Levothyroxine Side Effects: What’s Common & What’s Serious

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One of the most commonly prescribed medications for hypothyroidism is levothyroxine (brand name: Synthroid). This medication is made up of synthetic thyroid hormones and used when the thyroid gland is not producing enough thyroid hormone. 

Common side effects of levothyroxine include:

  • Fluctuations in weight
  • Mood changes
  • Changes in appetite
  • Being sensitive to heat
  • Headaches
  • Vomiting
  • Fatigue
  • Menstrual cycle changes

Side effects of thyroid medications in females are similar to that of males, but also include changes to the menstrual cycle.

Some patients experience autoimmune problems when taking Synthroid. 

Side effects from levothyroxine can occur, but they are not usually life-threatening. They can include both physical and emotional changes. If you find yourself experiencing side effects from levothyroxine, you should speak to your healthcare provider.

If you experience hypothyroidism, schedule a free phone consultation with PrimeHealth to see if our treatment center is right for you. We have years of experience treating and reversing hypothyroidism in our patients using individualized treatment plans and integrative tactics.

Common Side Effects of Levothyroxine

The most common side effects of taking levothyroxine are:

  • Weight gain
  • Weight loss
  • Headache
  • Nausea
  • Diarrhea
  • Fatigue
  • Changes in appetite
  • Fever
  • Excessive sweating
  • Menstrual changes
  • Sensitivity to heat
  • Hair loss
  • Joint pain
  • Leg cramps
  • Mood changes

Serious Side Effects of Levothyroxine

These side effects are not as common and can be more serious. In severe cases, levothyroxine may cause the following side effects:

  • Vomiting
  • Blurred vision
  • Dizziness
  • Eye pain
  • Pain in your hip or knee
  • Irregular heartbeat
  • Palpitations and arrhythmias
  • Tremors
  • Swelling
  • Hives
  • Skin rash
  • Severe fatigue
  • Severe headache
  • Abnormal physical growth in children

Rare Side Effects of Levothyroxine

If you experience any of the following side effects after taking levothyroxine, seek immediate care:

  • Chest pain, tightness, or discomfort
  • Trouble breathing
  • Dilated neck veins
  • Seizures

Symptoms of Levothyroxine Overdose

If you believe you or someone you are with has taken too much of their medication, seek immediate help.

What are the side effects of taking too much levothyroxine? The side effects of taking  too much levothyroxine include:

  • Irregular heart rate
  • Uncontrollable shaking of any part of the body
  • Sudden unconsciousness
  • Sudden cold feeling
  • Lightheaded feeling
  • Abrupt slurring of speech
  • Confusion
  • Loss of coordination

Levothyroxine Interactions With Other Medications

Levothyroxine is known to interact with many types of prescription and over-the-counter medications. In some cases, the medications cannot be taken at the same time, but can still be taken while using the other med. 

Here are the most common drug interactions with levothyroxine:

  • EpiPen (epinephrine)
  • Nexium (esomeprazole)
  • Metoprolol (metoprolol)
  • Questran (cholestyramine)
  • Colestid (colestipol)
  • Xenical (orlistat)
  • Coumadin (warfarin)
  • Carafate (sucralfate)
  • Welchol (colesevelam)
  • Claritin-D (pseudoephedrine)
  • Ketalar (ketamine)
  • Sudafed PE (phenylephrine)
  • Tums (calcium carbonate)
  • Gas-X (simethicone)
  • Kalexate, Kionex, and SPS (kayexalate)
  • Phosphate binders (calcium carbonate, ferrous sulfate, sevelamer, lanthanum)
  • Tegretol, Curatil (carbamazepine)
  • Certain types of Advil
  • Certain types of Benadryl
  • Certain types of Alka-Seltzer
  • Certain types of Mucinex
  • Certain types of DayQuil and NyQuil
  • Insulin
  • Antacids containing aluminum or magnesium
  • Testosterone
  • Estrogen hormone replacement therapy or contraceptive pills)

Levothyroxine Interactions With Herbal Supplements

In addition to medications, you should be mindful of the herbal and dietary supplements taken with levothyroxine. What should I avoid while taking levothyroxine? You should avoid supplements that can alter the efficacy of the drug. These include:

  • Soybean Products
  • Fiber Supplements
  • Calcium and Iron Supplements: Calcium and iron supplements can interfere with levothyroxine absorption. It’s recommended that levothyroxine should not be taken within four hours of consuming these supplements

It is also worth noting that why there is no study investigating taking biotin and levothyroxine together, biotin supplements have affected thyroid function test accuracy. Food high in iodine can also make hypothyroidism worse, so be aware of eating high-iodine foods.

