medicationPrescription only (US)
LevothyroxineBenefits, Dosage & Interactions
Also known as: synthroid
Levothyroxine, also known by brand names like Synthroid, is a synthetic form of the thyroid hormone thyroxine (T4). It is primarily used to treat hypothyroidism, a condition where the thyroid gland does not produce enough thyroid hormones. It is typically taken in the morning.
Researched by DoseRoutine R&D TeamReviewed for accuracy by Nicholas Alexander, RSELast updated
Evidence: Strong — 1 human randomized trial and regulatory approval on file.
Evidence strengthStrong · 4/4
PreclinicalStrong human evidence
1 human randomized trial and regulatory approval on file.
Plasma half-life
~168 h
Typical timing
morning
Food rule
empty stomach
Default unit
mcg
What published protocols report
Ranges documented in the literature, shown for reference. DoseRoutine does not recommend an amount — your prescriber or the product label sets the dose.
Reported for Levothyroxine in the cited sources. Published ranges are not dosing advice.
References & evidence
Documents the Levothyroxine entry is written from. Each number matches an inline marker above.
- [1]Thyroid Hormone Therapy for Older Adults with Subclinical HypothyroidismStott DJ, Rodondi N, Kearney PM, et al. · New England Journal of Medicine · 2017 · Peer-reviewed · PMID 28402245
- [2]Subclinical Hypothyroidism: A ReviewBiondi B, Cappola AR, Cooper DS · JAMA · 2019 · Peer-reviewed · PMID 31287527
How to track Levothyroxine doses
Tracking Levothyroxine well takes four fields and a habit. Here is what to record so the log is still useful in three months.
- 1
Add the dose and unit
Log Levothyroxine with its dose in mcg so totals stay comparable over time — including days when you split the dose or skip it.
- 2
Set the time of day and food rule
Typical timing is morning. Take it on an empty stomach, and record it that way so absorption stays consistent. Reminders fire at that time and can export to your calendar.
- 3
Check it against the rest of the stack
Run Levothyroxine through the interaction checker against everything else in the routine — supplements, peptides, hormones and prescriptions — before it becomes a daily habit.
- 4
Log adherence, not intentions
Mark each dose taken, skipped or delayed as it happens. Weeks of honest logs are what make an adherence rate or a trend line meaningful; retrospective guessing is not.
- 5
Review against outcomes
With a reported half-life around 168 h, effects and timing shifts show up over days rather than instantly. Review Levothyroxine alongside your logged metrics and any relevant blood work every few weeks before changing the dose.
What to log each time
- Dose in mcg
- Time of day and whether it was taken on an empty stomach
- Taken / skipped / delayed
- Any side effects or notable changes
What is Levothyroxine?
Levothyroxine, also known by brand names like Synthroid, is a synthetic form of the thyroid hormone thyroxine (T4). It is primarily used to treat hypothyroidism, a condition where the thyroid gland does not produce enough thyroid hormones. Thyroid hormones are crucial for regulating the body's metabolism, growth, and development. When the body lacks sufficient thyroid hormones, various bodily functions can slow down, leading to symptoms like fatigue, weight gain, constipation, and cold intolerance. Levothyroxine works by replacing the missing hormone, restoring normal metabolic function.
Levothyroxine is also used to treat or prevent various types of goiter (enlargement of the thyroid gland) and, in conjunction with surgery and radioactive iodine therapy, to manage thyroid cancer. It is a prescription medication and its use must be directed and monitored by a licensed clinician. (NIH ODS)
What does the research say about Levothyroxine?
Tap a section to expand.
Levothyroxine is chemically identical to the naturally occurring thyroid hormone thyroxine (T4). Once ingested, it is absorbed into the bloodstream. In the body's tissues, T4 is converted into its more active form, triiodothyronine (T3), primarily through the action of deiodinase enzymes. Both T4 and T3 exert their effects by binding to specific thyroid hormone receptors located in the nucleus of cells throughout the body.
Upon binding to these receptors, T3 influences gene expression, leading to the synthesis of various proteins that are essential for metabolic regulation. These proteins play roles in oxygen consumption, energy production (e.g., glucose and lipid metabolism), heat production, protein synthesis, and the growth and development of numerous organs, including the brain, heart, and skeletal muscle. By providing an exogenous source of T4, levothyroxine effectively raises the levels of thyroid hormones in the body, correcting the deficiency seen in hypothyroidism and restoring normal physiological functions. (DrugBank, NIH ODS)
The primary benefit of levothyroxine is the resolution of symptoms associated with hypothyroidism. By replacing the deficient thyroid hormones, it helps to restore normal metabolic function. Patients typically experience an improvement in their well-being, which can include:
* **Increased Energy Levels:** Combats fatigue and lethargy.
