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February 22, 2023 13 min read

If you have hypothyroidism or low thyroid hormone levels, chances are you're familiar with Synthroid. This prescription drug is one of the most commonly prescribed medications used to treat an underactive thyroid. But what is Synthroid, exactly, and how does it work?

In this article, we'll take a closer look at Synthroid, including its uses, potential side effects, dosage, and more. Whether you're considering starting this prescription medicine or you've been taking it for years, read on to learn what you need to know.

What is Synthroid, and what is it used for?

Synthroid is a brand name for the generic drug levothyroxine, which is a synthetic form of the thyroid hormone thyroxine (T4). Manufactured by AbbVie, Synthroid is one of the prescription drugs used to treat hypothyroidism, a condition in which the thyroid gland doesn't produce enough thyroid hormones. It leads to a wide range of symptoms, such as fatigue, weight gain, and even depression. In children, not having enough thyroid hormone can impair mental and physical development.

Aside from those with hypothyroidism, Synthroid is also used in people who need surgery and radioiodine therapy to manage a type of thyroid cancer called thyrotropin-dependent well-differentiated thyroid cancer. For this condition, the medication is used to suppress your levels of thyroid-stimulating hormone (TSH).

The drug comes in tablet form that you need to take orally once a day. Here are the main ingredients of Synthroid tablets:

  • Levothyroxine sodium – this is the active ingredient of Synthroid. It's chemically identical to the T4 hormone naturally produced by the thyroid gland.

  • Acacia

  • Confectioner's sugar (contains corn starch)

  • Lactose monohydrate

  • Magnesium stearate

  • Microcrystalline cellulose

  • Sodium starch glycolate

  • Colloidal silicon dioxide.

How does Synthroid work?

Our thyroid gland produces two main hormones: thyroxine (T4) and triiodothyronine (T3). T4 is the most abundant thyroid hormone, but it's inactive. T3, on the other hand, is the biologically active form of thyroid hormone that plays a key role in regulating metabolism and other bodily functions. T3 enters the cell nucleus and binds to specific receptors, which then activate or suppress the expression of certain genes.

When you have hypothyroidism, your thyroid doesn't make the amount of thyroid hormones required for your body to function normally. Synthroid works by replacing these missing thyroid hormones, helping to regulate metabolism so your body can return to normal functioning and easing the symptoms of hypothyroidism.

Synthroid is the synthetic form of the thyroid hormone thyroxine (T4). Therefore, it mimics the action of the natural T4. Once taken, Synthroid is absorbed into the bloodstream and transported to the cells throughout the body. In the cells, levothyroxine is converted into triiodothyronine (T3).


Who can and cannot use Synthroid?

Synthroid is generally safe and effective for most people with hypothyroidism. Here are some people that can use Synthroid:

  • People with hypothyroidism due to an underactive thyroid.

  • Individuals who have had their thyroid removed due to thyroid cancer or other thyroid conditions.

  • Those with an enlarged thyroid (goiter).

  • People with subclinical hypothyroidism who are experiencing symptoms of hypothyroidism.

However, there are some people who should not take Synthroid or who should take it with caution, including:

  • People with certain heart conditions, such as uncontrolled high blood pressure, angina, or coronary artery disease.

  • People with adrenal gland problems or have problems with their pituitary glands.

  • People with untreated or poorly controlled diabetes.

  • People with overactive and high thyroid hormone levels (hyperthyroidism).

  • People with an allergic reaction to any ingredients in Synthroid

  • Synthroid is not recommended or approved for treating benign thyroid nodules or nontoxic diffuse goiter in iodine-sufficient patients.

  • People that suffer from drug adjustment problems. As a study shows that it can lead to overdose.

  • People who intend to use it to treat infertility or as a weight loss tool but don't have hypothyroidism.

Can pregnant women and breastfeeding mothers take Synthroid?

Before taking Synthroid during pregnancy or while breastfeeding, discussing the risks and benefits with your doctor is important. Here are some specific considerations:


The American Thyroid Association recommends that pregnant women with hypothyroidism be treated with levothyroxine, such as Synthroid, to maintain normal thyroid hormone levels. A study shows that the need of thyroid hormones may increase during pregnancy and breastfeeding. Thus, your doctor may increase your dosage during pregnancy. You may also need to get regular monitoring to ensure you're not taking too much or too little of the medication.

An untreated hypothyroidism during pregnancy can lead to complications such as preterm birth, low birth weight, and developmental problems in the baby.


Levothyroxine, the active ingredient in Synthroid, does pass into breast milk in small amounts, but it is not usually enough to affect the baby's thyroid function. However, discussing the risks and benefits of taking Synthroid while breastfeeding with your doctor or pharmacist is still important.

