Key Takeaways
- The FDA prescribing information lists side effects across multiple body systems, including cardiovascular, neurological, gastrointestinal, and dermatological, most are dose-related and improve with adjustments.
- The FDA label lists hair loss as a dermatologic side effect; it typically occurs in the first few months of therapy and is usually temporary, per clinical guidance.
- A 2021 study published in PLOS One found that 27% of chronic levothyroxine users are outside their therapeutic TSH range, 17% overtreated and 10% undertreated, making dose-related side effects more common than many patients realize.
- Per the FDA prescribing information, most side effects of levothyroxine are signs of overtreatment (too high a dose) rather than reactions to the drug itself.
- A 2024 meta-analysis published in BMC Endocrine Disorders found that levothyroxine therapy shows only a slight adverse effect on lumbar spine bone mineral density in patients with overt hypothyroidism; postmenopausal women on TSH-suppressive therapy face the highest risk.
Three months after starting levothyroxine, Maria noticed clumps of hair on her pillow and was convinced something had gone wrong. Her doctor had recently adjusted her dose, and her TSH was finally in range. But the hair loss was alarming enough that she started questioning whether to stop taking the medication altogether.
What her doctor hadn’t had time to explain: the FDA prescribing information lists hair loss as a known dermatologic side effect, it typically occurs in the early months of treatment, and it usually resolves on its own. Maria wasn’t having an unusual reaction. She was experiencing something documented, expected, and temporary.
This is the problem with levothyroxine side effect information online: it exists, but it’s scattered, poorly organized, and rarely connected to what patients actually need to know. This guide compiles what the FDA label, published research, and ATA guidelines say about levothyroxine side effects, organized so you can understand what you’re experiencing, what causes it, and what to do.
What Are the Side Effects of Levothyroxine?
According to the FDA prescribing information, levothyroxine side effects span multiple body systems and are most commonly associated with excessive dosing. The FDA label states that adverse reactions “are generally those of hyperthyroidism due to therapeutic overdosage,” meaning most side effects occur when the dose is higher than the body needs.
Side effects fall into two broad categories:
- Dose-related effects: These occur when TSH is suppressed below normal range, effectively putting the body into a mild hyperthyroid state. They resolve when the dose is corrected.
- Ingredient-related reactions: Some patients react to inactive ingredients (fillers, dyes, or binders) in specific formulations rather than to levothyroxine itself.
Understanding which category applies to a specific symptom shapes what the right response is.
The first clue that something is off is often a change in how the patient feels, not a lab result
Symptoms like insomnia, tremor, and feeling overstimulated are what push many patients to ask whether the dose is wrong.
What Are the Most Common Side Effects of Levothyroxine?
Does Levothyroxine Cause Hair Loss?
Yes, according to the FDA prescribing information, which lists hair loss (alopecia) as a known dermatologic side effect. The label notes that hair loss during the first few months of thyroid hormone therapy is typically temporary.
The mechanism: levothyroxine can temporarily push hair follicles into a resting phase during early treatment or with dose changes, a process known as telogen effluvium. A 2021 review in Thyroid describes telogen effluvium as a known response to physiological stress, including hormonal shifts during thyroid stabilization.
Most patients see improvement once their TSH stabilizes. However, if hair loss persists beyond four to six months or worsens, it is worth discussing with a healthcare provider, as persistent hair loss may indicate undertreated hypothyroidism rather than a drug effect.
For a detailed breakdown of this topic, see our dedicated guide to levothyroxine and hair loss.
Can Levothyroxine Cause Weight Changes?
The FDA prescribing information lists weight changes in both directions. Weight loss appears in the general side effects list as a common effect associated with overtreatment. As for weight gain, the picture is more complicated.
The FDA label does not list weight gain as a direct side effect of properly dosed levothyroxine. However, patients who remain undertreated, with TSH above the therapeutic range, may continue experiencing hypothyroidism-related weight gain. In practice, when dose optimization corrects this, the expectation is gradual improvement, not dramatic loss.
Per ATA clinical guidelines, levothyroxine therapy in properly treated hypothyroidism may result in modest weight improvement, but many patients do not return to their pre-hypothyroid weight.
Our guide on levothyroxine and weight changes covers what the research says in detail.
Can Levothyroxine Cause Heart Palpitations?
Yes. The FDA prescribing information lists palpitations, tachycardia (rapid heart rate), arrhythmias including atrial fibrillation, increased pulse and blood pressure, and, in severe cases, heart failure and cardiac arrest among cardiovascular side effects.
