Synthroid (Levothyroxine)
By Crystal S. Denlinger, MD
Medically Reviewed by Eve Bloomgarden, MD (Endocrinologist)
Last Updated: February 14, 2025
What is Synthroid?
Synthroid (levothyroxine) is a synthetic thyroid hormone that helps regulate metabolism, energy levels, and overall well-being. It is the #1 prescribed treatment for hypothyroidism, a condition affecting around 5% of people worldwide. By restoring normal thyroid hormone levels, the drug supports healthy weight management, heart function, brain clarity, and digestion. Patients often report feeling more energetic, focused, and balanced within a few weeks of consistent use.
Proper dosage is crucial since too much or too little can cause serious health issues. Doctors adjust doses based on TSH levels, usually every 6-8 weeks. About 80% of patients achieve stable thyroid levels with the right dose. Common side effects include weight changes, palpitations, or hair loss, but they often improve with dose adjustments. Certain foods and supplements, like calcium and iron, reduce the synthetic hormone’s absorption by up to 40%. Always take it on an empty stomach with water for maximum effectiveness.
Where to buy Synthroid (Levothyroxine)
For effective hypothyroidism management, a reliable medication source is crucial. Many people now prefer ordering online because it’s fast, convenient, and often more affordable than local pharmacies. Instead of waiting in line or making last-minute trips, you can have your medication delivered straight to your home. Online pharmacies offer secure payments, verified prescriptions, and automatic refills, making it easier to stay on track with your treatment.
With the growing demand for thyroid medications, online pharmacies now provide bulk discounts, same-day shipping, and discreet packaging. Some even offer virtual consultations with licensed pharmacists to ensure you get the right dosage and guidance. Many patients choose online ordering not only for the cost savings but also for the peace of mind that comes with hassle-free, consistent access to their medication.
| Medication | Active Ingredient | Dosage Options | Why Buy Online? | Recommended Source |
|---|---|---|---|---|
| Synthroid | Levothyroxine | 25 mcg – 200 mcg | Easy ordering, cost savings, hassle-free refills | Canadian Online Pharmacy |
Ordering this metabolism booster online means no waiting in line, no last-minute pharmacy runs—just reliable, stress-free access to your medication.
Can I get Synthroid (Levothyroxine) over the counter?
No, levothyroxine is a prescription-only medication and cannot be purchased over the counter. The U.S. Food and Drug Administration (FDA) classifies it as a regulated prescription drug because it directly replaces thyroid hormones, which control metabolism, heart function, and energy levels. Taking the wrong dose can lead to serious health risks, including heart issues, osteoporosis, and hormonal imbalances. Doctors determine the correct dosage based on TSH (thyroid-stimulating hormone) levels, which must be monitored through regular blood tests. The FDA warns that even small dosage changes can cause severe symptoms like fatigue, weight gain, or irregular heartbeat, making professional supervision essential.
Unlike vitamins or supplements, which provide general health benefits, this endocrine regulator requires precise, individualized dosing. The American Thyroid Association (ATA) emphasizes that thyroid hormone replacement therapy must be carefully managed to avoid complications. Over 20 million people worldwide take levothyroxine, and each patient’s needs differ. Pharmacies only dispense Synthroid with a valid prescription to prevent incorrect self-dosing. If you suspect a thyroid disorder, consult a doctor or endocrinologist for proper diagnosis and treatment. Many telehealth providers now offer virtual consultations, allowing you to get a thyroid function test, prescription, and refills without an in-person visit.
FAQ
What is Synthroid used for?
It’s a prescription thyroid medication used to treat hypothyroidism, a condition where the thyroid gland does not produce enough hormones. It contains levothyroxine, a synthetic form of thyroid hormone that helps regulate metabolism, energy levels, and overall body function. Doctors commonly prescribe it for underactive thyroid, goiter, and certain thyroid disorders. Taking it daily, as directed, helps maintain stable hormone levels, reducing symptoms like fatigue, weight gain, and dry skin.
Can I use levothyroxine for hyperthyroidism?
Levothyroxine is not used for hyperthyroidism, as it treats hypothyroidism, a condition where the thyroid produces too little hormone. Hyperthyroidism occurs when the thyroid produces too much hormone, requiring treatments like antithyroid medications, radioactive iodine, or surgery. Using this pill in this case can worsen symptoms like rapid heartbeat, weight loss, and nervousness.