What are the precautions you need to know before taking Synthroid?

Before taking Synthroid, it's important to be aware of the following precautions:

Your medical history

You should inform your doctor of any medical conditions you have, such as heart disease, adrenal gland problems, a thyroid disorder called thyrotoxicosis, symptoms of a heart attack, or diabetes mellitus. You should also inform your doctor about any medications you are taking, including over-the-counter drugs and supplements.

In the elderly and patients with cardiovascular disease, Synthroid should be given at lower doses than those recommended in younger individuals or patients without cardiovascular disease. Moreover, you shouldn't take Synthroid if you suffer from uncorrected adrenal insufficiency. That's because this medication can affect your adrenal function, which can cause serious problems.


If you have had an allergic reaction to levothyroxine or any other ingredients in Synthroid, you should not take this medication. Signs of an allergic reaction may include hives, chest pain, difficulty breathing, or swelling of the face, lips, tongue, or throat. If you experience any of these symptoms, seek immediate medical attention.

What are the side effects of Synthroid?

Synthroid may help regulate your thyroid function, but it's not without its potential side effects. Here's what you need to know about the possible reactions you could experience while taking this medication.

Common side effects

Here are some of the common side effects of Synthroid:

  • Headache

  • Tremors

  • Nervousness

  • Insomnia

  • Changes in appetite

  • Weight loss or gain

  • Increased sweating

  • Heat intolerance

  • Fever

  • Diarrhea

  • Irregular menstrual periods

  • Temporary hair loss.

Some side effects will go away in a few weeks, but if they persist and become more severe, be sure to talk to your doctor. In most cases, the side effects may indicate a need for a change in medication or dose adjustment. Therefore, your doctor will monitor your symptoms and ensure that your treatment plan is safe and effective.

Serious side effects

Like almost all medications, there are some serious or even life-threatening side effects that you should be aware of.

  • Chest pain

  • Rapid or irregular heartbeat

  • Difficulty breathing

  • Seizures

  • Allergic reaction (swelling, itching, rash)

  • Worsening of diabetes control

  • Adrenal insufficiency (sudden and severe fatigue, muscle weakness, abdominal pain)

  • Rare cases of acute liver injury

These serious side effects are rare, but you must seek immediate medical help if you experience any of them.

How do doctors determine your dose?

The dose for Synthroid can be different from person to person. The dosage is based on your age, weight, medical condition, lab test results, response to treatment, and other medications you may be taking.

When you start taking Synthroid, your doctor will calculate the smallest possible dose that can provide the desired effect. They will monitor you closely to make sure that the medication is working effectively without causing any side effects.

It's worth noting that if you have certain health conditions, your doctor may prescribe a lower dosage than the typical starting dose. This is because some health conditions can make it harder for your body to absorb the medication or increase your risk of side effects, and a lower dose may be more effective for you.

For hypothyroidism

The typical starting dose for treating hypothyroidism in newly diagnosed adult patients is 1.6 mcg per kilogram of body weight per day or approximately 100 to 125 mcg per day.

Dosage adjustment for hypothyroidism

When you first start taking Synthroid, it takes time for the medication to have a noticeable effect on your thyroid hormone levels. Typically, it takes about 4 to 6 weeks for the medication to reach its full effect.

After this initial period, your doctor will order blood tests to check your thyroid hormone levels. The main lab test your doctor will perform to determine and monitor your dosage is the thyroid stimulating hormone (TSH) test.

TSH is a hormone released by the pituitary gland that regulates the production and release of thyroid hormones. When there isn't enough thyroid hormone in the blood, the pituitary gland secretes more TSH in order to bring them back to balance. Conversely, when thyroid hormone levels are high, the pituitary gland decreases TSH production. Therefore, if your test results show that your TSH level is high, it may indicate that you have low thyroid hormone levels and your dose needs to be increased.

In this case, your doctor may increase your daily dosage of Synthroid by 12.5 mcg to 25 mcg.

Another round of blood tests will be conducted after another 4 to 6 weeks. If your thyroid hormone levels are still not within the normal range, your doctor may further increase your daily Synthroid dose by 12.5 mcg to 25 mcg. This process will continue every 4 to 6 weeks until your thyroid hormone levels are within a normal range.

It's important to note that these dosage adjustments are individualized, and your doctor will tailor them to your specific needs. It's also important to always follow your doctor's instructions carefully and attend all follow-up appointments to ensure that your treatment plan is effective and safe.

For TSH Suppression

When treating thyroid cancer, one of the goals is to suppress the thyroid-stimulating hormone (TSH) levels in the body. To achieve this, your doctor will recommend a dosage of Synthroid that's tailored to your specific treatment goals and the severity of your cancer.

