Synthroid

By Alice Chung, MD
Medically Reviewed by Priya Shah, PharmD, MBA
Last Update: January 15, 2025

What is Synthroid?

Synthroid is a medication commonly used to treat hypothyroidism, a condition where the thyroid gland does not produce enough thyroid hormones. It contains levothyroxine, a synthetic version of the hormone thyroxine (T4), which helps regulate metabolism, energy levels, and other vital body functions. By normalizing thyroid hormone levels, the drug helps alleviate symptoms such as fatigue, weight gain, and sensitivity to cold. Both brand-name Synthroid and generic versions are widely available and equally effective.

The tablet is typically taken once daily on an empty stomach, at least 30 minutes to an hour before breakfast, to ensure proper absorption. It is also prescribed for managing goiter and as a hormone replacement therapy after thyroid surgery or radiation. Regular blood tests are important to monitor thyroid levels and adjust the dosage as needed, ensuring effective management of thyroid-related conditions and an improved quality of life.

FAQ

What is Synthroid used for?

It is a medication containing levothyroxine, a synthetic form of the thyroid hormone thyroxine (T4). It is used to regulate metabolism and energy levels in individuals with thyroid hormone imbalances. The drug is commonly prescribed for various thyroid-related conditions to restore hormone balance and alleviate associated symptoms.

What Synthroid is used for:

Hypothyroidism: To replace low thyroid hormone levels.
Goiter: To reduce the size of an enlarged thyroid gland.
Post-thyroid surgery: As hormone replacement after thyroid removal.
Radiation therapy: To maintain thyroid hormone levels after treatment.
Thyroid hormone suppression therapy: To prevent tumor growth or manage specific conditions.

This medication helps improve quality of life by addressing the underlying thyroid issues effectively.

What exactly does Synthroid do?

It helps replace a hormone called thyroxine (T4) that your thyroid gland normally makes. This hormone is essential for controlling how your body uses energy, which affects everything from your metabolism to your heart rate and overall energy levels. When your thyroid doesn’t produce enough of this hormone, Synthroid steps in to bring your levels back to normal.

By restoring thyroid hormone levels, the drug can relieve symptoms like tiredness, weight gain, feeling cold, and even hair loss. It helps your body work the way it should, improving your energy and keeping essential functions balanced. This makes it a key treatment for conditions like hypothyroidism and other thyroid-related problems.

What is the most common side effect?

The most common side effect of Synthroid (levothyroxine) is hyperthyroidism-like symptoms, which can occur if the dose is too high. These symptoms include rapid heartbeat, anxiety, sweating, weight loss, and trouble sleeping. Studies show that approximately 7-8% of patients may experience these effects, particularly during the initial adjustment period or if the dose is not carefully monitored.

To minimize the risk of side effects, doctors typically start with a low dose and gradually adjust it based on regular blood tests. If you notice any unusual symptoms while taking this thyroid hormone replacement, contact your healthcare provider to ensure your dosage is appropriate for your needs. Proper monitoring helps keep side effects minimal and ensures the medication works effectively.

How do I use Synthroid?

The drug should be taken carefully to ensure it works effectively and minimizes side effects. It is typically prescribed as a once-daily medication to help restore proper thyroid hormone levels. Following the correct usage guidelines is essential for the best results.

How to use Synthroid:

Take the tablet once daily on an empty stomach, ideally in the morning, at least 30-60 minutes before breakfast.
Swallow the tablet whole with a full glass of water.
Avoid eating or drinking anything (except water) for at least 30 minutes after taking the medication.
Do not take the tablet with calcium, iron supplements, or antacids, as they can interfere with absorption.
Take it at the same time every day to maintain consistent hormone levels.

For best results, follow your doctor’s instructions carefully and attend regular check-ups to monitor your thyroid hormone levels and adjust the dosage if needed. This ensures the medication is working effectively to manage your thyroid condition.

Why is Synthroid given at night?

The tablet is sometimes taken at night because it can improve absorption and maintain consistent thyroid hormone levels in certain individuals. Studies suggest that taking Synthroid on an empty stomach before bedtime may help avoid interactions with food or other medications consumed earlier in the day, which can interfere with how the body absorbs the medication. For some patients, this nighttime routine may lead to better hormone control.

However, the timing of T4 medication can vary based on individual needs. Most people are advised to take it in the morning on an empty stomach, but for those who have difficulty maintaining this schedule, nighttime dosing may be a good alternative. Always consult your healthcare provider before changing your medication routine to ensure the timing works best for your treatment plan.