* **Weight Management:** Helps reverse unexplained weight gain and promotes a healthy metabolism.
* **Improved Mood and Cognitive Function:** Addresses symptoms like depression, poor concentration, and memory issues.
* **Reduced Cold Sensitivity:** Improves the body's ability to regulate temperature.
* **Regular Bowel Movements:** Alleviates constipation.
* **Healthier Skin and Hair:** Can improve dry skin and hair thinning.
* **Cardiovascular Health:** Helps normalize heart rate and cholesterol levels, which can be affected by hypothyroidism.
* **Growth and Development:** Crucial for normal growth and brain development in infants and children with congenital hypothyroidism.
For goiter, levothyroxine can help shrink or prevent the enlargement of the thyroid gland. In the context of thyroid cancer, it is used to suppress thyroid-stimulating hormone (TSH), which can stimulate the growth of remaining thyroid cancer cells. (MedlinePlus, Mayo Clinic)
The efficacy of levothyroxine in treating hypothyroidism is well-established through extensive clinical research and long-standing clinical use. It is considered the gold standard treatment for this condition worldwide.
* **Clinical Trials:** Numerous studies have demonstrated that levothyroxine effectively normalizes TSH levels, a primary indicator of thyroid function, and resolves hypothyroid symptoms. Follow-up studies consistently show positive outcomes in patients receiving appropriate doses.
* **Meta-analyses:** Reviews and meta-analyses of clinical trials have consistently affirmed the benefits of levothyroxine in improving quality of life and biochemical parameters in hypothyroid patients. (Cochrane)
* **Clinical Guidelines:** Major professional organizations, including the American Thyroid Association, recommend levothyroxine as the first-line treatment for hypothyroidism based on strong evidence. Its use is supported by decades of experience and robust clinical data. (NIH ODS)
Evidence for its use in goiter and thyroid cancer suppression is also strong, based on clinical experience and studies showing its ability to reduce TSH production, thereby inhibiting goiter growth and recurrence of thyroid cancer. (Mayo Clinic)
Side effects from levothyroxine are primarily associated with overdosage, leading to symptoms of hyperthyroidism, or an allergic reaction to inactive ingredients. When the dose is properly adjusted, side effects are usually minimal. Symptoms of hyperthyroidism (too much thyroid hormone) can include:
* Palpitations, increased heart rate, or irregular heartbeat
* Chest pain
* Nervousness, anxiety, or irritability
* Tremors
* Diarrhea
* Heat intolerance and excessive sweating
* Weight loss
* Insomnia
* Headaches
* Hair loss (especially in the initial phase of treatment)
Less common side effects or those related to allergic reactions may include rash, itching, swelling (especially of the face, tongue, or throat), severe dizziness, or trouble breathing. It is important to report any new or worsening symptoms to a healthcare provider. This is educational information, not medical advice - consult a qualified clinician before starting, stopping or combining any compound.
Levothyroxine should be used with caution in certain populations and conditions.
* **Cardiovascular Disease:** Patients with pre-existing heart conditions, including coronary artery disease, angina, heart failure, or arrhythmias, should start levothyroxine at a low dose and have it increased gradually. Thyroid hormones can increase myocardial oxygen demand, potentially exacerbating cardiac symptoms. (NIH ODS)
* **Adrenal Insufficiency:** Untreated adrenal insufficiency should be corrected before starting levothyroxine to avoid precipitating an acute adrenal crisis. Thyroid hormones increase metabolic demand, which can overwhelm compromised adrenal glands. (EMA SmPC)
* **Diabetes:** Levothyroxine can increase blood glucose levels, potentially requiring dose adjustments of antidiabetic medications (e.g., insulin, oral hypoglycemics). Diabetic patients should monitor their blood glucose closely when starting or adjusting levothyroxine. (FDA label)
* **Osteoporosis:** Long-term overtreatment with levothyroxine, resulting in hyperthyroidism, has been associated with decreased bone mineral density and an increased risk of fractures, particularly in postmenopausal women. The dose should be carefully monitored to avoid supraphysiological levels of thyroid hormone. (MedlinePlus)
* **Elderly Patients:** Elderly patients may be more susceptible to the cardiovascular side effects of levothyroxine, and lower starting doses and slower titration are often recommended. (NIH ODS)
Regular monitoring of thyroid hormone levels (TSH, and sometimes T4) is crucial to ensure the correct dosage and prevent both hypo- and hyperthyroidism. Never adjust the dose without consulting a clinician. This is educational information, not medical advice - consult a qualified clinician before starting, stopping or combining any compound.
