What is the downside to taking levothyroxine?

October 11, 2025 •

3 min read

Over 20 million Americans take levothyroxine to treat hypothyroidism, but managing the medication can present challenges beyond simply taking a pill. While it is a life-changing treatment for many, understanding the potential downsides—including side effects, drug interactions, and long-term risks—is crucial for effective and safe management.

Quick Summary

Taking levothyroxine for hypothyroidism has potential downsides, primarily from incorrect dosing leading to symptoms like anxiety, palpitations, and weight fluctuations. Long-term risks include decreased bone density and cardiovascular issues, particularly in older adults. Medication absorption is also sensitive to timing with food and other supplements, necessitating careful management and regular monitoring to maintain optimal thyroid hormone levels.

Key Points

Dosage is critical: Most side effects, such as anxiety and heart palpitations, are directly related to the dose being too high.
Risk of osteoporosis: Long-term overtreatment can lead to decreased bone mineral density, increasing the risk of osteoporosis and fractures, particularly in older women.
Heart health concerns: Over-replacement of thyroid hormone can increase the risk of serious cardiovascular problems, including heart failure and irregular heartbeat.
Absorption interference: Taking the medication with food, certain supplements (calcium, iron), and antacids can significantly reduce its effectiveness.
Weight fluctuations: Weight gain is often a sign of under-dosing, while unintentional weight loss can signal an over-replacement of thyroid hormone.
Medication timing: For optimal absorption, levothyroxine should be taken on an empty stomach at a consistent time each day, separated from other medications and supplements.
Hair loss is often temporary: Some individuals may experience temporary hair loss in the initial months, which typically resolves as hormone levels stabilize.
Serious drug warning: Levothyroxine should never be used as a treatment for obesity or weight loss.

Common side effects of levothyroxine

Most side effects associated with levothyroxine occur when the dosage is not correctly balanced for an individual's needs. When the dose is too high, it can mimic the symptoms of hyperthyroidism, leading to a range of uncomfortable effects.

Cardiovascular issues: An incorrect dose can cause rapid or irregular heartbeats, palpitations, or chest pain. This risk is heightened in older patients or those with pre-existing heart conditions.
Neurological symptoms: Patients may experience anxiety, nervousness, irritability, or trouble sleeping. These effects can occur as the body adjusts to hormonal changes and typically subside once the dosage is optimized.
Hair loss: A temporary, diffuse hair loss can occur during the first few months of treatment as the body's hair growth cycle adjusts to new hormone levels. In most cases, this is mild and resolves with time.
Gastrointestinal problems: Some individuals may experience diarrhea, increased appetite, or stomach cramps. These symptoms are often linked to an overtreatment state where thyroid levels are too high.

Serious long-term risks

For individuals on long-term levothyroxine therapy, particularly older adults and post-menopausal women, certain long-term risks need careful consideration.

Bone mineral density loss: Long-term overtreatment can increase bone resorption, leading to decreased bone mineral density and an increased risk of osteoporosis and fractures. Regular monitoring of thyroid levels and bone health is recommended.
Cardiovascular complications: Over an extended period, an over-replacement of thyroid hormone can contribute to cardiovascular problems such as heart failure, atrial fibrillation, and an increased risk of heart attacks. This emphasizes the need for consistent and appropriate dosing.
Diabetes management: Levothyroxine can worsen glycemic control in diabetic patients. It may interact with diabetes medications, necessitating more frequent blood sugar monitoring and potential dosage adjustments by a healthcare provider.
Potential link to cancer and dementia: Some studies have observed an association between long-term levothyroxine therapy and an increased risk of certain cancers and dementia. However, findings on this topic are often controversial and require more conclusive research.

Drug and food interactions

Levothyroxine absorption can be significantly impacted by various factors, making consistent timing and careful attention to diet and other medications essential for efficacy.

