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

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Typical levothyroxine is made up of synthetic thyroid hormones. Conventional doctors prescribe levothyroxine to most hypothyroidism patients (not hyperthyroidism). However, some types of thyroid hormone medications are better for different people.

Common side effects of levothyroxine include fluctuations in weight, mood, appetite, body temperature, tiredness, and menstrual cycle. Some patients experience autoimmune problems when taking levothyroxine, so it’s wise to seek individualized treatment.

What are the benefits of taking levothyroxine? The benefit of taking levothyroxine is to treat hypothyroidism, which itself can lead to other medical conditions, such as heart problems, fertility issues, and goiters.

In this post, you’ll learn about levothyroxine side effects, as well as levothyroxine alternatives that might be better for some.

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.

What is levothyroxine?

Levothyroxine is a thyroid medication made up of synthetic thyroid hormone. It is the primary treatment for hypothyroidism (underactive thyroid), especially for anyone with a goiter or after thyroid cancer or thyroid resection.

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. Levothyroxine is meant as a long-term treatment for low thyroid hormone levels.

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:

  • Synthroid
  • Levothroid
  • Levoxyl
  • Tirosint
  • Unithroid

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. In some cases, we prescribe desiccated thyroid gland, which contains both levothyroxine (T4) and liothyronine (T3) at a ratio of 4:1 T4 to T3. Since your thyroid hormones are naturally produced together, combining T4 and T3 is generally preferred than either by itself. 

At PrimeHealth, 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.

Also, our 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.

Levothyroxine Side Effects

Levothyroxine can cause a long list of side effects — some are common, others severe.

Below, we have broken up levothyroxine’s side effects into “Common Side Effects”, “Serious Side Effects”, and “Symptoms of Levothyroxine Overdose”.

What should I avoid while taking levothyroxine? Avoid eating, taking supplements, or taking medications within an hour of taking levothyroxine. For proper absorption, it’s best to take your thyroid medication on an empty stomach, with water, an hour before any other food, drinks, medications, or supplements.

Common Side Effects

Be aware of these common side effects when taking levothyroxine:

  • 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

These side effects are rarer but more serious. In severe cases, levothyroxine may cause the following side effects:

  • Vomiting
  • Blurred vision
  • Dizziness
  • Eye pain
  • Pain in your hip or knee
  • Shortness of breath
  • Chest pain or tightness (angina)
  • Irregular heartbeat
  • Palpitations and arrhythmias
  • Tremors
  • Swelling
  • Hives
  • Skin rash
  • Severe fatigue
  • Severe headache
  • Abnormal physical growth in children

Symptoms of Levothyroxine Overdose

If you experience the following symptoms, you may have taken too much levothyroxine. If so, consult a healthcare professional right away.

  • Irregular pulse
  • Sudden unconsciousness
  • Sudden cold feeling
  • Lightheaded feeling
  • Abrupt slurring of speech
  • Confusion
  • Loss of coordination

Levothyroxine Interactions

Several drugs, activities, and conditions do not mix well with levothyroxine.

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)
  • Claritin-D (pseudoephedrine)
  • Ketalar (ketamine)
  • Sudafed PE (phenylephrine)
  • Tums (calcium carbonate)
  • Gas-X (simethicone)
  • 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
  • Testosterone
  • Multivitamins

Special Considerations for Taking Levothyroxine

Avoid eating within an hour of taking levothyroxine. 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 levothyroxine, and this can usually be reduced postpartum.

If you suffer from any of the following diseases, discuss with your healthcare provider the increased risk of taking levothyroxine:

  • 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.

Dosage for Hypothyroidism

Depending on your diagnosis and doctor 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.

Note: Certain medications and multivitamins also reduce the absorption of levothyroxine. Eating food within a few hours of taking levothyroxine may reduce its absorption. Take levothyroxine on an empty stomach.

Missed Doses

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.

Warnings & Outlook

Do 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 effects of thyroid medication include weight fluctuations, sensitivity to heat, joint pain, changes to your menstrual cycle, and possibly even autoimmune dysfunction.

If you’re worried about levothyroxine triggering autoimmune 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 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 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.

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