These effects are dose-dependent. According to the FDA label, they occur most often when TSH is suppressed below the therapeutic range, indicating that the dose is too high.
Notably, a 2024 systematic review published in PubMed on cardiac, neuropsychiatric, and musculoskeletal adverse events with levothyroxine confirmed that cardiovascular effects are primarily associated with overtreatment rather than appropriately dosed therapy.
If you experience palpitations after a dose increase, contact your healthcare provider. Persistent or severe cardiac symptoms require prompt evaluation.
For more, see our levothyroxine and heart palpitations guide.
Can Levothyroxine Cause Anxiety?
Yes. Nervousness, anxiety, irritability, and emotional lability are listed in the FDA prescribing information under central nervous system side effects. These effects are consistent with the stimulatory effects of excess thyroid hormone.
A 2024 systematic review in PubMed found neuropsychiatric adverse events, including anxiety and mood changes, are among the documented risks of levothyroxine, particularly in the context of overtreatment.
Patients who develop anxiety after a dose increase should discuss it with their provider. Specifically, a TSH check can confirm whether suppression is driving the symptoms.
See also our levothyroxine and anxiety overview.
Does Levothyroxine Cause Insomnia?
The FDA label lists insomnia as a central nervous system side effect. As with anxiety and palpitations, sleep disruption in levothyroxine patients typically reflects excess stimulation from an elevated dose.
Some patients find that taking levothyroxine at night worsens insomnia, while others report no difference. Per ATA guidance, timing adjustments are acceptable when morning dosing is not feasible, but the impact on sleep is individual.
Does Levothyroxine Cause Headaches or Tremors?
The FDA prescribing information lists both headache and tremor as central nervous system adverse reactions. Tremors, specifically fine tremors of the hands, are a classic sign of hyperthyroid excess and should prompt a review of the current dose.
The FDA label also lists muscle weakness, muscle spasm, and leg cramps as musculoskeletal side effects, and these are also more common in the context of overtreatment.
Can Levothyroxine Cause Digestive Problems?
The FDA prescribing information lists diarrhea, vomiting, and abdominal cramps as gastrointestinal adverse reactions. These symptoms are dose-related and typically improve when the dose is adjusted to maintain TSH in the therapeutic range.
📋 Looking for a quick-reference summary of levothyroxine side effects, interactions, and timing rules? Download our free Levothyroxine Patient Handbook, compiled from FDA prescribing information and ATA guidelines.
What Are the Less Common Side Effects of Levothyroxine?
Menstrual Changes
The FDA prescribing information lists menstrual irregularities as an endocrine system adverse effect. Irregular cycles may reflect either overtreatment (excess thyroid hormone accelerating the hormonal cycle) or undertreated hypothyroidism. A TSH check is the most direct way to distinguish between the two.
Heat Intolerance and Excessive Sweating
The FDA label lists heat intolerance, fever, and excessive sweating among general adverse effects. These symptoms mirror the presentation of hyperthyroidism and occur when levothyroxine suppresses TSH below the normal range.
Flushing and Rash
The FDA label identifies flushing as a dermatologic side effect. Rash may indicate either a dose-related effect or a reaction to inactive ingredients in a specific brand or generic formulation. We cover dye-related reactions in a dedicated section below.
Some side effects are really dose clues
Palpitations, shakiness, sweating, and new anxiety often point more toward over-replacement than an unpredictable medication.
What Are the Signs That Your Levothyroxine Dose Is Too High?
When levothyroxine is dosed above what the body requires, TSH is suppressed and symptoms of mild hyperthyroidism emerge. According to the FDA prescribing information, the clinical picture of overtreatment includes:
- Rapid or irregular heartbeat, palpitations
- Nervousness, anxiety, irritability
- Insomnia or disrupted sleep
- Tremors (particularly fine hand tremors)
- Excessive sweating and heat intolerance
- Unexplained weight loss despite adequate intake
- Diarrhea
- Headache
A 2021 study in PLOS One found that 17% of chronic levothyroxine users are overtreated, meaning their TSH is chronically below the therapeutic range. In fact, many of these patients experience symptoms without connecting them to dose excess.