It's important to note that the dosage for TSH suppression is typically higher than the dosage for treating hypothyroidism. Your doctor will monitor your TSH levels regularly through blood tests and adjust your Synthroid dosage as needed to ensure that your TSH levels are effectively suppressed.

How should you take Synthroid?

Synthroid should be taken exactly as prescribed by your doctor. Here are some general guidelines:

  • Take Synthroid on an empty stomach at least 30 minutes before breakfast or 4 hours after eating.

  • Swallow the tablet whole with a full glass of water.

  • Do not crush, chew, or break the tablet.

  • Do not take Synthroid with food, especially soybean products, as they may decrease its absorption.

  • Take Synthroid regularly every day to maintain a consistent level in your body.

  • Do not take Synthroid at the same time as other medications to avoid drug interactions. The general rule is to take other medications at least 4 hours before or after Synthroid.

  • Do not stop taking Synthroid without consulting your doctor, as it may cause your symptoms to return or worsen.

What happens if you miss a dose?

Forgetting to take your daily dose of Synthroid can result in a decrease in thyroid hormones. As a result, your hypothyroidism symptoms may come back. That's why it's essential that you take your dose at the same time every day.

But if you miss a dose of Synthroid, don't worry. Just take it as soon as you remember. However, if it's almost time for your next dose, skip the missed dose and take your next scheduled dose. Taking a double dose to make up for a missed dose is not recommended, as it can increase the risk of side effects, ruin your thyroid levels, and may cause harm.

If you consistently miss doses, talk to your doctor about creating a new schedule or setting reminders to help you remember to take your medication.

What happens if you take too much?

Taking too much thyroid medication can lead to excessive thyroid hormone levels in the body, which can be dangerous. Symptoms of Synthroid overdose are similar to symptoms of hyperthyroidism, which can include:

  • Rapid or irregular heartbeat

  • Chest pain

  • Shortness of breath

  • Tremors

  • Nervousness

  • Anxiety

  • Headache

  • Insomnia

  • Sweating

  • Diarrhea

  • Weight loss

  • Increased appetite

  • Muscle aches.

In severe cases, taking too much Synthroid can lead to serious and potentially life-threatening complications, such as seizures, cerebral embolism (blood clots in your brain), heart failure, bone loss, coma, and death. If you suspect you have taken too much Synthroid, seek immediate medical attention.

How should you store Synthroid?

Synthroid should be stored at room temperature, between 68°F to 77°F (20°C to 25°C), in a dry place and away from heat, moisture, and light. It's important to keep it out of the reach of children and pets. Don't store Synthroid in the bathroom or kitchen, where it could be exposed to moisture or heat. Additionally, don't use Synthroid after the expiration date on the packaging, and properly dispose of unused or expired medication.

Does Synthroid interact with other medications?

Yes, Synthroid can interact with other medications. Many drugs affect thyroid hormone pharmacokinetics and metabolism and may alter the therapeutic response to Synthroid. Therefore, it's important to tell your doctor about all the medications, supplements, and vitamins you are taking before starting Synthroid. Some medications that may interact with Synthroid include:

  • Antacids, iron supplements, andcalcium supplements can decrease the absorption of Synthroid.

  • Certain heartburn medications, such as omeprazole and pantoprazole, may change the amount of acid you have in your stomach, which in turn may decrease the level of Synthroid in your body.

  • Calcium carbonate (Tums) can bind to Synthroid in your stomach, stopping your body from absorbing the thyroid medication.

  • Certain medications for digestive issues, such as simethicone and sucralfate, may decrease the amount of Synthroid in your body, causing it to be less effective.

  • Some antidepressants, such as sertraline, can decrease Synthroid levels in your body. On the other hand, other types of antidepressants, like tricyclic antidepressants (TCA), may increase the levels of Synthroid, leading to hyperthyroidism.

  • Cholesterol medications, such as colestipol and cholestyramine, can decrease the level of Synthroid in your body.

  • Diabetes medication, such as metformin and insulin, may not work as well as usual if you take them with Synthroid.

In order to avoid drug interactions, your doctor may recommend you take Synthroid at least 4 hours before or after taking the drugs known to interfere with absorption.

How soon can you see improvements after taking Synthroid?

Generally, it takes about 4 to 6 weeks for Synthroid to have its maximum effect on your body. If you're experiencing symptoms related to your thyroid, such as fatigue or weight gain, you may notice improvements within this timeframe. After 4 to 6 weeks of treatment, your doctor will order blood tests to monitor your thyroid hormone levels and adjust your Synthroid dosage if necessary based on the results.

