Levothyroxine: Uses, Side-Effects, and Alternative Treatment

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Levothyroxine (brand name: Synthroid) is a synthetic thyroid hormone used for treating an underactive thyroid gland condition called hypothyroidism. It’s also used for other thyroid-related conditions, such as goiter, and in the management of thyroid cancer. 

Side effects from levothyroxine are common but not usually life-threatening. These can include both physical and emotional changes, such as weight loss, increased heart rate, mood swings, and anxiety.

If you are navigating hypothyroidism in the Colorado area, schedule a free phone consultation with PrimeHealth to see if our treatment center is right for you.

Side Effects of Levothyroxine

Levothyroxine often gives the body enough thyroid hormone to improve hypothyroid symptoms within weeks. However, it can come with a range of mild to severe side effects. Understanding these can help you know what is “normal” as well as when to seek urgent care.

Common Side Effects

Patients taking levothyroxine might experience a variety of side effects, which often vary in intensity. The most common side effects include:

  • Fever: Fever may present as part of an allergic reaction or hypersensitivity to the medication. In some cases, fever could also be a sign of an overactive thyroid state (thyrotoxicosis) induced by excessive levothyroxine dosage.
  • Weight fluctuations: Some individuals may notice weight loss, while others may experience weight gain due to metabolic changes including changes in appetite.
  • Increase in blood pressure and blood sugar: Increased metabolism may increase blood pressure and disrupt how the body processes glucose. This is significant for individuals with hypertension, heart disease, and diabetes and should be monitored.
  • Gastrointestinal issues: Nausea and diarrhea can occur, affecting digestive health.
  • Neurological effects: Headaches and mood changes, including irritability and anxiety, are relatively common.
  • Physical discomfort: Tiredness/mild fatigue, joint pain, and leg cramps can impact daily activities.
  • Thermoregulatory changes: Increased sensitivity to heat and excessive sweating may be experienced.
  • Dermatological changes: Hair loss is a concerning side effect for many.
  • Menstrual changes: Women may experience alterations in their menstrual cycle, due to hormonal replacement.
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Rare but Serious Side Effects

While less common, some side effects of levothyroxine are more severe and warrant immediate medical attention:

  • Cardiovascular Issues: Symptoms like irregular heartbeat, chest pain, palpitations, and dizziness indicate potential heart-related side effects that could indicate a heart attack or heart failure.
  • Vision and Pain: Blurred vision is a side effect that requires immediate medical attention. 
  • Significant pain: Seek medical attention for significant pain in areas like the hips or knees.
  • Allergic Reactions: Swelling, hives, and skin rashes are signs of possible allergic reactions to the medication.
  • Extreme Fatigue and Headaches: Severe fatigue and headaches, beyond normal levels, can be concerning.
  • Respiratory Problems: Trouble breathing or tightness in the chest requires urgent care.
  • Neurological Concerns: Instances of seizures or severe dizziness are critical and necessitate immediate medical intervention.

Special Considerations

  • Autoimmune responses: A subset of patients might encounter medical conditions such as onset autoimmune issues when on Synthroid, requiring close monitoring.
  • Adrenal insufficiency: Administering levothyroxine to someone with underlying adrenal gland insufficiency can exacerbate symptoms or lead to an adrenal crisis. It’s important to assess adrenal function before initiating levothyroxine therapy.
  • Growth concerns in children: Abnormal growth patterns in children on hormone replacement should be closely monitored by a healthcare professional to ensure healthy development.

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

How to Take Levothyroxine

Avoid eating within an hour of taking levothyroxine. The medication is absorbed best on an empty stomach. For that reason, you should wait several hours after eating a hypothyroidism-friendly meal before you take your next dose of levothyroxine. 

Staying consistent with your medication and meal schedule can help regulate your thyroid hormone levels. Many people tend to take their medication immediately upon waking for convenience, then eat 2 hours later.

Depending on your diagnosis and doctor’s recommendation, typical dosages of levothyroxine for hypothyroidism typically range from 25-300 mcg. 

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.

If you are pregnant, breastfeeding, or plan to become pregnant, seek medical advice about optimizing your dose of levothyroxine for you and the infant. During pregnancy, many women will benefit from a slight increase in their dose, 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

Interactions With Other Medications

Levothyroxine can interact with a range of medications, from over-the-counter antacids and pain relievers to prescription drugs like blood thinners and antidepressants. If you’re on levothyroxine, be sure to talk to your prescribing provider about prescription and over-the-counter medications to avoid.

The most common drug interactions with levothyroxine include:

  • 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)

Herbal supplements can also affect levothyroxine’s effectiveness. You should avoid any supplements that can alter the efficacy of the drug, including:

  • Soybean Products
  • Fiber Supplements
  • Calcium and Iron Supplements

Calcium and iron supplements can interfere with levothyroxine absorption. It’s recommended that levothyroxine not be taken within 4 hours of consuming these supplements.

It is also worth noting that while 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.

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

How It Works + Safer Alternative Treatments

Levothyroxine works by replacing thyroxine, a hormone your thyroid would 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 more metabolically active. Levothyroxine contains only T4. 

At PrimeHeath, we 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. 

Our providers 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 providers 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 as well as our thyroid supplements article.


What are the common symptoms of hypothyroidism?

Common symptoms of hypothyroidism (not to be confused with hyperthyroidism) 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 is the most common side effect of levothyroxine?

The most common side effect of levothyroxine is weight fluctuations, with some individuals experiencing weight gain and others weight loss.

What is the long-term damage from levothyroxine?

The long-term damage from levothyroxine includes 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. 

What are the benefits of taking levothyroxine?

The primary benefit of taking levothyroxine is the normalization of thyroid hormone levels, which helps alleviate symptoms of hypothyroidism, such as fatigue, weight gain, and cold intolerance.

Does levothyroxine cause weight gain?

While levothyroxine can lead to weight changes, it more commonly causes weight loss; weight gain is less typical and may indicate a need for dosage adjustment.

Can levothyroxine cause heart palpitations or other cardiac issues?

Yes, levothyroxine can cause heart palpitations and other cardiac issues, especially if the dosage is too high, leading to symptoms similar to hyperthyroidism.

Can levothyroxine cause hair loss or other dermatological side effects?

Levothyroxine can indeed cause hair loss, especially during the initial stages of treatment, as the body adjusts to the medication.

Can levothyroxine affect your sleep or cause insomnia?

Levothyroxine can affect sleep patterns and may cause insomnia in some individuals, particularly if taken in higher doses than needed.

Reverse Your Hypothyroidism

If you’re worried about the 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.

If you’re in Colorado, we’d love to help you treat the root cause of your thyroid issues. We have years of experience reversing hypothyroidism in our patients using individualized treatment plans and integrative tactics.


  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.
  3. Cancer Science, 112(6), 2533-2541.
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