James had been on 125 mcg of levothyroxine for four years when he started waking up at 3 a. m. with his heart pounding. His doctor had increased the dose six months earlier after a slightly elevated TSH reading. What neither of them tracked closely enough: his TSH had dropped to 0.1 mIU/L, well below the 0.4 mIU/L lower boundary of the normal range. A dose reduction back to 112 mcg brought his TSH to 1.8 mIU/L within eight weeks, and the palpitations stopped.
What to Do If You Suspect Overtreatment
If you experience several of the symptoms above, especially after a recent dose increase, contact your healthcare provider for a TSH check. Per our guide to dose adjustment and TSH monitoring, TSH should be checked six to eight weeks after any dose change.
What Are the Signs That Your Levothyroxine Dose Is Too Low?
Undertreatment means the dose is insufficient to normalize thyroid hormone levels. The symptoms of undertreatment are essentially the symptoms of hypothyroidism itself. Per ATA clinical practice guidelines and the FDA prescribing information, undertreated hypothyroidism presents as:
- Persistent fatigue and low energy
- Weight gain or difficulty losing weight
- Cold intolerance
- Brain fog, poor concentration, memory problems
- Dry skin and hair
- Constipation
- Depressed mood or emotional flatness
- Slow heart rate
The 2021 PLOS One study found that 10% of chronic levothyroxine users are undertreated. Importantly, many of these patients attribute ongoing symptoms to the medication itself when the correct interpretation is that the dose needs to be higher.
If your symptoms resemble hypothyroidism despite being on levothyroxine, ask your healthcare provider to check your TSH. “Normal” varies by patient and clinical context; ATA guidelines acknowledge that the optimal TSH target differs between patient populations.
A refill that looks different can be a meaningful clue
When symptoms begin right after the product changes, the problem may be dyes or fillers rather than the hormone itself.
Can Inactive Ingredients in Levothyroxine Cause Side Effects?
Yes, and this is one of the most underrecognized sources of adverse reactions in levothyroxine patients.
Levothyroxine tablets contain inactive ingredients (excipients) including fillers, binders, colorants, and sometimes synthetic dyes. Patients sensitive to any of these may experience reactions that are not caused by the levothyroxine molecule itself.
The FDA prescribing information for standard levothyroxine tablets acknowledges the presence of these ingredients. Specifically, reactions can include:
- Rash or hives
- Gastrointestinal upset
- Symptoms that do not improve with dose adjustment
Different brands and generics use different inactive ingredients. For example, Synthroid uses corn starch, acacia, lactose, sucrose, and FD&C dyes for color-coding by dose. By contrast, some generics use different fillers. Tirosint (levothyroxine soft gel capsule) was formulated specifically for patients with absorption concerns or sensitivities, containing only four ingredients: levothyroxine, gelatin, glycerin, and water.
When Linda switched from Synthroid to a generic after her insurance changed, she developed intermittent nausea and a skin rash within a few weeks. Her TSH was stable. Her provider identified a likely sensitivity to one of the generic’s inactive ingredients. Switching to Tirosint resolved the symptoms without any dose change.
If your side effects appeared after switching brands or generics, or do not respond to dose adjustment, discuss the possibility of an ingredient reaction with your healthcare provider. See our comparison of Tirosint and standard levothyroxine tablets for more on filler-free formulation options.
Think your symptoms may be dose-related? Understanding your TSH is the first step. Use our Thyroid Lab Result Explainer to decode what your numbers mean, based on published clinical reference ranges.
Is Levothyroxine Safe Long Term?
What Does the Research Show About Bone Density?
The relationship between long-term levothyroxine use and bone health has been studied extensively. A 2024 meta-analysis published in BMC Endocrine Disorders found the following:
- Levothyroxine therapy shows a slight adverse effect on lumbar spine bone mineral density (BMD) in patients with overt hypothyroidism
- Researchers found no significant BMD effect in patients with subclinical hypothyroidism
- Postmenopausal women on TSH-suppressive doses (doses that keep TSH below normal range, common in thyroid cancer treatment) are at the highest risk
- Premenopausal women and men show generally safe bone profiles under standard replacement therapy
The key variable is dose. Specifically, TSH-suppressive therapy, intentionally prescribed to suppress TSH in patients treated for thyroid cancer, carries higher bone risk than standard replacement therapy aimed at keeping TSH in the normal range.
Per ATA guidelines, postmenopausal women on levothyroxine should receive routine bone density monitoring, particularly those on higher doses.
What Does the Research Show About Cardiovascular Safety?
