Synthroid is consistently in the top three most commonly prescribed medications in the United States, with nearly 102 million prescriptions written for the drug in 2017.


Synthroid is a synthetic form of a hormone that is naturally produced in the body, and it is used to treat conditions associated with the thyroid gland, including hypothyroidism. Hypothyroidism is estimated to impact approximately 4.6 percent of people in the United States ages 12 and older.


Hypothyroidism is more likely to affect women than men, and it is more likely to affect people over the age of 60. However, anyone can be affected by hypothyroidism at any age. Medications like Synthroid help people with thyroid disorders regulate the hormone levels in their bodies, but what is Synthroid and how does it work?



What is Synthroid?


Synthroid is a brand name prescription medication that is used for the treatment of underactive thyroid, also known as hypothyroidism. Synthroid was first approved by the U.S. Food and Drug Administration (FDA) in 2002 and is also available under the generic name of its active ingredient, levothyroxine sodium.


Synthroid is a synthetic form of a thyroid hormone naturally produced in the body called T4. The medication can be used in patients of all ages, including newborns, but should be carefully monitored to avoid side effects, particularly in children and the elderly. 



What conditions are treated with Synthroid?


Synthroid is primarily used for the treatment of two conditions: hypothyroidism and TSH suppression in patients with certain thyroid conditions. The two conditions are discussed briefly below.




Hypothyroidism is a condition in which the body does not produce enough thyroid hormones on its own. Thyroid hormones are necessary for a number of bodily functions, including regulating your metabolism, assisting with digestion, and growth and repair of the body’s cells.


There are three types of hypothyroidism, including primary hypothyroidism, in which the body does not produce enough thyroid hormones; secondary hypothyroidism, in which the pituitary gland fails to direct the thyroid gland to make hormones; and tertiary hypothyroidism, in which the hypothalamus fails to direct the pituitary gland to tell the thyroid to produce more hormones. Regardless of the type of hypothyroidism a person has, the symptoms of the condition are the same, and Synthroid treats them in the same way.


Symptoms of hypothyroidism include:

  • Fatigue
  • Constipation
  • Weight gain
  • Hoarseness
  • Elevated blood cholesterol level
  • Pain, stiffness, or swelling in the joints
  • Thinning hair
  • Depression
  • Enlarged thyroid gland
  • Increased sensitivity to cold
  • Dry skin
  • Puffy face
  • Muscle weakness
  • Muscle aches, tenderness, and stiffness
  • Heavier than normal or irregular menstrual periods
  • Slowed heart rate
  • Impaired memory


TSH Suppression


Patients with certain conditions, such as specific types of thyroid cancers, may have a need to suppress production of thyroid stimulating hormone, or TSH.


Patients with thyrotropin-dependent well-differentiated thyroid cancer can have their condition worsen if levels of TSH are not suppressed.


Although early forms of thyroid cancer typically have no symptoms, patients may experience the following symptoms as thyroid cancer progresses:

  • A lump in the throat
  • Hoarseness
  • Difficulty swallowing
  • A cough
  • Pain in the throat and neck
  • Swollen lymph nodes in the neck



How does Synthroid work?


The manner in which Synthroid works depends on the condition it is treating. When treating hypothyroidism, Synthroid is used to replace the body’s T4. The two main thyroid hormones produced by the body are T3 and T4. When the body has sufficient levels of T4, it can use the hormone to make T3, which brings the hormone levels back into balance. Bringing hormone levels into a normal range helps to reduce the symptoms associated with hypothyroidism.


Synthroid works differently when used to reduce TSH levels in people who have thyrotropin-dependent well-differentiated thyroid cancer. TSH is a hormone that directs the body to make more thyroid hormones and causes the growth of cells in the thyroid gland. While this is a good thing when the thyroid only contains healthy cells, it is dangerous for patients with certain types of thyroid cancer. Low levels of thyroid hormone typically direct the body to make more TSH, but patients who take Synthroid trick their bodies into keeping TSH levels low because the body believes thyroid hormone levels are normal. Keeping the TSH level low can help reduce the growth rate of some kinds of thyroid cancer.



What side effects are associated with Synthroid?


Synthroid is associated with numerous side effects, the majority of which occur as a result of using too much of the medication. Therefore, it is extremely important that patients only take as much Synthroid as prescribed by their doctor and follow the prescribing instructions carefully.


Side effects of Synthroid that usually do not require medical attention include:

  • Abdominal or stomach cramps
  • Crying
  • False or unusual sense of well being
  • Feeling unwell or unhappy
  • Feeling of warmth
  • Feelings of suspicion and distrust
  • Headache
  • Mental depression
  • Quick to react or overreact emotionally 
  • Redness of the face, neck, arms, and occasionally upper chest
  • Trouble getting pregnant
  • Unusual tiredness or weakness
  • Weight gain or loss
  • Change in appetite
  • Diarrhea
  • Fear or nervousness
  • Feeling of discomfort
  • Feeling things are not real
  • Hair loss
  • Increased appetite
  • Muscle weakness
  • Rapidly changing moods
  • Restlessness
  • Trouble sitting still
  • Vomiting


Other side effects of Synthroid can be serious and require immediate medical attention. If you experience any of the following side effects, it’s important to seek medical attention immediately:

  • Chest pain or discomfort
  • Difficult or labored breathing
  • Dilated neck veins
  • Fainting
  • Fever
  • Hives or welts, skin itching, rash, or redness
  • Irritability
  • Nausea
  • Sweating
  • Tightness in the chest
  • Decreased urine output
  • Difficulty with swallowing
  • Extreme fatigue
  • Fast, slow, irregular, pounding, or racing heartbeat or pulse
  • Heat intolerance
  • Irregular breathing
  • Menstrual changes
  • Pain or discomfort in the arms, jaw, back or neck
  • Swelling of the eyes, face, lips, throat, or tongue
  • Tremors
  • Blurred or double vision
  • Eye pain
  • Limp or walk favoring one leg
  • Seizures
  • Dizziness
  • Lack or slowing of normal growth in children
  • Pain in the hip or knee
  • Severe headache


Although it is not common, it is possible to overdose on Synthroid if too much of the medication is taken. Synthroid overdose can be an emergency, so patients should seek immediate medical attention if they experience any of the following symptoms:

  • Change in consciousness
  • Confusion
  • Fast or weak pulse
  • Loss of consciousness
  • Sudden loss of coordination
  • Cold, clammy skin
  • Disorientation
  • Lightheadedness
  • Sudden headache
  • Sudden slurring of speech


Most of the side effects of Synthroid are caused by taking too much or too little of the medication and having the wrong amount of thyroid hormone in the body. Changing your Synthroid dose even a small amount can cause the hormone levels in the body to change dramatically, causing side effects.


Side effects caused by taking too much Synthroid tend to mirror symptoms of hyperthyroidism, including fast heart rate, heart palpitations, muscle weakness, feeling nervous or anxious, trouble sleeping, and weight loss.


Taking too little Synthroid can cause symptoms commonly associated with hypothyroidism, including tiredness, weight gain, hair loss, constipation, and muscle weakness. In order to avoid side effects, take your Synthroid dose regularly at the same time each day, and do not change your dose without your doctor’s approval. 






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