Levothyroxine is generally contraindicated in individuals with certain medical conditions.
* **Untreated Adrenal Insufficiency:** As mentioned in warnings, if untreated, starting levothyroxine can worsen adrenal crisis.(EMA SmPC)
* **Untreated Thyrotoxicosis:** Levothyroxine should not be used in individuals with uncorrected hyperthyroidism (thyrotoxicosis) as it would further elevate thyroid hormone levels, exacerbating the condition.(FDA label)
* **Acute Myocardial Infarction (Heart Attack):** While not an absolute contraindication for long-term use, initiating levothyroxine directly after an acute heart attack may be contraindicated due to the increased metabolic demand it places on the heart. Treatment should be deferred until the patient's condition stabilizes, and then initiated very cautiously if indicated.(NIH ODS)
* **Hypersensitivity:** Known hypersensitivity to levothyroxine or any inactive ingredients in the formulation is a contraindication. Allergic reactions can range from skin rashes to anaphylaxis. (EMA SmPC)
Always ensure a complete medical history is provided to your clinician before starting levothyroxine. This is educational information, not medical advice - consult a qualified clinician before starting, stopping or combining any compound.
Levothyroxine can interact with numerous medications, supplements, and even foods, affecting its absorption or metabolism. It's crucial to inform your clinician and pharmacist about all products you are taking.
* **Calcium Supplements & Antacids (e.g., calcium carbonate, aluminum hydroxide):** These can bind to levothyroxine in the gut, reducing its absorption. Separate administration by at least 4 hours. (FDA label)
* **Iron Supplements:** Similar to calcium, iron can impair levothyroxine absorption. Separate administration by at least 4 hours. (NIH ODS)
* **Proton Pump Inhibitors (PPIs) & H2 Blockers:** Medications like omeprazole or ranitidine can reduce gastric acidity, potentially affecting levothyroxine absorption. While not an absolute contraindication, close monitoring of TSH levels may be required. (FDA label)
* **Bile Acid Sequestrants (e.g., cholestyramine, colestipol):** These cholesterol-lowering drugs can bind levothyroxine, significantly reducing its absorption. Administer levothyroxine several hours apart (usually 4-5 hours) from these medications. (DrugBank)
* **Soy Products & High-Fiber Foods:** Large amounts of soy or dietary fiber can interfere with levothyroxine absorption. While complete avoidance isn't typically necessary, consistency in intake and monitoring of thyroid levels are recommended if significant dietary changes occur. (NIH ODS)
* **Certain Seizure Medications (e.g., phenytoin, carbamazepine, phenobarbital):** These can increase the metabolism of thyroid hormones, requiring an increased dose of levothyroxine. (DrugBank)
* **Warfarin:** Levothyroxine can enhance the anticoagulant effect of warfarin, increasing the risk of bleeding. Close monitoring of INR and potential warfarin dose reduction is necessary when initiating or adjusting levothyroxine. (FDA label)
* **Estrogen-Containing Products (e.g., oral contraceptives, hormone replacement therapy):** Estrogens can increase serum thyroxine-binding globulin (TBG) levels, reducing the amount of free (active) thyroid hormone. This may necessitate an increase in levothyroxine dosage. (MedlinePlus)
This is not an exhaustive list. Always consult your prescribing clinician or pharmacist before starting, stopping, or mixing any medication or supplement with levothyroxine. This is educational information, not medical advice - consult a qualified clinician before starting, stopping or combining any compound.
