Evidence Based

Levothyroxine Uses, Side Effects, Dosage + Thyroid Support

Written by Mathew Eng, PharmD | Reviewed by Ana Aleksic, MSc (Pharmacy) | Last updated:
Medically reviewed by
Jonathan Ritter, PharmD, PhD (Pharmacology) | Written by Mathew Eng, PharmD | Reviewed by Ana Aleksic, MSc (Pharmacy) | Last updated:

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In the early 1900s, a synthetic thyroid hormone called levothyroxine was developed. Today, it is the standard treatment for hypothyroidism and is one of the most prescribed drugs in the U.S. What makes levothyroxine so popular? And what natural thyroid support supplements work? Read on to find out.

Disclaimer: By writing this post, we are not recommending this drug. Some of our readers requested that we commission a post on it, and we are providing an overview of the information available in the clinical and scientific literature. Please discuss your medications with your doctor.

What is Levothyroxine?

Levothyroxine is one of the most commonly prescribed medications in the United States. Brand names of levothyroxine include Synthroid, Levoxyl, and Unithroid [1].

It’s mostly used to treat hypothyroidism, a condition where the body doesn’t create enough thyroid hormones [1].

Levothyroxine acts as a synthetic version of T4 (thyroxine), one of the main hormones that the thyroid gland releases [1].

It is also sometimes used to treat other thyroid disorders, such as goiter and thyroid cancer [1].

How Does it Work?

Before we get into how levothyroxine works, we first have to discuss thyroid hormones.

Our thyroid gland produces and secretes two important hormones, thyroxine (T4) and triiodothyronine (T3) [2].

T3 is the active form of the two – it’s the hormone that primarily affects our body. The effects of T3 include changes to heart rate, breathing, and metabolism, to name a few [2].

Although T3 does most of the work, there is actually much more T4 in our blood. In fact, the thyroid gland releases about 14 times more T4 than T3 [3].

Why is this?

T4 is useful because it has a longer half-life than T3. This allows the body to convert T4 into T3 when needed. In other words, T4 acts as a stable source of T3 in the body [4].

It’s important for the body to regulate the levels of thyroid hormones in the blood. Having too much or too little can cause health problems [4].

Now back to levothyroxine.

Levothyroxine is a man-made version of T4. When the drug is absorbed into the blood, it is converted into T3 just like normal T4 [4].

Simply put, levothyroxine is a T4 substitute for those that cannot make enough of their own [4].

What Is Hypothyroidism?

It’s hard to talk about levothyroxine without talking about hypothyroidism as well.

People with hypothyroidism produce low amounts of thyroid hormones. This is usually due to problems with the thyroid gland [5].

A common cause of hypothyroidism is Hashimoto’s disease, an autoimmune disorder that destroys the thyroid gland [5].

Some other causes include a lack of iodine in the diet and pregnancy [5].

What happens when thyroid hormone levels are low?

The effects of hypothyroidism can vary and some people may experience no symptoms at all [5].

Some common symptoms include [5]:

Uses of Levothyroxine

Important Warning: Levothyroxine should not be used for weight loss nor in combination with any weight loss or obesity therapy. Please see the “Levothyroxine Is Not for Weight Loss” section below.

1) Hypothyroidism

Levothyroxine is primarily used to treat thyroid hormone deficiency, also known as hypothyroidism [1].

It is usually the first choice for thyroid hormone replacement therapy [6, 1].

Both the American Thyroid Association and the American Association of Clinical Endocrinologists recommend levothyroxine as the preferred medication for hypothyroidism [1].

This recommendation is based on strong clinical evidence of its effectiveness and safety [1].

Other factors like good absorption, a long half-life, and low cost also support its use [1].

It wasnt always like this, though.

For the longest time, desiccated thyroid (brand name Armour Thyroid) was the first choice for hypothyroidism. This drug is an animal extract that contains both T4 and T3 (the active form of T4) [6].

There are several reasons why levothyroxine came to be the standard.

One reason was the discovery that most T3 in the body comes from T4 conversion. Researchers also found that the thyroid gland mostly secretes T4 [6].