Considerations While Taking Levothyroxine

Avoid eating within an hour of taking it. Schedule your hypothyroidism diet meals and levothyroxine dosage hours apart. A consistent schedule is easier to keep to. Many people tend to take their medication immediately upon waking for convenience, then eat an hour later.

If you are pregnant, breastfeeding, or plan to become pregnant, talk to your doctor about optimizing your dose of levothyroxine for you and the infant. During pregnancy, many women will benefit from a slight increase in their dose of it, and this can usually be reduced postpartum.

If you suffer from any of the following medical conditions, discuss with your healthcare provider the increased risk of taking it:

  • High blood pressure (hypertension)
  • Cardiovascular disease
  • Adrenal insufficiency
  • Pituitary gland problems
  • Thyrotoxicosis (too much thyroid hormone in the body)
  • Osteoporosis
  • Diabetes
  • Seizures

Check out this complete list of 200 medications that may interact badly with levothyroxine. Always discuss drug information and interactions with your healthcare provider.

When To Contact Your Healthcare Provider

Although they may not be a cause for major concern, any side effects you experience when taking Synthroid should be reported to your prescribing physician. If you experience serious side effects that interfere with your daily life, don’t wait to call the doctor to discuss your options.

If you experience severe symptoms or signs of an overdose after beginning levothyroxine, call your provider right away. You may need to seek emergency medical care.

What is levothyroxine?

Levothyroxine (Synthroid) is an oral tablet, capsule, or injectable made up of synthetic thyroid hormone. It is the primary treatment for hypothyroidism, an underactive thyroid. This medication is also used for people dealing with a goiter, after thyroid cancer, or thyroid resection.

There are different types and different brands of thyroid hormone medication. Some types have worse associated side effects than others.

Common FDA-approved brand names of levothyroxine or levothyroxine sodium include:

  • Synthroid
  • Levothroid
  • Levoxyl
  • Tirosint
  • Unithroid

Why is it used?

People with hypothyroidism (not to be confused with hyperthyroidism) have a thyroid gland that doesn’t produce sufficient thyroid hormones. These hormones are critical in regulating the body’s energy and metabolism. 

Not having enough thyroid hormone can result in symptoms such as:

  • Poor growth
  • Slow speech
  • Lack of energy
  • Excessive tiredness
  • Constipation
  • Weight gain
  • Hair loss
  • Increased sensitivity to cold
  • Joint and muscle pain
  • Heavy or irregular menstrual periods
  • Depression

What happens to your body when you start taking levothyroxine? When you start taking levothyroxine, the hormones are replaced and alleviate hypothyroidism symptoms. You may notice more stable blood sugar levels and improved body temperature regulation.

How long do you need to take levothyroxine before it starts working? You may need to take levothyroxine for two weeks before you notice it starts working. It is meant as a long-term treatment for low thyroid hormone levels.

Levothyroxine is also used to treat or prevent goiter, an enlarged thyroid gland. This can be caused by hormone imbalances, radiation treatment, surgery, or cancer. It’s also used in conjunction with surgery and radioactive iodine therapy to treat thyroid cancer.

How does levothyroxine work?

Levothyroxine works by replacing thyroxine, a hormone your thyroid would naturally produce if it were functioning optimally. Thyroxine (T4) is the most abundant thyroid hormone in your body. Triiodothyronine (T3) is the second most abundant although T3 is much more metabolically active. 