Levothyroxine is typically taken once daily in the morning, on an empty stomach, at least 30 to 60 minutes before breakfast or any other medications, and ideally at least 4 hours apart from calcium or iron supplements. Taking it consistently at the same time each day is important for maintaining stable thyroid hormone levels. Some clinicians may recommend taking it at bedtime, at least 3-4 hours after the last meal, as an alternative for patients who struggle with morning adherence or absorption issues. The specific timing and food interactions must be directed by a licensed clinician. (FDA label)
What interacts with Levothyroxine?
| Interacts with | Severity | Type | What happens | What to do |
|---|---|---|---|---|
| Calcium | Caution | Compound pairConfidence: theoretical | Calcium reduces absorption of thyroid medication when taken together.Source pending | Separate by about 4 hours.Suggested separation: 4 h |
| Iron (ferrous bisglycinate) | Caution | Compound pairConfidence: theoretical | Iron reduces absorption of thyroid medication when taken together.Source pending | Separate by about 4 hours.Suggested separation: 4 h |
| Any supplement + medication | Note | Category ruleConfidence: theoretical | Some supplements change how the body processes medications (absorption, liver enzymes, or additive effects).Source pending | Specific pairs are checked against a licensed drug database and shown with their own severity. Always confirm with your provider or pharmacist. |
| Any vitamin + medication | Note | Category ruleConfidence: theoretical | Certain vitamins interact with specific medications (e.g., vitamin K with blood thinners).Source pending | Specific pairs are checked against a licensed source; confirm with your provider. |
| Ashwagandha | Note | Compound pairConfidence: theoretical | Ashwagandha can raise T4/T3 levels and may amplify thyroid medication effect.Source pending | Monitor TSH when starting ashwagandha on levothyroxine. |
| Magnesium Glycinate | Caution | Compound pairConfidence: theoretical | Divalent cations chelate levothyroxine in the gut and reduce absorption by up to 30%.Source pending | Take levothyroxine on empty stomach; separate mineral supplements by at least 4 hours.Suggested separation: 4 h |
| Magnesium Citrate | Caution | Compound pairConfidence: theoretical | Divalent cations chelate levothyroxine in the gut and reduce absorption by up to 30%.Source pending | Take levothyroxine on empty stomach; separate mineral supplements by at least 4 hours.Suggested separation: 4 h |
| Magnesium Oxide | Caution | Compound pairConfidence: theoretical | Divalent cations chelate levothyroxine in the gut and reduce absorption by up to 30%.Source pending | Take levothyroxine on empty stomach; separate mineral supplements by at least 4 hours.Suggested separation: 4 h |
| Zinc | Caution | Compound pairConfidence: theoretical | Divalent cations chelate levothyroxine in the gut and reduce absorption by up to 30%.Source pending | Take levothyroxine on empty stomach; separate mineral supplements by at least 4 hours.Suggested separation: 4 h |
| Zinc Picolinate | Caution | Compound pairConfidence: theoretical | Divalent cations chelate levothyroxine in the gut and reduce absorption by up to 30%.Source pending | Take levothyroxine on empty stomach; separate mineral supplements by at least 4 hours.Suggested separation: 4 h |
| Zinc Bisglycinate | Caution | Compound pairConfidence: theoretical | Divalent cations chelate levothyroxine in the gut and reduce absorption by up to 30%.Source pending | Take levothyroxine on empty stomach; separate mineral supplements by at least 4 hours.Suggested separation: 4 h |
| Omeprazole | Caution | Compound pairConfidence: theoretical | Reduced gastric acid impairs levothyroxine absorption.Source pending | Monitor TSH after starting/stopping PPI; dose adjustment may be needed. |
| Omeprazole (Prilosec) | Caution | Compound pairConfidence: theoretical | Reduced gastric acid impairs levothyroxine absorption.Source pending | Monitor TSH after starting/stopping PPI; dose adjustment may be needed. |
| Pantoprazole | Caution | Compound pairConfidence: theoretical | Reduced gastric acid impairs levothyroxine absorption.Source pending | Monitor TSH after starting/stopping PPI; dose adjustment may be needed. |
| Esomeprazole | Caution | Compound pairConfidence: theoretical | Reduced gastric acid impairs levothyroxine absorption.Source pending | Monitor TSH after starting/stopping PPI; dose adjustment may be needed. |
| Lansoprazole | Caution | Compound pairConfidence: theoretical | Reduced gastric acid impairs levothyroxine absorption.Source pending | Monitor TSH after starting/stopping PPI; dose adjustment may be needed. |
Frequently asked questions about Levothyroxine
Levothyroxine is a medication. It is also known as synthroid. What follows is drawn from peer-reviewed literature indexed on PubMed, FDA label text where a label exists, and NIH reference records.
Levothyroxine is commonly discussed in the context of the goals discussed on its profile page. Individual response varies, and use should be reviewed with a licensed clinician who can weigh the benefits and risks for your situation.
For Levothyroxine, it is typically taken in the morning, it is usually taken on an empty stomach, and its approximate half-life is 168 hours, which influences dosing frequency. Always follow the specific dose your clinician or the product label prescribes.