Do you gain or lose weight with this pill?

Weight changes with Synthroid depend on how it balances your thyroid hormone levels. For individuals with hypothyroidism, where the thyroid gland is underactive, symptoms often include weight gain due to a slowed metabolism. The medication helps restore normal hormone levels, which can lead to modest weight loss as metabolism improves. Studies show that patients typically lose about 5-10% of body weight that was gained due to hypothyroidism after starting treatment.

However, this drug is not a weight-loss medication. Once thyroid hormone levels are normalized, it does not cause significant weight changes. If taken in doses higher than prescribed, it can lead to symptoms of hyperthyroidism, such as unintentional weight loss, but this is unsafe and can cause serious health problems. Always take the medication as directed by your healthcare provider to manage your thyroid condition effectively and safely.

What should I avoid when taking Synthroid?

When taking levothyroxine tablets, there are certain things to avoid to ensure the medication works effectively. Synthroid’s absorption can be affected by food, supplements, and other medications, so following these guidelines is essential for the best results.

What to avoid when taking Synthroid:

Food: Avoid eating for at least 30-60 minutes after taking the tablet, as food can reduce its absorption.
Calcium and iron supplements: These can interfere with absorption if taken within four hours of thyroid health medication.
Antacids or medications with aluminum or magnesium: Avoid these for at least four hours after taking your dose.
Soy and high-fiber foods: These may decrease how much medication your body absorbs.
Grapefruit juice: This can affect how the medication is metabolized in your body.

For the best results, take the pill on an empty stomach with a full glass of water and maintain a consistent routine.

What happens to your body when you start taking Synthroid?

When you start taking this T4 hormone supplement, it works to replace the thyroid hormone your body is missing. In simple words, it helps your body regulate energy, metabolism, and temperature, which may improve symptoms like tiredness, weight gain, and feeling cold. Over time, your body adjusts as the hormone levels return to normal.

For better understanding, think of this medication as a helper that fills the gap your thyroid gland can’t. It usually takes a few weeks to feel the full effects, such as increased energy and better overall health. Regular blood tests will ensure your dose is just right, helping you feel your best.

How quickly does it make you feel better?

Most people begin to feel improvements within 1 to 2 weeks of starting Synthroid, with noticeable changes in energy levels and reduced symptoms like fatigue and sluggishness. However, it can take up to 6 to 8 weeks to experience the full benefits as your thyroid hormone levels gradually stabilize. How quickly you feel better depends on factors like the severity of your hypothyroidism, your overall health, and whether your dosage is properly adjusted.

For better understanding, the medication starts working immediately to replace the missing thyroid hormone, but changes in your body happen gradually. Symptoms like weight gain, dry skin, or hair loss may take longer to improve, often requiring consistent treatment over weeks or months. Regular blood tests and follow-ups with your doctor are essential to ensure your dosage is effective. With time and proper monitoring, most people find significant relief and a return to normal energy and health.

Is it safe to take levothyroxine while breastfeeding?

Yes, it is generally safe to take levothyroxine while breastfeeding. According to the American Thyroid Association (ATA), this drug is considered safe for both the mother and the baby when used to treat hypothyroidism during breastfeeding. Only a very small amount of the medication passes into breast milk—less than 0.1% of the maternal dose—which is not enough to affect the infant’s thyroid function.

Research and clinical studies support its safety, showing no significant risks for babies whose mothers take this thyroid hormone replacement. Maintaining proper thyroid hormone levels is essential for the mother’s health and milk production, and untreated hypothyroidism can lead to complications like fatigue or low milk supply. Always consult your healthcare provider to ensure the dosage is appropriate for your needs while breastfeeding.

Can Synthroid be used for hyperthyroidism?

This drug is not typically used to treat hyperthyroidism, as this condition involves an overactive thyroid gland producing too much hormone. However, in certain cases, Synthroid may be prescribed following treatments for hyperthyroidism. For example, if a patient undergoes radioactive iodine therapy or thyroid surgery to address hyperthyroidism, they may develop hypothyroidism as a result. In such cases, this thyroid therapy is used to replace the thyroid hormone the body can no longer produce.

