Synthroid side effects and how to avoid them

Common Synthroid side effects include weight changes, headache, and hair loss

By Georgia C. Yalanis, MD | Sep. 18, 2021 Medically reviewed by Anne Jacobson, MD, MPH

Synthroid (levothyroxine sodium) is a thyroid-replacement hormone. You may also see this medication manufactured under the brand name Levoxyl. It is a synthetic version of the thyroid hormone called thyroxine, or T4, which naturally circulates in the body. The thyroid gland releases thyroid hormones in response to thyroid-stimulating hormone (TSH) release from the brain. Thyroid hormones act on nearly every tissue in the body and function to regulate aspects of human metabolism including proper development and differentiation of all cells in the body.

The U.S. Food and Drug Administration (FDA) has approved Synthroid for use in adults and children as young as infants for the treatment of primary, tertiary, congenital, and hypothalamic hypothyroidism (types of underactive thyroid), as well as for suppression of hormone release from the pituitary gland in individuals with thyroid cancer prior to undergoing high-stress states such as surgical procedures.

Synthroid is a prescription drug in tablet form, available in a wide range of doses (12-300 mcg) depending on usage and patient characteristics.

Synthetic hormone use, including the use of Synthroid, may result in a range of adverse effects. In this article, we will discuss some of the possible side effects, warnings, and drug interactions associated with levothyroxine use as well as how to avoid them.

Common side effects of Synthroid

As with any medication, Synthroid can cause some very common, non-serious side effects. Many side effects of Synthroid are self-limiting (they go away on their own). If any of the following adverse effects become bothersome, notify your healthcare provider.

Heart palpitations and tachycardia (increased heart rate)
Increased nervousness, anxiety, and tremor
Increased appetite
Diaphoresis (excessive sweating)
Dyspnea (shortness of breath)
Nausea, abdominal cramping or pain, and diarrhea
General feelings of malaise including headache and fever
Insomnia or trouble sleeping
Alopecia (hair loss)
Heat intolerance
Irregular menstrual periods
Skin rash
Muscle spasms
Weight loss

Weight changes

Hypothyroidism is a condition where the body does not produce enough thyroid hormones, or, the organs and tissues in the body do not respond to the thyroid hormones acting on them. Individuals with hypothyroidism may experience symptoms such as weight gain, intolerance to cold, tiredness and fatigue, GI complaints including constipation, muscle weakness, dry skin, brittle hair, depression, impaired memory, or an enlarged thyroid gland referred to as a goiter.

Once individuals with clinical hypothyroidism begin taking levothyroxine, their metabolism appears to “speed up,” but it is actually normalizing to the homeostatic baseline for that individual. As this process occurs, it is common for individuals to have an improved mood with reduced appetite and subsequent weight loss.

The amount of weight loss varies significantly between individuals, is usually moderate, and in some cases for individuals with obesity being treated for hypothyroidism, the weight loss is largely attributable to loss of water weight rather than fat. For some individuals, weight loss does not occur while taking Synthroid, and some even continue to gain weight. There are many factors that contribute to overall thyroid function, and while levothyroxine is very effective, when used alone it is not a “cure-all” for thyroid disease.

It is important to note that levothyroxine compounds, including Synthroid, should never be taken for the purpose of weight loss. Doses beyond the range of daily hormonal requirements may produce serious or even life-threatening manifestations of toxicity. If you believe you may be experiencing an underactive thyroid or have concerns about weight and weight loss methods, speak with your healthcare provider about lifestyle modifications and other medication options.

Headache

Headache is a common side effect of many medications. It is so common, the exact incidence of its occurrence is not recorded accurately in the literature. Interestingly, it is actually not uncommon for individuals, especially children, with subclinical hypothyroidism to have primary headaches including migraines. Some healthcare providers even recommend blood tests to check thyroid hormone levels in children who experience migraines.

However, it is important to notify your healthcare provider if you are taking Synthroid and begin to experience new-onset or worsening headaches or migraines. It is possible your medication could be adjusted to minimize these side effects of Synthroid or it is possible that there is something more serious going on. Seek medical care if you experience any of the following symptoms of a severe headache:

Sharp or sudden-onset headache with extremely intense pain
Headache associated with:
- Pressure or a stiff neck, nausea, and vomiting
- A fever over 101 degrees Fahrenheit
- Bleeding from the eyes, ears, nose, or mouth
- Extreme pain or redness in one eye
- Severe pain at the temples or upper cheekbones
- Chest pain or other signs of a heart attack
- Confusion, slurring speech, numbness or weakness on one side of your body, or a droopy face, eyes, or arms
If you experience syncope, dizziness, or fainting
Headache that increases with movement
Headache after a head injury or accident
Headache that progressively worsens
A “regular” headache that persists after 24 hours of treatment at home
Any severe headache in an individual with cancer or autoimmune conditions
Headache preceded by or followed by:
- Seizures
- Changes in mentation or personality