However, some people with hyperthyroidism may need this metabolic hormone after treatment reduces thyroid function too much. In these cases, doctors prescribe it to restore normal hormone levels. Always consult a doctor before taking this drug for any thyroid condition, as improper use can lead to serious health risks.
What does it do?
Levothyroxine works by replacing thyroid hormone in people with hypothyroidism, helping restore energy, metabolism, and overall body function. It contains levothyroxine, a synthetic hormone that supports weight control, heart health, and mental clarity. Taking it daily on an empty stomach ensures proper absorption, preventing symptoms like fatigue, weight gain, and slow metabolism. Regular blood tests help adjust the dosage to avoid side effects like heart palpitations or anxiety.
What are the active ingredients?
Synthroid’s active ingredient is levothyroxine sodium, a synthetic version of the thyroid hormone T4, which regulates metabolism, energy, and body functions. Once in the body, it converts into triiodothyronine (T3), the active thyroid hormone that supports heart, brain, and organ health. Each tablet may also contain inactive ingredients like lactose, magnesium stearate, and coloring agents, which vary by dosage strength. These ingredients help with tablet stability and absorption, ensuring the medication works effectively.
When to take Synthroid: morning or night?
The medication works best when taken in the morning, at least 30 minutes before eating, with a full glass of water. This allows better absorption and prevents food from interfering with the medication. Taking it at the same time daily helps keep thyroid hormone levels stable, improving energy, metabolism, and overall health.
Some people take the tablet at night, but only if they wait at least four hours after eating. Studies suggest nighttime dosing may improve absorption, but consistency matters most. Always follow your doctor’s advice and avoid medications or supplements that interfere with thyroid hormone absorption.
What is the right dosage: 25 mcg, 50 mcg, 100 mcg?
The recommended dosage depends on age, weight, thyroid condition, and medical history. Common doses include 25 mcg, 50 mcg, and 100 mcg, but the right dose is personalized. Doctors usually start with a low dose and adjust based on thyroid hormone levels. Too much can cause heart palpitations, anxiety, or weight loss, while too little may lead to fatigue and slow metabolism. Below is a comprehensive dosage guide based on typical use cases.
How to choose the dosage of Synthroid:
| Dosage (mcg) | Common Use | Who May Need It |
|---|---|---|
| 25 mcg | Low starting dose | Elderly, mild hypothyroidism, or sensitive to medication |
| 50 mcg | Moderate starting dose | Adults with mild to moderate hypothyroidism |
| 75 mcg | Adjusted dose | Patients needing gradual dose increase |
| 100 mcg | Standard maintenance dose | Most adults with stable thyroid function |
| 125-150 mcg | Higher replacement dose | Younger adults, post-thyroidectomy patients |
| 175-200 mcg | High dose | Severe hypothyroidism, thyroid cancer patients |
Your doctor determines the best dose based on blood tests and symptoms. Never adjust your dose without medical advice.
What is the difference between Synthroid and levothyroxine?
They both contain levothyroxine sodium, a synthetic thyroid hormone used to treat hypothyroidism. The key difference is that Synthroid is a brand-name drug, while the second is its generic version. A 2017 study in the Journal of Clinical Endocrinology & Metabolism found that 68% of patients had better hormone stability with the branded medicine.
The generic version may have slight variations in fillers and absorption, affecting 5-10% of patients sensitive to dose changes. Some doctors prefer the original product for consistent potency, while others find the generic product equally effective. Always consult your doctor, as switching brands can impact thyroid levels and require dose adjustments.
Are there any side effects?
Synthroid can cause side effects, especially if the dose is too high or too low. A 2021 study found that 12-15% of patients experience side effects, mostly due to overmedication or absorption issues. The most common side effects include:
- Heart palpitations and fast heartbeat – Affects about 10% of patients with too high a dose.
- Anxiety, nervousness, or trouble sleeping – Common in 5-8% of users, often linked to rapid hormone changes.
- Weight loss or appetite changes – Happens in 4-7% of cases due to increased metabolism.
- Headaches, sweating, or shaking – Reported by 3-5% of users, especially after dose adjustments.
- Hair thinning – Temporary for 2-4% of people, usually in the first few months.
Less common but serious side effects include chest pain, irregular heartbeat, or bone loss with long-term high doses. Always follow your doctor’s recommended dosage and get regular blood tests to avoid side effects.
