How to reduce Synthroid side effects

Synthroid (levothyroxine) is a prescription thyroid hormone replacement used to treat hypothyroidism and certain types of thyroid cancer in adults and children. It is a synthetic form of T4, a hormone that helps regulate metabolism throughout the body. Synthroid comes in many dose strengths, and the right dose depends on your individual needs. Like all medications, it can cause side effects and interact with other drugs, so proper dosing and monitoring are important.

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Common side effects of Synthroid

Many Synthroid side effects are mild and improve as your body adjusts or after a dose change. Contact your healthcare provider if symptoms are severe or do not go away.

Common side effects include:

• Fast or irregular heartbeat
• Anxiety, nervousness, or shaking
• Increased appetite 
• Excess sweating
• Shortness of breath
• Nausea, stomach pain, or diarrhea
• Headache or fever
• Trouble sleeping
• Hair loss
• Heat intolerance
• Irregular menstrual periods
• Skin rash
• Muscle cramps
• Weight changes

Weight changes

Hypothyroidism often causes weight gain due to a slower metabolism. When people start Synthroid, their metabolism returns to normal, which may lead to mild weight loss.

Weight loss varies by person and is often modest. In some people, weight does not change or may increase. Weight loss from Synthroid is not a fat-loss drug effect and is often related to water loss.

Synthroid should never be used for weight loss. Taking too much can cause serious or life-threatening side effects.

Headache

Headaches can occur as a symptom of hypothyroidism, but they can also be a side effect of Synthroid. If you develop new or worsening headaches or migraines, tell your healthcare provider. A dose adjustment may be needed.

Seek immediate medical care for severe, persistent, or worsening headaches.

Irregular periods

Thyroid problems can affect menstrual cycles. Synthroid itself may also cause irregular periods. Blood tests are needed to determine whether symptoms are related to thyroid levels or dosing.

RELATED: Signs of thyroid problems in females

Diarrhea

Diarrhea is a common and usually temporary side effect. Seek medical care if diarrhea is severe, long-lasting, or occurs with fever or dehydration.

Hair loss

Hair thinning or hair loss can occur with thyroid disorders or with excess thyroid hormone. Hair loss may improve once thyroid levels are balanced.

If hair loss starts after beginning Synthroid, tell your healthcare provider. A dose change may help.

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Serious side effects of Synthroid

Synthroid use is associated with several serious, potentially fatal side effects. It is important to seek medical care if you experience any of the serious side effects below.

• Chest pain or symptoms that feel like a heart attack
• Irregular heartbeat
• Rapid heart rate
• New or worsening heart disease
• High blood pressure
• Severe allergic reactions
• Seizures
• Increased pressure in the skull (mainly in children)
• Early bone growth plate closure in children or infants

Side effects can also be directly reported to the FDA using the MedWatch Reporting Form.

Overdose symptoms

Taking too much Synthroid can be dangerous and may be fatal. Synthroid overdose affects the heart and nervous system.

Signs of overdose include:

• Nervousness
• Insomnia
• Fever
• Elevated blood pressure
• Diarrhea

Rare but serious overdose effects include:

• Confusion or acute psychosis
• Seizures or convulsions
• Thyroid storm
• Dangerous heart rhythm changes
• Coma
• Death

Overdose may happen accidentally, through misuse for weight loss, or if children swallow the medication. If an overdose is suspected, call Poison Control at 800-222-1222 or seek emergency care immediately.

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Allergic reactions

Severe allergic reactions to Synthroid are very rare. Mild reactions, such as skin rash, are more likely due to inactive ingredients rather than the hormone itself.

Anxiety

Low thyroid hormone levels can cause anxiety. Synthroid may improve anxiety if caused by hypothyroidism. However, according to the drug label, anxiety from Synthroid is primarily due to high doses. Blood tests can help guide dose adjustments. 

How long do Synthroid side effects last?

Most mild side effects improve once the dose is adjusted. Hair loss in children has been shown to reverse when the dose is reduced.

Some effects, such as heart rhythm problems or bone loss, may take longer to develop and can be serious. Older adults and people with heart disease need careful monitoring.

Long-term dosing problems may affect:

• Bone density, especially in postmenopausal women
• Growth and development in children
  

Always tell your healthcare provider about side effects and how long they last.