Diarrhea

Diarrhea is also a common side effect associated with Synthroid use. However, this side effect is self-limiting for most individuals receiving Synthroid therapy. If you experience any of the following signs and symptoms of severe diarrhea, you should seek medical attention right away:

Diarrhea lasting more than 48 hours
Blood or pus in the stool
Stool with a black or “tarry” appearance
Extremely foul-smelling stool
Diarrhea accompanied by:
- Severe abdominal or rectal pain
- A fever over 101 degrees Fahrenheit
- Signs of severe dehydration

Hair loss

Thyroid hormones directly alter the human hair follicle structure and function. Hair abnormalities including thin hair, brittle hair, coarse hair, and even hair loss are associated with both hypothyroidism and hyperthyroidism (overactive thyroid) in both adults and children.

Hair loss is also associated with levothyroxine overdose, especially in children. The effects that thyroid hormones exert on hair and the skin can be found here. Since hair loss may be associated with hypothyroidism itself, treating the condition with medications like Synthroid may help. But if you had not experienced hair loss prior to taking Synthroid and begin to notice it after beginning treatment, be sure to notify your healthcare provider as it is possible your dose of Synthroid may need to be adjusted.

Serious side effects of Synthroid

Synthroid use can cause several serious, potentially fatal side effects. It is important to seek medical care if you experience any of the serious side effects below. Additionally, adverse events can be directly reported to the FDA using the MedWatch Reporting Form.

Heart symptoms that may mimic heart attack including angina or chest pain
Arrhythmia (irregular heartbeat) or heart palpitations
Tachycardia or rapid heart rate
New-onset heart disease including congestive heart failure (CHF)
Hypertension or elevated blood pressure
Hypersensitivity or allergic reactions
Seizures
Pseudotumor cerebri (increased pressure in the skull) in pediatric patients
Craniosynostosis (early fusion of the bones in the skull) in infants
Premature epiphyseal closure (early fusion of the growth plates in bones)

Overdose symptoms

It is possible to overdose on this medication and for that overdose to result in death. Thyroid hormones act on most tissues in the body, including cardiac tissues, and overdose could cause life-threatening cardiac arrhythmias. The following is a list of signs and symptoms associated with over-ingestion of levothyroxine. A complete article dedicated to levothyroxine poisoning can be found here.

Common side effects of Synthroid overdose:

Nervousness
Insomnia
Fever
Elevated blood pressure
Loose stools

Rare effects:

Coma
Convulsions
Acute psychosis
Thyrotoxicosis or thyroid storm
Tachycardia
Cardiac arrhythmia
Death

It is important to remember that overdose of this medication may occur when people take too much in an attempt to lose weight or to cause self-harm. Overdose of this medication could also be accidental, or by children ingesting medication that was not intended for them. Overdose treatment is more effective the earlier it is given. If you believe you have taken too much medication or ingested this medication accidentally, contact the Poison Control Centers at 800-222-1222 or seek emergency medical attention right away.

RELATED: Synthroid dosages

Allergic reactions

There are no reports of anaphylaxis with this medication in the 50 years it has been available. There are some case reports that document vocal cord dysfunction (VCD) mimicking an anaphylactic drug reaction. Any other serious allergic responses to this medication are extremely rare as this is a synthetic analog of a hormone that is naturally occurring in the body. There are occasional case reports of hypersensitivity reactions to this medication which include skin rashes. It is more likely these responses are to other ingredients in the medication and not the hormone analog itself. Therefore, it is possible that anyone experiencing an allergic reaction while taking Synthroid, may not have the same effect when taking another levothyroxine preparation.

How long do Synthroid side effects last?

Most mild side effects associated with Synthroid use will eventually go away or become intermittent with adjustments to Synthroid doses. Hair loss has been reported to be reversed with discontinuation of the medication in children, but this is not always the case in adults. Most other mild and severe symptoms described in this article respond to medication dose adjustments. Some severe side effects such as arrhythmia could result in death. It is very important to communicate with your healthcare provider about any side effects you experience and their duration while taking Synthroid.

