not individualized advice. Ask a pharmacist or prescriber about pregnancy or breastfeeding on this medication.
Missed Dose
If you miss a dose, take it as soon as you remember. If it is almost time for your next dose, skip the missed dose and continue with your regular schedule.
This is a plain-language summary of levothyroxine's FDA labeling, not individualized advice. Ask a pharmacist or prescriber what to do about your specific missed dose.
Storage
Store levothyroxine at room temperature, away from light and moisture.
Side Effects (from patient reports)
Based on 304,465 FDA adverse event reports.
Most-reported reactions
Adverse reactions in FAERS for levothyroxine, by number of reports
- Tiredness
Tiredness
25,847 reports
- Feeling sick to your stomach
Feeling sick to your stomach
22,021 reports
- Head pain
Head pain
18,798 reports
- Loose stools
Loose stools
18,178 reports
- Difficulty breathing
Difficulty breathing
16,216 reports
- General pain
General pain
15,936 reports
- Feeling lightheaded
Feeling lightheaded
15,429 reports
- Weakness
Weakness
13,164 reports
- Joint pain
Joint pain
12,986 reports
- Feeling unwell
Feeling unwell
12,116 reports
What this shows Bars show how often each reaction was reported, not how likely it is to happen, a report records a temporal association, never proof that the drug caused it.
Reports over time
Adverse-event reports filed for levothyroxine each year to the FDA Adverse Event Reporting System (FAERS).
Year-to-year volume tracks usage, prescribing, and scrutiny, not a change in per-patient risk. Source: FDA FAERS.
Where levothyroxine sits
levothyroxine has more FDA adverse-event reports than 98% of the drugs FAERS tracks. A high position reflects how widely a drug is used and watched, not how dangerous it is.
Percentile across all drugs PlainMeds tracks by FAERS report volume. The dot is levothyroxine; the line is the median (50th percentile).
FDA Adverse Event Report Analysis
Detailed analysis of 304,465 reports from the FDA Adverse Event Reporting System (FAERS). Reports span 1997–2025.
Total Reports
304,465
Reports Mentioning Death
19,579
6.4% of reports — not proof of cause
Hospitalization Reports
88,368
Top Indication
Product Used For Unknown Indication
Gender Distribution
Age Distribution
Most Reported Adverse Reactions (FAERS)
| # | Reaction | Reports |
|---|---|---|
| 1 | FATIGUE | 25,847 |
| 2 | NAUSEA | 22,026 |
| 4 | HEADACHE | 18,797 |
| 5 | DIARRHOEA | 18,176 |
| 6 | DYSPNOEA | 16,216 |
| 7 | PAIN | 15,934 |
| 8 | DIZZINESS | 15,424 |
| 9 | ASTHENIA | 13,159 |
| 10 | ARTHRALGIA | 12,987 |
| 12 | MALAISE | 12,116 |
| 13 | VOMITING | 11,622 |
| 14 | FALL | 11,464 |
| 15 | WEIGHT DECREASED | 9,798 |
| 16 | RASH | 9,623 |
| 17 | PAIN IN EXTREMITY | 9,622 |
Reactions in Death Reports
Reactions in Hospitalization Reports
Source: FDA FAERS (Adverse Event Reporting System) FDA FAERS (Adverse Event Reporting System) Reports are voluntary and do not establish causation
Serious Warnings
Thyroid hormones, including levothyroxine, should not be used for weight loss or to treat obesity. Using high doses can cause serious and life-threatening side effects, especially when taken with stimulant medicines.
Known Drug Interactions
Administration of sertraline in patients stabilized on levothyroxine sodium may result in increased levothyroxine sodium requirements. Administration of sertraline in patients stabilized on levothyroxine sodium may result in increased levothyroxine sodium requirements.
Mechanism: Taking sertraline can make your thyroid medication less effective, meaning your body might need more of it to stay healthy.
What to do: Your doctor should check your thyroid levels after you start or change your dose of sertraline. They may need to increase your thyroid medication dose.
Other drugs: Carbamazepine Furosemide (> 80 mg IV) Heparin Hydantoins Non-Steroidal Anti-inflammatory Drugs - Fenamates These drugs may cause protein-binding site displacement. Furosemide has been shown to inhibit the protein binding of T4 to TBG and albumin, causing an increase free T4 fraction in serum. Furosemide competes for T4-binding sites on TBG, prealbumin, and albumin, so that a single high dose can acutely lower the total T4 level.