Remember that Synthroid is a long-term treatment, and it's important to continue taking it as prescribed even if you don't notice immediate improvements. By working closely with your doctor and regularly monitoring your thyroid hormone levels, you can find the right dose of Synthroid to manage your hypothyroidism effectively.

What are the alternatives to Synthroid?

If you feel like Synthroid doesn't work well for you, there are other thyroid hormone replacement medications that you can consider. These include:

Generic levothyroxine

Synthroid and generic levothyroxine contain the same active ingredient, which is levothyroxine sodium. Both medications are considered equally effective in treating hypothyroidism and other thyroid conditions.

However, the two drugs aren't exactly identical. This is because there may be some differences in the inactive ingredients used to formulate the two medications, which can affect the way the body absorbs and processes the drug. Some people may feel better with Synthroid, while others may experience relief from symptoms with generic levothyroxine.

Liothyronine (Cytomel)

This medication contains the synthetic version of triiodothyronine (T3) and is used for patients who do not respond well to levothyroxine or have persistent symptoms. Liothyronine is often used in conjunction with levothyroxine.

Natural desiccated thyroid (NDT)

Also known as desiccated thyroid extract (DTE), NDT is made from the desiccated (dried) porcine (pig) thyroid glands and contains both T3 and T4 hormones. It is an alternative to levothyroxine and is often used in patients who have difficulty converting T4 to T3. Some brands of NDT are Armour Thyroid, NP Thyroid, and VitaliThy.

How does Synthroid compare with Cytomel?

Synthroid and Cytomel are both synthetic forms of hormones. Synthroid is an artificial form of thyroxine (T4). It functions in the body similarly to naturally produced T4, providing additional amounts of it where needed. On the other hand, Cytomel (liothyronine) is an artificial form of triiodothyronine (T3), which works differently than T4 by helping to regulate metabolism in cells more quickly than T4 does.

How does Synthroid compare with natural desiccated thyroid (NDT)?

Synthroid and natural desiccated thyroid (NDT) are two types of oral medications used to treat hypothyroidism. However, they're very different from each other.

Synthroid is synthetic and made of the active drug levothyroxine sodium. It only consists of one hormone: T4. NDT, on the other hand, is natural as it is derived from pig thyroid glands. It contains both T3 and T4, so it closely mimics your thyroid.

Since levothyroxine is the standard treatment of hypothyroidism, doctors usually prescribe Synthroid to those dealing with the condition. In fact, Synthroid is among the most commonly prescribed medications in the United States. But while Synthroid is well tolerated and works effectively for some people, it might not work as well for others.

And for those who don't see any improvement with Synthroid, NDT is a popular option. A study found that most patients prefer NDT over levothyroxine, like Synthroid.

One reason for this may be because NDT contains both T4 and T3 hormones, whereas Synthroid only contains T4. T4 is inactive and needs to be converted to T3 in order for our body to use it. Unfortunately, the conversion process can be impaired in some people. For these individuals, taking T4-only medications like Synthroid may not be effective.

In addition to its T3 content, NDT is also popular among those who prefer a more natural approach to treatment.

There are various NDT brands out there. One great option if you want to start trying NDT is VitaliThy, a natural desiccated thyroid you can buy online. Not only does VitaliThy contain complete thyroid hormones, but it's also free from common allergens like gluten and lactose. Additionally, it doesn't contain fish, shellfish, eggs, or artificial coloring and flavoring. This can be an excellent option for those with specific dietary restrictions or sensitivities.

Can you switch from Synthroid to another medication?

Yes, it's possible to switch from Synthroid to another medication. However, it's important to keep in mind that each medication can have differences. Even Synthroid and generic levothyroxine, which essentially contain the same active ingredient, can be different.

That's why it's better to stick with one type of medication since switching between different brands, or formulations of levothyroxine may cause changes in thyroid hormone levels. However, if you need to switch medications, it's important to talk to your doctor first to ensure a smooth transition and avoid unwanted side effects. Your doctor can help you determine the appropriate way to switch your medication, such as gradually adjusting the dosage over time.

Conclusion: Synthroid is an effective medication for hypothyroidism

Synthroid is the synthetic version of the T4 hormone. It's a prescription medicine for hypothyroidism, a condition in which the body doesn't have enough thyroid hormone. When taking Synthroid, following proper dosing and monitoring are important to avoid potential side effects or complications.

While Synthroid is a highly effective medication for treating hypothyroidism and other thyroid conditions, it may not work for everyone. Fortunately, there are alternatives available such as natural desiccated thyroid (NDT). NDT contains both T3 and T4 hormones, making it a popular option for those who don't see improvement with Synthroid. One such NDT brand is VitaliThy, a natural desiccated thyroid you can buy online.

Wojciech Majda
Wojciech Majda

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