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Thyroid Medication Side Effects

Learn about possible side effects of common thyroid hormone replacement medications.
Thyroid Medication Side Effects
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If you have a thyroid disorder, your doctor may prescribe thyroid hormone replacement medication as part of your treatment plan. While it can make a world of difference in your symptoms, thyroid medication can also cause side effects. 

What is thyroid hormone replacement medication?

The thyroid gland is a butterfly-shaped gland located at the base of your neck. It produces hormones that regulate your body's energy use, along with many other essential functions. As part of the endocrine system, the thyroid gland makes and stores hormones that help regulate the body's metabolism in blood pressure, blood temperature, and heart rate.

When your thyroid hormone production drops, your body processes slow down and change, affecting virtually every system in your body.

There are many different formulations of thyroid hormone replacement medication. Thyroid hormone therapy uses human-made thyroid hormones to raise replace low levels of natural thyroid hormones in the body. 

While exploring the benefits and differences between these many options, it's important to remember that there is no one-size-fits-all option. We are all unique with individual sensitivities. Our bodies will not all react the same way to a specific medication or dosage.

Work with your doctor to determine the best treatment plan for your specific needs, including medication, nutrition, and lifestyle modifications. While there is no cure for hypothyroidism, you can manage the condition and your symptoms successfully with thyroid hormone replacement therapy.

Side effects of thyroid hormone medicationa

Most medications have at least one or two side effects, while many have a daunting list. Ahead, some of the side effects of the three different drug treatment options for an underactive thyroid.

Note that many different brands manufacture synthetic thyroid replacement hormones. We have listed the side effects of two of the most common medications. Always read the package insert that is given to you when you pick up your prescription to learn about the specific side effects of the particular drug you are taking.

Before diving into medication-specific side effects, please be aware of allergic reactions, specifically anaphylaxis. If you take any medication and notice a severe allergic response that includes any of the following, contact your doctor or get medical help immediately:

  • Difficulty breathing
  • Rash
  • Hives
  • Chest tightness
  • Swelling of the face, mouth, lips or tongue

Side effects of levothyroxine

Levothyroxine is a synthetic form of one thyroid hormone, thyroxine (T4). T4 is mostly inactive but needed for the body to convert into the thyroid hormone, triiodothyronine (T3), to be used by cells.

Your physician may specify a brand name to treat your thyroid condition because the potency of generic thyroxine has a history of varying considerably. These variations come from many manufacturers and formulations used by different companies. 

Some of the most common side effects of levothyroxine include:

  • Changes in the menstrual cycle
  • Diarrhea
  • Excessive sweating
  • Fever
  • Headache
  • Increased appetite
  • Insomnia
  • Irritability
  • Nervousness
  • Nausea
  • Stomach cramps
  • Tremor
  • Sensitivity to heat
  • Temporary hair loss (especially among adolescent patients)
  • Weight loss
  • Vomiting

If you have a rapid or irregular heartbeat or pulse, chest pain, or confusion/disorientation, discontinue use and get medical treatment immediately.

Levothyroxine interacts with some medications, meaning that some drugs (including some common over-the-counter meds, like aspirin) may cause changes in thyroid hormone levels and side effects. These interactions can affect the effectiveness and toxicity profiles of other medications. The extra five minutes you'll spend at the pharmacy counter for the pharmacist consult will be well worth it to educate you about any interactions fully!

Side effects of Liothyronine

Our bodies convert thyroxine (T4) into triiodothyronine (T3) to be used by cells. If your body has difficulty converting T4 to T3, T4-only formulations may not be sufficient for you. In this instance, the addition of a T3 hormone replacement medication may help.

Liothyronine is the generic formulation of thyroid hormone, triiodothyronine (T3). T3 is the active thyroid hormone that works at the cellular level to help deliver oxygen and energy to cells, tissues, and glands throughout the body.

Unusually, someone is prescribed T3-only medication without T4. Liothyronine is typically used in combination with T4 medicines to balance the ratio of the two main thyroid hormones present in the body. 

Generic liothyronine has a shortlist of mild side effects listed, which include:

  • Mild nausea
  • Temporary hair loss (more often in adolescents)

If the above side effects do not go away or become severe, reach out to your practitioner.

Thyroid hormone replacement drugs are powerful. For this reason, it is critical to be under careful medical supervision when on these drugs, especially when starting a new brand or increasing dosage level.

Side effects of natural desiccated thyroid 

Natural desiccated thyroid (NDT) derives from the dried (desiccated) thyroid glands of pigs or cows. It provides T3, T4, and other thyroid hormones found in the human thyroid. Pigs produce T4 to T3 at a 4:1 ratio, which is higher than the human proportion of 14:1.

Desiccated animal thyroid was the most common form of thyroid therapy before the discovery of individual thyroid hormones. Because there is no evidence that NDT has any advantage over synthetic T4, it is not currently FDA-approved.

Side effects of NDT that you should report to your healthcare provider include:

  • Allergic reactions
  • Anxiety or breathing problems
  • Changes in menstrual periods
  • Chest pain
  • Diarrhea
  • Fast or irregular heartbeat
  • Swelling of ankles, feet, or legs
  • Tremors
  • Trouble sleeping
  • Vomiting

Other, less urgent side effects of NDT may include:

  • Changes in appetite
  • Headache
  • Irritability
  • Nausea
  • Weight loss

A note from Paloma Health

This article does not contain a complete list of possible side effects from each type or brand of thyroid medication. If you notice symptoms not listed on the medication pamphlet insert, contact your doctor or pharmacist for medical advice. You may also report side effects to the FDA at 1-800-FDA-1088 or at


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Katie Wilkinson

Katie Wilkinson, previously serving as the Head of Content and Community at Paloma Health, fervently explores the nexus between healthcare and technology. Living with an autoimmune condition, she's experienced firsthand the limitations of conventional healthcare. This fuels both her personal and professional commitment to enhancing patient accessibility to superior care.

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