Levothyroxine carries potential side effects and drug interactions, documented in NIH, Mayo Clinic, and FDA label sources. Levothyroxine is classified as a controlled substance in some jurisdictions, so legal status and prescribing rules apply. Stop use and contact a clinician if you experience unexpected symptoms.
Levothyroxine is a medication, and interactions are possible with prescriptions, hormones, peptides and other supplements in the same routine. Because Levothyroxine is usually taken in the morning, most avoidable conflicts come from what else lands in that same window. With a reported plasma half-life near 168 hours, separating doses can change the picture as much as removing one. Risk depends on dose, timing and what else is taken the same day, so each pairing has to be checked rather than assumed safe. The free checker at https://doseroutine.com/interaction-checker covers Levothyroxine with no sign-up.
Combining a medication like Levothyroxine with TRT is usually a question of labs rather than a blanket yes or no: the ester, injection interval and any aromatase inhibitor all change the answer. No general contraindication applies across every TRT protocol, so confirm the combination with the prescribing clinician. See the free TRT interaction reference: https://doseroutine.com/trt-supplement-interactions
Each peptide class carries its own profile against Levothyroxine: a healing peptide raises different questions than a GLP-1 agonist or a growth-hormone secretagogue. Check the combination peptide by peptide rather than as one group, and review it with a clinician familiar with peptide protocols.
A long plasma half-life of about 168 hours means Levothyroxine is commonly dosed every few days or weekly rather than daily. It is typically taken in the morning. Most protocols have it taken on an empty stomach. Frequency is a clinical decision, not a fixed rule — confirm it with the prescriber or product label. Comparison of apps that keep a schedule like this: https://doseroutine.com/best-dose-tracking-apps
With a plasma half-life near 168 hours, a single missed dose of Levothyroxine usually has a smaller effect on overall exposure than an irregular pattern of missed doses does. Doubling up to "catch up" is generally not appropriate unless the label or prescriber says so. Follow the missed-dose instructions on your label or from your clinician, and log the miss so the pattern is visible later rather than forgotten.
For Levothyroxine the useful record is the dose, the time it was actually taken, and what else was taken in the same window. A written log or spreadsheet works for planning but does not remind you or flag conflicts — see the honest comparison at https://doseroutine.com/vs/spreadsheet and the wider roundup at https://doseroutine.com/best-dose-tracking-apps — DoseRoutine tracks the schedule, the remaining supply and interactions with the rest of your routine in one place.
Verify at
Publisher search links for Levothyroxine. These are places to check the information — they are not citations, so they are not numbered.
- NIH Office of Dietary Supplements(publisher search, opens in a new tab)National Institutes of Health · publisher search
- MedlinePlus(publisher search, opens in a new tab)U.S. National Library of Medicine · publisher search
- PubChem(publisher search, opens in a new tab)National Center for Biotechnology Information · publisher search
- Mayo Clinic(publisher search, opens in a new tab)Mayo Clinic · publisher search
- FDA-approved labeling (DailyMed)(publisher search, opens in a new tab)U.S. Food and Drug Administration · publisher search
- European Medicines Agency(publisher search, opens in a new tab)European Medicines Agency · publisher search
- Cochrane Library(publisher search, opens in a new tab)Cochrane · publisher search
- DrugBank(publisher search, opens in a new tab)DrugBank Online · publisher search
- PubMed literature search(publisher search, opens in a new tab)National Library of Medicine · publisher search
- Wikipedia entry(publisher search, opens in a new tab)Wikipedia · publisher search
DoseRoutine compiles summaries from publicly available scientific and regulatory references. Always verify important decisions with a licensed clinician. How we source and review this information.
What is the short answer on Levothyroxine?
Plain-text summary, safe to quote verbatim:
Levothyroxine, also known by brand names like Synthroid, is a synthetic form of the thyroid hormone thyroxine (T4). It is primarily used to treat hypothyroidism, a condition where the thyroid gland does not produce enough thyroid hormones. It is typically taken in the morning.
Source: DoseRoutine — https://doseroutine.com/library/levothyroxine
Cite this page
Using this in an article, AI answer, or research note? Please attribute:
DoseRoutine. (2026). Levothyroxine — Overview, Benefits & Side Effects. Retrieved from https://doseroutine.com/library/levothyroxine
What compounds are similar to Levothyroxine?
Others in the medication category or studied for the same goals.