These findings imply that replacement with T4 alone may be better than products that contain both T3 and T4 [1].

Other factors include safety concerns over the inconsistency of desiccated thyroid products [6].

2) Subclinical Hypothyroidism

Sometimes people can have normal thyroid hormone levels, but mildly elevated TSH levels. When this occurs it is referred to as subclinical hypothyroidism [7].

This condition can develop into actual hypothyroidism. It also may be a risk factor for developing heart issues [7].

There is some debate on whether we should treat this mild form of hypothyroidism.

For instance, a comprehensive review of 12 studies including 350 people found no benefit with levothyroxine. It did not reduce the risk of death or prevent heart problems associated with subclinical hypothyroidism [8].

On the other hand, research also shows that levothyroxine improves heart function and cholesterol. These two effects could potentially help with heart health, but the evidence remains unclear [9, 8].

3) Myxedema Coma

Myxedema coma is a severe case of hypothyroidism that often requires immediate thyroid replacement and hospitalization [1].

Research shows that high doses of levothyroxine can reduce the risk of death associated with myxedema coma. It can also help restore heart and lung function [10, 11, 1].

4) Thyroid Cancer and Goiter

Levothyroxine is sometimes used to help with thyroid cancer and goiter. In these conditions, the thyroid gland can swell up or develop small nodules [12, 13].

If this happens, a strategy called TSH suppression therapy is sometimes used. Levothyroxine suppresses TSH levels in the blood, which helps control the size of the thyroid gland [12, 13].

Clinical trials show that TSH suppression with levothyroxine is effective at reducing thyroid size [14, 15, 16].

However, there are some risks as well.

TSH suppression with levothyroxine may increase the risk of heart disease as well as osteoporosis [17, 18, 14].

Levothyroxine Is Not for Weight Loss

One of the well-known symptoms of hypothyroidism is weight gain. Restoring thyroid hormones to normal levels can reverse this.

This has led some people to believe that levothyroxine can be used to lose weight.

There are a few problems with this.

One issue is that high levels of thyroid hormone can cause life-threatening toxicity. This is especially true for people with normal thyroid hormone levels who take levothyroxine [19].

Beyond that, levothyroxine may not even cause significant weight loss.

A study of 101 patients found that levothyroxine reduces body weight in only 52% of people. For those that did see a benefit, the effects were mild – the average weight loss was about 8 lbs [20].

Bottom line: Using levothyroxine for weight loss is dangerous and ineffective.

The FDA has placed a statement on all levothyroxine products explicitly warning against using it for weight loss purposes [21].

Levothyroxine Side Effects & Safety

Common Side Effects

Levothyroxine is generally considered safe. Side effects are usually mild [1].

Some common side effects include [21]:

  • Headache
  • Sleep problems
  • Tiredness
  • Changes in appetite and weight
  • Fever
  • Sweating

Often, side effects are due to imbalances in thyroid hormones and not from the medication itself. If thyroid hormone levels are within normal range, side effects are uncommon [1].

Risk of Bone Loss

Based on initial reports, the FDA warns that levothyroxine may cause bone loss, especially in postmenopausal women [21].

Recent research has revealed conflicting results.

For example, a large review of 63 studies concluded that the scientific evidence was unclear, even in postmenopausal women [22].

Many recent clinical trials have also found no effects on bone health [23, 24, 25].

Based on a trial of 74 women, levothyroxine does not increase the risk of fracture nor does it affect bone density [24].

Risk of Heart Disease

Those with cardiovascular diseases should be careful when using levothyroxine [26].

Thyroid hormones play a major role in heart function. Overtreatment with levothyroxine can lead to an irregular heartbeat and potential heart attack [26].

Safety in Pregnancy and Breastfeeding

Levothyroxine is considered safe during pregnancy [27].

Clinical trials have not revealed any safety risks to the fetus. For this reason, the FDA considers it a low risk during pregnancy (former Pregnancy Category A) [28].

Pregnancy can cause hypothyroidism in some women, as thyroid hormone requirements increase during pregnancy. This means that if you’re pregnant, your doctor may need to increase your regular dose of levothyroxine [27, 29].