A 2024 systematic review in Systematic Reviews assessed cardiovascular and bone health outcomes in older adults with subclinical hypothyroidism treated with levothyroxine. The review found no significant association between appropriately dosed levothyroxine and major cardiovascular outcomes in patients 65 and older, with a hazard ratio of 0.89 (95% CI 0.71-1.12).
A 2019 study published in JAMA looking at adults aged 80 and older found that a 29.5% adverse event rate in treated patients was similar to the rate seen in the placebo group, suggesting that levothyroxine itself does not meaningfully increase adverse event risk when the dose is appropriate.
Taken together, the pattern across the literature is clear: properly dosed levothyroxine, keeping TSH within the target range, does not appear to carry significant long-term cardiovascular risk. Overtreatment, with chronic TSH suppression below the lower limit of normal, is associated with atrial fibrillation and cardiovascular strain, particularly in older patients.
The Bottom Line on Long-Term Safety
What the Evidence Supports
Levothyroxine has been prescribed for decades and is one of the most-dispensed medications in the United States. The body of research supports that it is safe for long-term use when:
- TSH remains within the appropriate target range
- A provider reviews the dose regularly (annually for stable patients, more frequently if clinical factors change)
- Higher-risk groups receive regular bone density monitoring (postmenopausal women, especially those on higher doses)
For a comprehensive review, see our levothyroxine long-term safety guide.
Some symptoms belong in an urgent workup, not a wait-and-see approach
Chest pain, significant palpitations, swelling, wheezing, or rapidly worsening symptoms deserve timely medical evaluation.
When to Contact Your Healthcare Provider
Contact Your Provider If You Experience
Per the FDA prescribing information and ATA clinical guidance, the following symptoms warrant a call to your healthcare provider:
- Palpitations, irregular heartbeat, or rapid pulse after a dose increase
- New or worsening anxiety, tremors, or insomnia
- Persistent hair loss beyond four to six months of stable therapy
- Weight loss without dietary changes
- Excessive sweating or heat intolerance
- Unexplained diarrhea or abdominal cramping
- Symptoms that suggest your dose may be too low (fatigue, cold intolerance, brain fog, constipation) despite adherence to your regimen
- Menstrual changes that have appeared or worsened since starting or adjusting levothyroxine
Seek Immediate Medical Attention If You Experience
⚠️ Per the FDA label: Contact emergency services or go to the nearest emergency room immediately if you experience chest pain, severe shortness of breath, severe rapid or irregular heartbeat, or seizures while taking levothyroxine.
These symptoms may indicate acute thyrotoxicosis or a serious cardiovascular event and require urgent evaluation.
Frequently Asked Questions
What are the most common side effects of levothyroxine?
According to the FDA prescribing information, the most commonly reported side effects include hair loss, weight changes, heart palpitations, anxiety, insomnia, tremors, headache, and diarrhea. Most are dose-related and improve when TSH is brought into the target range.
Does levothyroxine cause hair loss?
Yes, the FDA label lists hair loss (alopecia) as a known side effect. Per clinical guidance, it typically occurs in the first few months of treatment or after a dose change and is usually temporary. Persistent hair loss beyond four to six months warrants a TSH check to rule out undertreatment.
Can levothyroxine cause weight gain?
Weight gain is not a listed direct side effect of levothyroxine. However, undertreated hypothyroidism, in which the dose is insufficient to normalize TSH, can cause continued weight gain. Per ATA guidelines, properly treated patients may see modest improvement, though full return to pre-hypothyroid weight is not guaranteed.
What are the signs that my levothyroxine dose is too high?
Per the FDA prescribing information, overtreatment signs include palpitations, rapid heartbeat, anxiety, tremors, insomnia, heat intolerance, excessive sweating, and unexplained weight loss. A TSH below 0.4 mIU/L confirms overtreatment and signals that a dose reduction should be discussed with your provider.
What are the signs that my levothyroxine dose is too low?
Undertreated hypothyroidism produces the classic hypothyroid symptom set: fatigue, weight gain, cold intolerance, brain fog, dry skin, constipation, depressed mood, and slow heart rate. Per the 2021 PLOS One study, 10% of chronic users are undertreated. These symptoms should prompt a TSH check.
Is levothyroxine safe to take long term?
Published research supports long-term safety when levothyroxine is properly dosed. A 2024 meta-analysis in BMC Endocrine Disorders found minimal bone density effects at standard replacement doses, with higher risk in postmenopausal women on suppressive doses. A 2019 JAMA study found adverse event rates comparable to placebo in patients aged 80 and older.