Levothyroxine contains only T4. At PrimeHeath, we will prescribe desiccated thyroid gland in certain cases, which contains both levothyroxine (T4) and liothyronine (T3) at a ratio of 4:1 T4 to T3. We do this because your thyroid hormones are naturally produced together and combining T4 and T3 is generally preferred than either by itself. 

How will I know if the levothyroxine is working? You will know if the levothyroxine is working by having regular blood tests. 

Our highly qualified specialists may also prescribe compounded synthetic thyroid hormones because these are much less likely to trigger an autoimmune reaction. By prescribing compounded thyroid hormones, a doctor can prescribe whatever ratio of dose you need — not just 4:1.

PrimeHealth specialists prefer not to prescribe levothyroxine as a first-line treatment. We only recommend thyroid hormones for patients in an otherwise good place. Check out our unique and effective approach to treating hypothyroidism or our thyroid supplements article.

How To Take Levothyroxine

You should take levothyroxine and all other medications according to your healthcare professional’s instructions and the listed dosage and timing on the prescription bottle.

Typically, patients are told to take levothyroxine on an empty stomach about an hour before eating.

Dosage and Strength

Depending on your diagnosis and doctor’s recommendation, typical dosages of levothyroxine for hypothyroidism include:

  • 25 mcg
  • 50 mcg
  • 75 mcg
  • 88 mcg
  • 100 mcg
  • 112 mcg
  • 125 mcg
  • 137 mcg
  • 150 mcg
  • 175 mcg
  • 200 mcg
  • 300 mcg

The less thyroid hormone that your body naturally produces, the higher your prescription drug dosage will likely be. The more body weight you have, the higher your dosage may be.

The more susceptible to levothyroxine side effects, the lower your dosage should be. History of heart disease, diabetes, or seizures would likely necessitate a lower dosage.

Forgetting a Dose

If you missed a dose of levothyroxine, take it as soon as you remember.

If it is almost time for your next dose, simply skip the last dose and continue on your regular dosage schedule.

Do not take two doses at once for any reason. Talk to your doctor if you have concerns about missed doses.

Long-Term Effects Of Levothyroxine

Levothyroxine is one of the most commonly prescribed medications in the U.S. It is important to work with a healthcare provider that will not over-treat or under-treat with levothyroxine. Adverse effects can drastically impact your quality of life if you veer from the prescribed dosage.

What are the long-term effects of thyroid medication? The long-term side effects of thyroid medication include weight fluctuations, decreases in bone mineral density, sensitivity to heat, joint pain, changes to your menstrual cycle, and possibly even autoimmune dysfunction.

A potential increased risk of cancer was associated with long-term levothyroxine use according to a retrospective population-based study conducted in Taiwan. Levothyroxine users showed a 50% higher risk of cancer at any site compared with non-users. 

Reverse Your Hypothyroidism

If you’re worried about unacceptable side effects of levothyroxine, triggering autoimmune reactions, or allergic reactions, seek medical advice about compounded synthetic thyroid hormones instead of desiccated porcine thyroid hormones.

Do not use levothyroxine unless recommended by your doctor — and only for the purpose for which it was prescribed. Do not use levothyroxine to combat obesity or for weight loss

Contrary to what most conventional doctors will tell you, hypothyroidism is often reversible. First, you must diagnose the root cause of your thyroid function problems with blood tests that measure TSH and thyroid hormone levels. Second, you treat the root cause.

Denver/Colorado Residents: If you have concerns about your levothyroxine prescription or other questions about hypothyroidism, schedule a free phone consultation with PrimeHealth. We specialize in hypothyroidism and other conditions often caused by autoimmune problems.


  1. Karimifar, M., Esmaili, F., Salari, A., Kachuei, A., Faragzadegan, Z., & Karimifar, M. (2014). Effects of Levothyroxine and thyroid stimulating hormone on bone loss in patients with primary hypothyroidism. Journal of research in pharmacy practice, 3(3), 83.
  2. Wu, C. C., Islam, M. M., Nguyen, P. A., Poly, T. N., Wang, C. H., Iqbal, U., … & Yang, H. C. (2021). Risk of cancer in long‐term levothyroxine users: Retrospective population‐based study.Cancer Science, 112(6), 2533-2541.
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