Uncontrolled hypothyroidism during pregnancy can increase the risk of pregnancy complications [27].

Treatment with levothyroxine can decrease the risk of pregnancy loss, premature delivery, and low birth weight [30].

Levothyroxine is also considered safe during breastfeeding. Small amounts of thyroid hormones are found in breast milk, but no safety concerns have been found with levothyroxine. In fact, thyroid hormones play an important role in maintaining lactation [31].

However, you should talk to your doctor before using it while breastfeeding.

Let your doctor know if you are taking levothyroxine and are pregnant or plan to become pregnant. Your doctor may need to monitor your hormone levels or adjust the dosage.


There are several cases where levothyroxine should not be taken.

Levothyroxine should be avoided in conditions that raise thyroid hormones (such as thyrotoxicosis). This combination can lead to dangerously high levels of thyroid hormone [32].

This drug can have negative effects on the heart. Individuals who have recently experienced a heart attack should not take levothyroxine [33].

People with uncorrected adrenal insufficiency should also avoid levothyroxine. In this disorder, thyroid hormones can trigger an adrenal crisis, a life-threatening condition [34].

And of course, those who are allergic to any of the ingredients inside levothyroxine should not take it.

Drug Interactions

Levothyroxine has many major interactions with other drugs and substances. It is important to recognize interactions to ensure thyroid hormone levels stay stable [35].

Some common interactions include [35]:

  • Calcium carbonate (Tums)
  • Antacids that contain aluminum and magnesium
  • Sucralfate
  • Iron supplements
  • Heartburn drugs (Prilosec, Nexium, Prevacid)
  • Dietary fiber

A recent small study suggests there is an interaction with milk as well [36].

Many of these interactions reduce the absorption and effectiveness of levothyroxine. If a drug interaction cannot be avoided, usually the dose and timing of levothyroxine are changed [35].

There are many other interactions that exist as well. It’s important to discuss with your doctor about all the drugs and supplements you are currently taking.

Levothyroxine Dosage & How to Take


Levothyroxine is usually taken as a tablet – strengths range from 25 to 300 micrograms.

Other forms like capsules, liquid solutions, and injections also exist. They are typically reserved for special situations (such as solutions for those who can’t swallow large tablets).

Dosing in hypothyroidism is individualized and based on TSH and T4 lab tests [1].

Lab values are usually monitored every 6 – 8 weeks and the dose is adjusted as needed. Once TSH and T4 become stable on a dose, monitoring is usually extended to every 6 – 12 months [1].

Levothyroxine can take a while to work, peak effects might not be seen for 4 – 6 weeks [1].

The average dose of levothyroxine is about 100 to 125 micrograms per day for an adult. This can vary depending on a number of factors including age, health, and drug interactions [37].

How to Take Levothyroxine?

Generally, levothyroxine should be taken with water 30 minutes to 1 hour before breakfast each day. This is done to prevent any potential interactions with food.

Levothyroxine should be taken 4 hours apart from drugs that may affect its absorption. Some examples include antacids, heartburn medications, and calcium carbonate supplements.

There’s some evidence that levothyroxine may be equally effective when taken at bedtime as well [38].

Brand vs. Generic

Not too long ago, different brands of levothyroxine were not interchangeable because they were not regulated by the FDA [6].

Why was this?

Thyroid extracts from animals have been used for hypothyroidism since before the FDA was formed [6].

When levothyroxine was developed, the FDA grouped the drug with animal thyroid extracts, which were not regulated [6].

It wasn’t until 1997 that the FDA decided to regulate thyroid medications, due to safety concerns from inconsistent products [6].

Today, drug standards have greatly improved.

Most levothyroxine products are now compatible with each other (including brand and generics). However, some products are still not interchangeable [6].

Experts typically recommend sticking with one form of levothyroxine if possible. If a change is necessary, lab tests are usually monitored more frequently until levels become stable again [6].


The Good:

  • One user with Hashimoto’s disease from an early age stated that levothyroxine helped with losing weight, decreasing irritability, and increased concentration.
  • Another person with Hashimoto’s disease said that it lowered their TSH and helped them feel great and full of energy.
  • One user with underactive thyroid said that it helped combat feelings of being cold and fatigued.
  • One user with hypothyroidism felt that it made them feel much more energetic and active.