Can levothyroxine cause anxiety?
Yes. The FDA prescribing information lists nervousness, anxiety, irritability, and emotional lability as central nervous system side effects. These are most commonly associated with overtreatment (TSH below normal range) and typically improve when the dose is adjusted.
Does levothyroxine affect bone density?
Per a 2024 meta-analysis in BMC Endocrine Disorders, levothyroxine shows a slight adverse effect on lumbar spine bone mineral density in patients with overt hypothyroidism. Risk is highest in postmenopausal women on TSH-suppressive therapy. Standard replacement doses in premenopausal women and men show minimal bone impact.
Can different brands or generics of levothyroxine cause different side effects?
Yes, differences in inactive ingredients between formulations can cause reactions in sensitive patients. If your symptoms changed after switching brands or generics, discuss the possibility of an ingredient sensitivity with your provider. Tirosint, which contains only four ingredients, is an option specifically formulated for patients with absorption issues or sensitivities.
What should I do if I miss a dose of levothyroxine?
Per the FDA prescribing information, 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. Do not double up doses. For the complete FDA-sourced protocol, see our missed dose guide.
The Bottom Line
Levothyroxine side effects are common, well-documented, and in most cases manageable. The FDA prescribing information makes clear that most adverse effects are dose-related rather than inherent to the drug. When TSH is maintained in the therapeutic range, the side effect risk profile drops substantially.
The three most important things to understand:
- Most side effects point to a dose problem, not a drug problem. Palpitations, anxiety, insomnia, tremors, and heat intolerance are signs of overtreatment. Fatigue, brain fog, and cold intolerance are signs of undertreatment. Both are fixable with dose adjustment.
- Hair loss in the first few months is expected and usually temporary. The FDA label documents it. It typically resolves as TSH stabilizes.
- Long-term safety is supported by evidence. At standard replacement doses, the research does not show significant cardiovascular or bone risk in appropriately dosed patients.
For a comprehensive reference you can bring to your next appointment, download our free Levothyroxine Patient Handbook, compiled directly from FDA prescribing information and ATA clinical guidelines. And for questions specific to your situation, consult your healthcare provider.
Sources & References
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FDA Prescribing Information — Levothyroxine Sodium Tablets. U. S. Food and Drug Administration. Approved 2019. https://www. accessdata. fda. gov/drugsatfda_docs/label/2019/021116s017lbl. pdf
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Pimentel AL, et al. “Prevalence and factors associated with chronic use of levothyroxine in a population of adults.” PLOS One. 2021;16(12): e0261629. PMC. https://pmc. ncbi. nlm. nih. gov/articles/PMC8687586/
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Stott DJ, et al. “Association Between Levothyroxine Treatment and Thyroid-Related Symptoms Among Adults Aged 80 Years and Older With Subclinical Hypothyroidism.” JAMA. 2019;322(20):1977-1986. https://jamanetwork. com/journals/jama/fullarticle/2753909
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Zhao X, et al. “Effects of levothyroxine therapy on bone mineral density and bone metabolism markers: a systematic review and meta-analysis.” BMC Endocrine Disorders. 2024. https://link. springer. com/article/10.1186/s12902-024-01819-7
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Aziz M, et al. “Cardiovascular and bone health outcomes in older people with subclinical hypothyroidism: a systematic review.” Systematic Reviews. 2024. https://link. springer. com/article/10.1186/s13643-024-02548-7
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Livadariu R, et al. “Risk of cardiac, neuropsychiatric and musculoskeletal adverse events with levothyroxine: Systematic review and meta-analysis.” PubMed. 2024. https://pubmed. ncbi. nlm. nih. gov/41559017/
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American Thyroid Association. “Hypothyroidism: Booklet for Patients and Their Families.” thyroid. org. https://www. thyroid. org/hypothyroidism/
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American Thyroid Association. “2012 ETA/ATA Guidelines for the Management of Thyroid Dysfunction During Pregnancy and Postpartum.” thyroid. org.
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Mayo Clinic. “Levothyroxine (Oral Route) — Side Effects.” https://www. mayoclinic. org/drugs-supplements/levothyroxine-oral-route/side-effects/drg-20072133
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MedlinePlus. “Levothyroxine.” U. S. National Library of Medicine. https://medlineplus. gov/druginfo/meds/a682461. html