According to the 2025 data, about 90% of patients treated for hyperthyroidism with radioactive iodine therapy or surgery develop hypothyroidism and require lifelong hormone replacement with medications like this one. This approach helps restore normal thyroid function and prevent symptoms like fatigue, weight gain, or low energy. If you have been treated for hyperthyroidism and require T4 medication, your healthcare provider will closely monitor your hormone levels to ensure the correct dosage.

How do Synthroid 75 mcg and 50 mcg differ?

50 mcg and 75 mcg differ in the amount of levothyroxine they provide. The dose you need depends on your thyroid hormone levels, overall health, and specific treatment goals.

When to use 50 mcg:

For mild hypothyroidism.
Starting dose for older adults or those with heart conditions.

When to use 75 mcg:

For moderate to severe hypothyroidism.
When blood tests show the need for more thyroid hormone.

Regular blood tests help your doctor decide the best dose to keep your thyroid function balanced.

Can you drink coffee with levothyroxine?

Drinking coffee shortly after taking synthetic thyroxine can interfere with the medication’s absorption. A recent study found that consuming coffee simultaneously with levothyroxine significantly reduced its absorption, leading to decreased effectiveness.

To ensure optimal absorption of the drug, it’s recommended to wait at least 30 to 60 minutes after taking the medication before drinking coffee. This allows the body to properly absorb the hormone without interference. Alternatively, some studies suggest that taking the pill at bedtime, well after the last meal or beverage, can also be effective.

Is 125 mcg Synthroid a lot?

125 mcg of levothyroxine is considered a moderate dose, but whether it is “a lot” depends on individual factors such as your thyroid hormone levels, body weight, age, and overall health. Synthroid doses are personalized, with typical starting doses ranging from 25 mcg to 75 mcg for mild hypothyroidism, while doses like 125 mcg or higher are often prescribed for moderate to severe thyroid hormone deficiencies.

For better understanding, a dose of 125 mcg is usually recommended when blood tests show a significant need for thyroid hormone replacement. The appropriate dose is often calculated based on body weight, with an average dose being 1.6 mcg per kilogram per day. For someone weighing around 77 kg (170 lbs), 125 mcg would be within the standard range. Regular blood tests ensure your dose is correctly adjusted to meet your needs and maintain healthy thyroid function. Always take the medication as directed by your healthcare provider.

What are signs of too much levothyroxine?

Taking too much thyroid health medication can lead to symptoms of hyperthyroidism, as the body has excess thyroid hormone. These signs indicate that your dose may need adjustment. If you experience any of these symptoms, consult your healthcare provider promptly.

Signs of too much levothyroxine:

Increased heart rate: Feeling your heart racing or palpitations.
Anxiety or nervousness: Feeling unusually jittery or restless.
Weight loss: Unexplained loss of weight despite no changes in diet.
Sweating: Excessive perspiration or feeling unusually warm.
Tremors: Shaking hands or difficulty with steady movements.
Sleep problems: Insomnia or difficulty staying asleep.
Fatigue: Feeling overly tired despite getting enough rest.
Muscle weakness: Weakness or discomfort in your muscles.

If left untreated, too much T4 hormone supplement can lead to serious complications, such as heart problems or bone loss. Regular blood tests and open communication with your doctor are essential to ensure your dosage is safe and effective.

Is there a big difference between Synthroid and levothyroxine?

These drugs are essentially the same in terms of active ingredients, as both contain synthetic thyroxine (T4), which is used to treat hypothyroidism. The key difference lies in the brand name (Synthroid) versus the generic version (levothyroxine). While the active ingredient is identical, the inactive ingredients, fillers, and dyes can vary between the two, potentially affecting how the medication is absorbed or tolerated by different individuals.

For better understanding, here’s a comparison table:

Feature	Synthroid	Levothyroxine (Generic)
Active ingredient	Levothyroxine sodium	Levothyroxine sodium
Cost	Typically higher	More affordable
Availability	Brand-name only	Multiple generic manufacturers
Inactive ingredients	Specific to the brand version	Varies by manufacturer
Absorption consistency	More consistent for some	May vary between brands
Doctor preference	Sometimes preferred for sensitive patients	Often prescribed for cost savings

In most cases, generic medication works as effectively as the brand-name medicine. However, if you experience symptoms after switching brands or notice inconsistencies in your thyroid levels, your doctor may recommend staying on one specific version to ensure stability. Always consult your healthcare provider for guidance on which option is best for you.

Does this medication affect sleep?