The Bad:

  • One user with hypothyroidism listed many negative side-effects such as headaches, insomnia, and feeling depressed.
  • One user with Hashimoto’s disease said that after using levothyroxine they felt much more inflammation and that it went away after stopping levothyroxine.
  • One person with underactive thyroid said that levothyroxine caused lip swelling and irritation and so they stopped taking it.

Natural Thyroid Support

This section will give you an overview of supplements and nutrients that support the thyroid.

The listed compounds are in no way meant to replace your thyroid medication.

Talk to a doctor before taking supplements alongside prescription medication. Do not change the dosage of your medication or stop taking it without consulting a doctor first.

1) Iodine

Our body needs iodine to make thyroid hormones. Our body also doesn’t produce iodine, we get all of it from food [39].

It’s not surprising then that iodine deficiency is one of the leading causes of hypothyroidism [39].

So should we take iodine supplements?

The answer is a bit unclear.

People in the U.S. typically get more than enough iodine from their diets. Especially since the creation of iodized salt, iodine deficiency has not been a major problem [40].

Another issue is that consuming too much iodine is dangerous. The upper limit of iodine is about 1,100 micrograms per day, doses above that can be lethal [41].

Ironically, a study of 256 healthy adults found that iodine supplements can cause hypothyroidism. This happened when subjects took more than 800 micrograms each day [42].

Additionally, too much iodine increases Th1 and Th17 immune responses. In turn, excess iodine supplementation may worsen inflammation and autoimmune issues [43].

Who should receive supplements then?

Pregnant women and children are particularly vulnerable to iodine deficiency [44].

This is because iodine is important for brain development in kids. Deficiencies during pregnancy can cause a number of complications [44].

However, a large review of 14 studies including over 2,700 pregnant women did not find any beneficial effects with iodine supplements [45].

A different review of 26 studies including over 29,000 children also had mixed results. They did find that supplementation with iodized oil can reduce rates of goiter [46].

Bottom Line

The recommended daily intake of iodine is 150 micrograms. Consuming too much iodine can have negative effects [39].

We usually get enough iodine through our diet. If you want to boost your intake, some good food sources include [39]:

  • Seafood
  • Seaweed and kelp
  • Bread
  • Milk
  • Iodized salt

Children and pregnant women may benefit from taking iodine supplements, but clinical evidence is mixed [45, 46].

2) Selenium

Selenium is another important trace element for our health. It is used by the enzymes responsible for converting T4 into T3 [47].

Research shows that selenium supplementation may help fight infections and some types of cancer [48, 49].

As far as thyroid disorders go, it’s unclear how helpful selenium is [50, 51].

The issue is we require a very small amount of selenium for proper thyroid function. People who receive adequate selenium through their diet may not benefit from supplements [48].

The World Health Organization recommends a maximum daily intake of 70 micrograms of selenium. Doses over 400 micrograms per day can be toxic [52].

Bottom Line

Selenium plays many important roles in our body, including thyroid hormones.

For those concerned they aren’t getting enough selenium, it’s probably best to try to include more in the diet.

Some great dietary sources of selenium include [52]:

  • Brazil nuts
  • Garlic
  • Onion
  • Salmon
  • Eggs
  • Beef

3) Tyrosine

Research has not looked at the benefits of tyrosine supplements on thyroid disorders, but we do know it is important.

Tyrosine is an amino acid that is used to make T3 and T4 [53].

Low levels of tyrosine are linked to lower levels of thyroid hormones, at least in patients with severe infections [53].

As an added bonus, tyrosine supplements have been shown to enhance cognitive performance as well [54].

Foods that contain high levels of tyrosine are [55]:

  • Chicken and turkey
  • Fish
  • Dairy products
  • Peanuts
  • Bananas

4) Fish Oil

A few animal studies show that fish oil supplements may benefit those with thyroid problems.

One rat study found that the PUFAs in fish oil can enhance the action of thyroid hormones [56].