Medications and supplements

Calcium and iron supplements: These supplements can interfere with the absorption of levothyroxine. It is recommended to take them at least four hours apart from your thyroid medication.
Antacids and proton pump inhibitors (PPIs): These can also reduce absorption, requiring careful scheduling or dosage adjustments.
Cholesterol-lowering drugs: Certain bile acid sequestrants, such as cholestyramine and colesevelam, must be taken several hours apart from levothyroxine.

Foods and beverages

Soy products, walnuts, and dietary fiber: These can hinder absorption and should be consumed several hours away from the medication.
Coffee and grapefruit juice: Drinking coffee within an hour of taking levothyroxine can reduce its absorption, while grapefruit juice may also have an inhibitory effect.

A comparison of under-dosing vs. over-dosing

Finding the right dosage is a fine balance. The table below outlines the potential consequences of getting the dosage wrong, which is a key downside to taking levothyroxine.


Issue	Under-dosing (Persistent Hypothyroidism)	Over-dosing (Symptoms of Hyperthyroidism)
Energy	Continued fatigue, low energy levels	Jittery sensations, restlessness, and anxiety
Weight	Potential weight gain and difficulty losing weight	Unexpected weight loss, increased appetite
Heart	Increased risk of high cholesterol and heart disease	Rapid or irregular heartbeat, palpitations, potential heart failure
Mental Health	Mood changes, depression	Irritability, anxiety, insomnia
Bone Health	No immediate risk related to dosage	Decreased bone mineral density, especially in post-menopausal women

Adherence and lifestyle factors

Consistency is critical for levothyroxine's effectiveness. Taking the medication at the same time every day, ideally on an empty stomach, helps maintain stable thyroid hormone levels. Forgetting doses or taking them inconsistently can lead to fluctuating hormone levels and a return of hypothyroid symptoms. Additionally, lifestyle factors like diet and other medications can interact with levothyroxine, so a comprehensive health review with your doctor is essential.

The bottom line on the downside

While the downsides of taking levothyroxine are real, they are predominantly tied to the challenges of finding and maintaining the correct dose and adhering to proper medication practices. For most individuals, the benefits of treating hypothyroidism far outweigh the potential risks, especially when managed proactively with a healthcare provider. Regular monitoring through blood tests is the most effective tool for minimizing side effects and preventing long-term complications.

Disclaimer: This article is for informational purposes only and does not constitute medical advice. Consult with a qualified healthcare professional before making any decisions about your treatment.

Frequently Asked Questions

Yes, an incorrect dosage of levothyroxine can cause anxiety, nervousness, irritability, and other mood changes. These are often signs that your dosage may be too high and should be discussed with your doctor to find the right balance.

Some people may experience temporary hair loss during the first few months of starting levothyroxine. This usually happens as the body adjusts to the new hormone levels and typically resolves as your thyroid function stabilizes.

Taking levothyroxine on an empty stomach ensures that your body absorbs the medication properly and consistently. Many foods and supplements can interfere with absorption, making the medication less effective if not separated by at least 30-60 minutes from a meal.

Over-replacement of levothyroxine can lead to a rapid or irregular heartbeat, palpitations, chest pain, and, in rare cases, heart failure or a heart attack. These risks are higher for older patients or those with pre-existing heart conditions.

Yes, long-term overtreatment with levothyroxine can increase the rate of bone resorption, which may lead to decreased bone mineral density and an increased risk of osteoporosis and fractures, especially in post-menopausal women.

Levothyroxine does not directly cause weight gain. However, weight changes can indicate an improper dose. Unexplained weight gain may mean your dose is too low, while unintended weight loss could indicate it is too high.

Yes, levothyroxine is considered safe during pregnancy. Maintaining healthy thyroid hormone levels is essential for both the mother and the fetus, and the dosage may need to be adjusted during and after pregnancy.

Due to levothyroxine's long half-life, a single missed dose typically won't have a major impact. However, consistent dosing is crucial for stable hormone levels. If you miss a dose, take it as soon as you remember, unless it is almost time for your next dose, in which case you should skip the missed dose.