Synthroid contraindications and warnings

Levothyroxine is contraindicated for use in individuals with uncorrected adrenal insufficiency. Thyroid hormone increases metabolic clearance of glucocorticoids. Beginning therapy with levothyroxine prior to initiating glucocorticoid therapy in individuals with adrenal insufficiency may precipitate an adrenal crisis. In any individual with adrenal insufficiency requiring thyroid hormone replacement, glucocorticoids must be initiated first. Levothyroxine is also contraindicated in people with acute myocardial infarction and those whose TSH is suppressed naturally or by medication.

Abuse and dependence

Synthroid is not habit-forming or addictive. There is potential for abuse in individuals who misuse the mediation for weight loss purposes. There are no withdrawal symptoms with discontinuing use, however, individuals should not abruptly stop taking this medication without speaking with their healthcare provider.

Restrictions

This medication comes with a black box warning. This medication is not for the treatment of obesity or weight loss and should not ever be used for weight loss. Doses beyond the therapeutic range for an individual’s daily hormone requirements may result in life-threatening toxicity.

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

Overtreatment with Synthroid can cause an increase in heart rate, cardiac wall thickness, and cardiac contractility which may precipitate angina or arrhythmia. Use lower doses of Synthroid in this population.

Benign thyroid nodules or non-toxic diffuse goiter in iodine-sufficient individuals

Do not use levothyroxine in individuals with benign thyroid nodules or with non-toxic diffuse goiter as there is no clinical benefit for this group and administration of Synthroid in these individuals may result in overtreatment and induce hyperthyroidism.

Prevention of hypothyroidism or incomplete treatment of hypothyroidism

Synthroid is not indicated for the treatment of hypothyroidism during the recovery phase or for subacute thyroiditis.

Worsening diabetic control

Therapy in individuals with diabetes may worsen diabetic control and result in the need for increased insulin or other antidiabetic medications.

Decreased bone mineral density

Overreplacement of thyroid hormone can increase bone resorption and decrease bone mineral density. Use caution in individuals with osteoporosis.

Pregnancy

Women who are pregnant may require higher doses of Synthroid. Maternal hypothyroidism is associated with significant risk to the fetus with higher rates of complications including spontaneous abortion, stillbirth, and premature delivery. Speak with your healthcare provider about a dose adjustment if you are taking Synthroid and become pregnant.

Breastfeeding

There is insufficient information available to determine the effects of Synthroid on breastfed infants. Inadequate thyroid hormone levels may affect milk production in lactating mothers. The developmental and health benefits associated with breastfeeding should be considered along with any potential adverse effects to the infant when considering using this medication in breastfeeding mothers.

Synthroid interactions

There are several prescription and over-the-counter drugs (such as antacids) that may interact with thyroid hormone transport, serum levels, metabolism, and conversion. It is important to disclose a full medication history to your healthcare provider prior to beginning this medication to avoid any potential drug interactions.

How to avoid Synthroid side effects

Follow medical advice while taking this medication. Take only the dose prescribed by a healthcare professional—no more or less. If there is a missed dose, take the next dose as soon as you remember. Continue taking the medication as prescribed even if you feel well after the first few doses.
Do not discontinue this use of this medication without seeking medical advice. You should not discontinue synthetic hormone use unless you with your healthcare provider have decided that is the right decision.
Disclose a full medication list, all medical conditions, and any allergies before taking Synthroid. This includes all medications and supplements, any medical conditions, or any allergic reactions to medications in the past.
Store the medication correctly and out of the reach of children. Synthroid should be stored at room temperature (68 to 77 degrees Fahrenheit), away from children’s medications, and out of the reach of children, as most fatal overdoses occur in pediatric populations.
Swallow the tablet whole. Do not crush, break or chew the tablet.
Avoid taking this medication with antacids. Antacids and other magnesium salts reduce the amount of thyroxine in the systemic circulation. If you must take antacids, take them at least two hours before or two hours after Synthroid administration.
Avoid taking this medication with grapefruit products. Grapefruit and grapefruit juice can interfere with the metabolism of the medication and the amount that is circulating in the bloodstream.
Avoid taking this medication with food. Food affects the absorption of this medication. Take Synthroid in the morning, on an empty stomach, at least two hours prior to the first meal of the day.
Talk with a healthcare provider about contraceptive methods. Contraceptives containing estrogen may interfere with the transport of thyroid hormone.
Avoid anabolic steroid use or abuse. Steroid compounds affect the transport of thyroid hormones.
Patients should immediately seek medical care if they experience any type of allergic reaction, serious side effects, unexplained muscle pain. Allergic responses should be reported and treated as early as possible. Any type of serious adverse effect or potential overdose should receive medical attention immediately.