Supplements may also protect against cognitive impairment caused by hypothyroidism [57].

5) Vitamin B12

Vitamin B12 deficiencies are very common in those with thyroid disorders. According to some estimates, about 40% of people with hypothyroidism have low levels of vitamin B12 [58].

Correcting this deficiency can improve certain symptoms, including weakness, memory difficulties, and depression [59].

A study of 116 people found that vitamin B12 supplements improve symptoms in almost 60% of subjects [59].

6) Ashwagandha

Ashwagandha, also known as Indian ginseng or poison gooseberry, is a plant commonly used in traditional Ayurvedic medicine [60].

A study of 50 people suggests that root extracts help with subclinical hypothyroidism. They found that the extract improves TSH, T4, and T3 levels compared to placebo [60].

7) Thyroid Supplements

There are a number of different supplements that are marketed as promoters of thyroid health.

Thyroid Support

For example, multiple products called Thyroid Support are available. These supplements typically contain a variety of vitamins and nutrients, including iodine, tyrosine, selenium, and even ashwagandha.

Most of these ingredients are indeed important for thyroid health, but it’s unclear how beneficial supplementation is [45, 50].

Thyroid Glandular and Desiccated Thyroid Gland

Thyroid Glandular or Raw Thyroid are the names of another popular type of supplement.

These products contain thyroid gland tissue usually taken from cows. Some products also include extracts from other glands, like the pituitary and adrenal tissue.

One study from the ‘90s analyzing three thyroid gland supplements revealed that none contained T4 and only two might’ve contained some T3. At doses two times higher than recommended on the supplement label, these products had no effect on thyroid lab markers in healthy volunteers [61].

In another analysis, similar over-the-counter (OTC) thyroid gland supplements contained only negligible amounts of both T3 and T4 [62].

Based on these studies, it’s highly questionable if OTC thyroid gland supplements work.

Armour Thyroid and NP Thyroid, on the other hand, contain the crushed (desiccated) thyroid glands of pigs. Based on the manufacturers’ claims, both products should provide a consistent amount of T3 (9 mcg) and T4 (38 mcg) per grain of thyroid.

Clinical reports suggest Armour Thyroid may be useful for mild (sub-clinical) or diagnosed (clinical) hypothyroidism [63, 64].

Both Armour Thyroid and NP Thyroid require a prescription in the US. Despite this, some people use them for weight loss without medical supervision. In one case, Armour Thyroid use along with testosterone caused serious side effects in a 32-year-old bodybuilder who wanted to lose weight [65].

Consult a doctor before using Armour Thyroid or NP Thyroid and provide them with a full list of all the supplements and medications you’re taking.

Limitations and Caveats

There is strong evidence that levothyroxine is safe and effective for hypothyroidism.

Research on other conditions, like subclinical hypothyroidism, is less clear.

Additionally, many natural supplements have not been studied in clinical trials. Their safety and effectiveness cannot be confidently evaluated until more research is done.


Levothyroxine is a synthetic form of the thyroid hormone T4. It is typically used to treat hypothyroidism. Less commonly, it is used for subclinical hypothyroidism, goiter, and thyroid cancer. Levothyroxine should not be used for weight loss or alongside anti-obesity therapies.

This medication works by restoring T4 blood levels. Side effects are usually mild (headache, sleep problems, and weight changes were common).

Consult your doctor before using levothyroxine and mention all the drugs and supplements that you take to avoid dangerous interactions. Those with thyrotoxicosis, recent heart attack, and uncorrected adrenal insufficiency should not take this medication.

Natural thyroid support options include iodine, selenium, tyrosine, fish oil, vitamin B12, and ashwagandha. It’s best to get the ones you can from a healthy nutritious diet.

About the Author

Mathew Eng

Mathew received his PharmD from the University of Hawaii and an undergraduate degree in Biology from the University of Washington.
Mathew is a licensed pharmacist with clinical experience in oncology, infectious disease, and diabetes management. He has a passion for personalized patient care and believes that education is essential to living a healthy life. His goal is to motivate individuals to find ways to manage their chronic conditions.

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