Mechanism: High doses of this water pill can knock thyroid hormones off the proteins they usually travel on in your blood. This changes how much active thyroid hormone is available for your body to use.
What to do: Your doctor may need to monitor your thyroid levels closely if you are taking high doses of furosemide. They will check to see if your medication dose needs to be changed.
Drug or Drug Class Effect Beta-adrenergic antagonists (e.g., Propranolol > 160 mg/day) In patients treated with large doses of propranolol (> 160 mg/day), T3 and T4 levels change, TSH levels remain normal, and patients are clinically euthyroid. Drug or Drug Class Effect Beta-adrenergic antagonists (e.g., Propranolol > 160 mg/day) In patients treated with large doses of propranolol (> 160 mg/day), T3 and T4 levels change, TSH levels remain normal, and patients are clinically euthyroid.
Mechanism: Taking large doses of this blood pressure medicine can change the levels of thyroid hormones in your blood. However, it usually does not change how your thyroid actually functions or how you feel.
What to do: Your doctor may monitor your thyroid blood tests if you take high doses of propranolol. Usually, no change in your thyroid medication is needed.
Drug or Drug Class Effect Phosphate Binders (e.g., calcium carbonate, ferrous sulfate, sevelamer, lanthanum) Phosphate binders may bind to levothyroxine. Drug or Drug Class Effect Phosphate Binders (e.g., calcium carbonate, ferrous sulfate, sevelamer, lanthanum) Phosphate binders may bind to levothyroxine.
Mechanism: Ferrous sulfate can stick to levothyroxine in your stomach, which prevents your body from absorbing the thyroid medicine correctly.
What to do: Take these two medications at least four hours apart to make sure your thyroid medicine works as it should.
7.5 Antidepressant Therapy Concurrent use of tricyclic (e.g., amitriptyline) or tetracyclic (e.g., maprotiline) antidepressants and levothyroxine sodium may increase the therapeutic and toxic effects of both drugs, possibly due to increased receptor sensitivity to catecholamines. 7.5 Antidepressant Therapy Concurrent use of tricyclic (e.g., amitriptyline) or tetracyclic (e.g., maprotiline) antidepressants and levothyroxine sodium may increase the therapeutic and toxic effects of both drugs, possibly due to increased receptor sensitivity to catecholamines.
Mechanism: Using these drugs together can make your body more sensitive to certain chemicals, which may increase the side effects of both medications.
What to do: Your doctor should monitor you closely for any unusual symptoms or increased drug effects while taking this combination.
This is a plain-language summary of interactions documented in FDA labeling, not individualized advice. Ask a pharmacist or prescriber before combining medications.
Common Questions
Can I use levothyroxine for weight loss?
What should I do if I experience chest pain while taking this medicine?
Can other medicines affect levothyroxine?
How long does it take for levothyroxine to work?
Will I need regular blood tests?
Can I stop taking this medicine if I feel better?
What if I have adrenal insufficiency?
Can I take levothyroxine with food?
What are the symptoms of taking too much levothyroxine?
Is there gluten in levothyroxine?
What are the common side effects of levothyroxine?
Does levothyroxine interact with other medications?
What drug class is levothyroxine?
Is levothyroxine safe during pregnancy?
Has levothyroxine been recalled?
Is levothyroxine currently in shortage?
Active Recalls
Subpotent Drug
MACLEODS PHARMA USA, INC
Product Mix-Up: A single bottle of this lot was reported by a pharmacy to contain 88 mcg tablets instead of 150 mcg tablets.
Alvogen, Inc
Subpotent Drug: Assay below the approved specification
ACCORD HEALTHCARE, INC.
Subpotent Drug: Assay below the approved specification
ACCORD HEALTHCARE, INC.
Subpotent
ACCORD HEALTHCARE, INC.
Super-Potent Drug: Out of specification potency results were obtained.
Mylan Institutional, Inc.
Subpotent and Superpotent Drug
Mylan Institutional, Inc.
Subpotent and Superpotent Drug
Mylan Institutional, Inc.