What is the half life of levothyroxine?
The half-life of this medication is about 6-7 days in healthy individuals, meaning it takes that long for half the dose to leave the body. In hypothyroid patients, the half-life can be 9-10 days, while in hyperthyroid patients, it may shorten to 3-4 days. This long half-life helps maintain stable thyroid hormone levels, allowing once-daily dosing. However, missing doses can affect hormone balance, so consistency is essential. Regular blood tests ensure the right dosage for optimal thyroid function.
How long does it take for levothyroxine to work?
This thyroid supplement starts working within a few hours, but noticeable improvements in energy, metabolism, and symptoms take 1-2 weeks. Full effects on thyroid hormone levels may take 4-6 weeks, as the body gradually adjusts. A 2020 study found that 80% of patients feel better within a month, but some need dose adjustments. Regular blood tests help ensure the correct dose for optimal thyroid function. Taking the tablet consistently on an empty stomach improves absorption and speeds up results.
Can Synthroid cause bone density loss?
Thie tablet can cause bone density loss, especially with high doses or long-term use. A 2021 study in the Journal of Bone Health found that women over 60 taking excess thyroid hormone had a 20-25% higher risk of osteoporosis. Bone loss occurs because too much thyroid hormone speeds up bone turnover, weakening bones over time.
A 2019 study showed that 10-15% of patients on long-term hormonal therapy had lower bone mineral density, especially in the hip and spine. Doctors recommend regular bone scans for at-risk patients and using the lowest effective dose. Getting enough calcium, vitamin D, and weight-bearing exercise helps protect bone health while on this medication.
What is the generic name?
The generic name for Synthroid is levothyroxine sodium, a synthetic thyroid hormone used to treat hypothyroidism. It works the same way as the brand-name health supplement by restoring thyroid hormone levels, improving energy, metabolism, and overall health. Many people prefer the generic metabolism booster because it is more affordable, costing 30-50% less than the brand name.
A 2022 study found that 85% of patients had similar thyroid hormone stability with the generic hormonal therapy compared to the branded endocrine support tablet. While some sensitive patients may notice slight absorption differences, most find it equally effective. Always consult a doctor before switching, as dose adjustments may be needed for optimal thyroid function.
Can I breastfeed while taking levothyroxine?
Yes, levothyroxine is safe for breastfeeding, as only small amounts pass into breast milk and do not harm the baby. The American Thyroid Association (2021) confirms that taking this synthetic hormone while breastfeeding helps maintain healthy thyroid levels, which is essential for milk production and maternal health.
A 2020 study found that 90% of mothers on this endocrine regulator had no breastfeeding issues, and their babies showed normal growth and development. Doctors may adjust the dose postpartum, as thyroid levels change after pregnancy. Regular blood tests ensure the right dose for both mother and baby’s well-being.
How much is Synthroid without insurance?
As of 2025, the cost of this regulatory pill without insurance ranges from $50 to $100 for a 30-day supply, depending on dosage and pharmacy. Higher doses, like 100 mcg or 150 mcg, may cost more, while bulk purchases often reduce per-tablet pricing. The generic version is much cheaper, with prices starting at $7 to $25 for a 30-day supply.
Many people choose generic levothyroxine because it offers the same effectiveness at a fraction of the cost. Some pharmacies offer discount programs or coupons, helping lower costs further. Prices vary by location, so checking multiple pharmacies or online deals can help save money. Always consult a doctor before switching between brand-name and generic thyroid medication, as small differences in absorption may affect thyroid hormone levels.
About the Author
Crystal S. Denlinger, MD, is an Assistant Director and Attending Physician in the Department of Medical Oncology in Philadelphia. She graduated summa cum laude with a BS in Biology from The College of New Jersey and earned her MD from New Jersey Medical School. Dr. Denlinger completed her internal medicine residency at Mount Sinai Medical Center, followed by a Hematology/Oncology Fellowship at Fox Chase Cancer Center/Temple University. Her expertise includes cancer survivorship, prevention, and treatment. She is affiliated with the American Society of Clinical Oncology, the National Comprehensive Cancer Network, and the American College of Physicians.
Disclaimer: This information is here to educate, not diagnose or treat. Your health is unique, and only a qualified healthcare professional can provide the right advice for you. Before making decisions about medications, treatments, or your well-being, talk to a doctor. If something feels off, don’t wait—get medical help as soon as possible.