Levothyroxine can affect sleep patterns, particularly if the dosage is not properly balanced. When taken in excessive amounts, it may cause symptoms of hyperthyroidism, such as insomnia, restlessness, and an increased heart rate, making it harder to fall or stay asleep. On the other hand, if the dosage is too low, it can leave thyroid hormone levels insufficient, leading to fatigue and excessive sleepiness. Research shows that maintaining the right dosage is crucial for balancing energy levels and promoting better sleep quality.

A study published in the Journal of Clinical Sleep Medicine found that low-dose levothyroxine treatment improved total sleep time and reduced daytime sleepiness in patients with specific sleep disorders, highlighting its potential impact on sleep regulation. To minimize sleep disturbances, it is essential to take teh drug as prescribed and undergo regular blood tests to monitor thyroid hormone levels. If sleep issues persist, consult your healthcare provider for a possible dosage adjustment or further evaluation. Proper sleep hygiene, including a consistent bedtime routine and a comfortable environment, can also help improve sleep quality while on hormone replacement therapy.

Can I take vitamin D while on Synthroid?

Yes, you can take vitamin D while on this thyroid health medication, but timing is important to avoid potential interactions. Vitamin D itself does not interfere with Synthroid, but supplements like calcium, often combined with vitamin D, can affect how your body absorbs the medication. To ensure the tablet works effectively, it’s recommended to take it on an empty stomach at least 30-60 minutes before breakfast and wait at least 4 hours before taking any vitamin D or calcium supplements.

Keeping your vitamin D levels within a healthy range is essential for bone health and overall well-being, especially if you have thyroid issues. Research suggests that vitamin D deficiency is common in people with hypothyroidism, making supplementation beneficial. Always consult your healthcare provider for personalized advice on managing your levothyroxine medication and vitamin D routine to ensure the best results for your health.

Why do you have to wait 30 minutes after taking the pill?

You need to wait 30 minutes after taking Synthroid because food and certain beverages can interfere with the medication’s absorption. This tablet is most effective when taken on an empty stomach, as even small amounts of food can reduce how much of the hormone your body absorbs. Studies show that taking T4 hormone supplements with food can lower absorption by up to 40-80%, making the medication less effective at managing your thyroid condition.

Waiting at least 30-60 minutes allows the medication to be fully absorbed into your bloodstream before eating or drinking anything other than water. This ensures that your thyroid hormone levels stay stable and your treatment works as intended. For best results, take the tablet with a full glass of water at the same time every day, and follow your doctor’s instructions for any other medications or supplements.

Does it cause hair growth?

Levothyroxine can help restore normal hair growth by balancing thyroid hormone levels. Hair loss is a well-known symptom of hypothyroidism, and starting thyroid health medication often addresses this issue as hormone levels normalize. However, temporary hair shedding can occur during the first few months of treatment. This is a common adjustment period as the body adapts to the medication and thyroid levels stabilize.

Hair regrowth typically takes time, with noticeable improvements often appearing after several months of consistent treatment. If hair loss continues or worsens, it may indicate that your thyroid hormone levels still need adjustment, so it’s important to consult your doctor. A nutrient-rich diet, including iron, zinc, and biotin, can also support healthier hair growth while on levothyroxine. Regular follow-ups and monitoring ensure the treatment is working effectively for both your thyroid health and overall well-being.

About the Author

Dr. Alice Chung Dr. Alice P. Chung, MD, is a renowned surgical oncologist in Beverly Hills, California, with over 20 years of experience. Affiliated with Cedars-Sinai Medical Center, she specializes in oncology and surgical oncology, with expertise in mastectomy and complex oncologic conditions. A 2000 graduate of Chicago Medical School, Dr. Chung completed her surgical residency at Cedars-Sinai, mastering advanced techniques. Board-certified in surgery and a Fellow of the American College of Surgeons (FACS), she is recognized for her compassionate care and clinical excellence. Licensed in California and New York, she is also certified in CMS Meaningful Use Stage 1 through EpicCare EMR. Known for her dedication to innovative surgical solutions and patient-centered care, Dr. Chung is highly regarded by patients and colleagues for her expertise and personalized approach. She remains committed to advancing the field of surgical oncology through both her clinical practice and advocacy for cutting-edge treatments.

Disclaimer: The information provided on this page is for general informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a licensed healthcare provider with any questions you may have regarding your health or medical conditions. Do not disregard or delay seeking professional advice based on the information presented here. For specific concerns or guidance, consult your doctor or a qualified healthcare professional.