Subpotent and Superpotent Drug
Mylan Institutional, Inc.
Subpotent and Superpotent Drug
Mylan Institutional, Inc.
Subpotent and Superpotent Drug
Mylan Institutional, Inc.
Subpotent and Superpotent Drug
Mylan Institutional, Inc.
Superpotent Drug and Subpotent Drug: potency failures obtained
Viatris Inc
Superpotent Drug and Subpotent Drug: potency failures obtained
Viatris Inc
Superpotent Drug and Subpotent Drug: potency failures obtained
Viatris Inc
Superpotent Drug and Subpotent Drug: potency failures obtained
Viatris Inc
Superpotent Drug and Subpotent Drug: potency failures obtained
Viatris Inc
Subpotent drug
ACCORD HEALTHCARE, INC.
Related Medications in Thyroid Hormone
Other drugs grouped near levothyroxine - same-class peers and common alternatives.
liothyronine
Cytomel
Liothyronine (Cytomel) is a medicine that replaces a thyroid hormone called T3.
Compare with levothyroxine →
methimazole
Tapazole
Methimazole (Tapazole) is a medicine that lowers the amount of thyroid hormone your body makes.
Compare with levothyroxine →
propylthiouracil
PTU
Propylthiouracil (PTU) is a medicine that treats an overactive thyroid.
Compare with levothyroxine →
thyroid (desiccated)
Armour Thyroid, Nature-Throid
Armour Thyroid is a natural thyroid hormone medicine.
Compare with levothyroxine →
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What the FDA Data Shows for levothyroxine
The FDA label for levothyroxine (sold under brand names such as Synthroid, Levoxyl, Tirosint) classifies it as a prescription-only medication in the Thyroid Hormone class. This medicine treats hypothyroidism, a condition where the thyroid gland doesn't make enough thyroid hormone. Official labeling lists 20 commonly reported side effects, including Fatigue, Increased appetite, Weight loss.
Post-market surveillance from the FDA Adverse Event Reporting System (FAERS) captures real-world experience. For this drug, FAERS contains 304,465 voluntary reports. The database also lists 22 documented drug interactions derived from FDA labeling, with the top-flagged interaction rated minor severity. NADAC pricing from CMS shows a generic unit cost of $3.07 versus $0.92 for the brand.
Report counts do not establish causation, a FAERS entry documents a temporal association, not proof that the drug produced the outcome. Widely prescribed medications naturally accumulate more reports than niche therapies, so raw totals must be interpreted alongside total exposure. Shortage status, recall history (currently 18 recall records on file), and patent information further shape supply and switching decisions. This page summarizes public FDA data for educational purposes only and is not a substitute for professional medical advice, always consult a licensed healthcare provider before starting, stopping, or changing any medication.
Data Sources
Drug labeling: FDA Drug Labels (SPL/DailyMed). Adverse events: FDA Adverse Event Reporting System (FAERS). Pricing: CMS National Average Drug Acquisition Cost (NADAC). Shortage status: FDA Drug Shortages Database.
FAERS reports are voluntary and do not establish causation. Drug interactions are derived from FDA labeling and clinical references. Always consult a healthcare professional before making medication decisions.
Last updated: July 22, 2023
Read our methodology - how this data is sourced, computed, and verified.
Data currency: FDA FAERS adverse-event reports through 2025, CMS NADAC acquisition-cost pricing effective December 2024, compiled and last refreshed May 2026. See our methodology for per-source dates and refresh cadence. Spot a figure that looks wrong? Report a correction.
All federal data sources used on this page
- FDA Orange Book - approved drug products with therapeutic equivalence. accessdata.fda.gov/cder/ob
- FDA DailyMed - NIH-hosted drug labeling for FDA-approved meds. dailymed.nlm.nih.gov
- FDA Adverse Event Reporting System (FAERS) - post-marketing safety surveillance. fda.gov/drugs/faers
- NLM RxNorm - standardized clinical drug nomenclature. nlm.nih.gov/research/umls/rxnorm
- CMS Medicare Part B Drug Average Sales Price Files - federal drug pricing data. cms.gov/medicare/part-b-drugs/asp
- FDA Drug Shortages Database - current and resolved drug shortage tracking. accessdata.fda.gov/drugshortages