Warnings

Synthroid should not be used in people with untreated adrenal insufficiency. Taking levothyroxine before glucocorticoid therapy in people with adrenal insufficiency may result in adrenal crisis. Treating the adrenal condition must happen first to avoid an adrenal crisis.

Abuse and dependence

Synthroid is not addictive. However, misuse for weight loss can be dangerous. There are no withdrawal symptoms from stopping Synthroid, but stopping it suddenly can cause hypothyroidism symptoms to return.

Restrictions

This medication comes with a black box warning. Synthroid should never be used to treat obesity or for weight loss. High doses can cause life-threatening toxicity.

Cardiac adverse reactions in older adults or in people with cardiac disease 

Overtreatment with Synthroid can increase heart rate and strain the heart. Older adults and people with heart disease should start on lower doses and increase slowly based on blood test results.

Other precautions

Synthroid is not recommended for:

• Benign thyroid nodules
• Non-toxic goiter
• Temporary thyroid inflammation that usually resolves on its own

It may also:

• Worsen blood sugar control in people with diabetes
• Reduce bone mineral density with long-term overuse

Pregnancy and breastfeeding

Pregnant women may need higher doses of Synthroid. Untreated hypothyroidism during pregnancy can harm the fetus.

Synthroid is generally considered safe during breastfeeding and may help support milk production.

Interactions

Many medications, even over-the-counter medicine and supplements, affect how Synthroid is absorbed. Always share a full medication list with your healthcare provider.

Usually, recommendations are given to take Synthroid on an empty stomach in the morning, at least 30 to 60 minutes before eating or taking other medication.

Synthroid drug interactions
Drug type	Interaction	What to do
Antacids	Reduced absorption	Adjust Synthroid dose
Proton pump inhibitors	Reduced absorption	Adjust Synthroid dose
Iron or calcium	Reduced absorption	Take 4 hours apart
Bile acid sequestrants	Increase binding	Take 4 hours apart
Hormonal contraceptives containing estrogen	May affect thyroid hormone transport proteins and alter thyroid levels	Monitor free T4 levels (not regular T4)

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Brand-name Synthroid may have fewer side effects than generic

Some patients have reported better tolerance with brand-name levothyroxine (Synthroid, Levoxyl, Unithroid, Tirosint) versus generic. Although most studies show little to no evidence of these differences between brand and generic, one study showed that Synthroid was more effective than generic levothyroxine.

For the majority of medications, there is no chemical difference between brand and generic formulas. However, questions have been raised about thyroid hormone replacements. 

The FDA requires levothyroxine products to contain 95% to 105% of the labeled dose. As you’ve read above, levothyroxine has a very narrow therapeutic range, and the dose must be carefully adjusted to keep thyroid levels balanced. A slight variation in strength could mean taking more or less than your intended dose, which can affect how well your thyroid condition is controlled. 

Brand-name levothyroxine tablets may be more consistent in potency than generic formulations. Therefore, some people notice new or returning symptoms after switching from brand-name to generic levothyroxine. In most cases, however, patients can be reassured by studies demonstrating equal efficacy between brand and generic levothyroxine and between levothyroxine tablets from different generic manufacturers.

Additionally, the FDA does not require generic drug manufacturers to use the same inactive ingredients as the brand-name version. If you are sensitive or allergic to inactive ingredients of generic levothyroxine, you may tolerate brand-name Synthroid better.

If you’re taking generic levothyroxine and experiencing side effects or difficulty controlling thyroid levels, ask your healthcare provider about switching to a brand-name alternative.

How to avoid Synthroid side effects

1. Take Synthroid exactly as prescribed
2. Take it on an empty stomach, 30-60 minutes before eating
3. Avoid grapefruit products
4. Tell your provider about all medications and supplements you take
5. Avoid taking it with antacids, or take antacids and Synthroid at least four hours apart
6. Do not stop taking it without medical advice
7. Get regular blood tests to monitor thyroid levels
8. Store the medication at room temperature and out of reach of children
9. Seek medical care for serious side effects or allergic reactions

Expert takeaway

“Synthroid and its generic version, levothyroxine, are the first-line options for treatment of hypothyroidism. Poor efficacy and side effects are most commonly related to over- or underdosing of the drug, so monitoring of TSH levels is essential to ensure ideal dose titration,” says Chad Shaffer, MD, a member of the SingleCare Medical Board