Synthroid Side Effects

What is Synthroid?

Synthroid or levothyroxine is also known as L-thyroxineis an artificial thyroid hormone, thyroxine, which is prescribed to treat hypothyroidism. It belongs to a class of medications known as synthetic thyroid hormones and was first made in 1927. [1, 2, 4, 5]

Synthroid side effects

It is highly recommended that Synthroid is taken on an empty stomach, 30 to 60 minutes before breakfast because absorption of this drug is increased on an empty stomach. Many people with hypothyroidism who frequently need lifelong thyroid hormone therapy prefer Synthroid. This medication is also available under brand names including:

  • Unithroid
  • Novothyrox
  • Levothroid
  • Tirosint
  • Levoxyl

Uses of Synthroid

Decreased thyroid hormone levels can occur naturally or when the thyroid gland is removed by surgery or injured by medications or radiation. Synthroid provides more thyroid hormones and is used for the following:[1, 2, 4, 5, 6]

  • Synthroid is used to treat hypothyroidism by providing more thyroid hormone which is usually produced by the thyroid gland.
  • It is also used to treat the severe form of thyroid hormone deficiency called myxedema coma.
  • It may also be used for other thyroid diseases including certain types of goiter and thyroid cancer to suppress secretion of thyroid-stimulating hormone (TSH).

Synthroid Dosage

synthroid dosage

An average dosage guideline for taking Synthroid include:[5, 7]

  • For adults, the starting dose is 25-50 mg daily and is increased gradually after 6-8 weeks.
  • For senior patients, the starting dose is 12.5-25mg daily which is gradually increased after 4-6 weeks.
  • For children, the starting dose is 25mg daily and is increased gradually every 2-4 weeks until the effect desired is attained.
  • Synthroid may be prescribed for infants or children who cannot swallow. The tablet can be crushed and suspended in a small amount like 5-10ml of water and administered using a dropper or a spoon.

Synthroid (Levothyroxin) Side Effects

Many medications cause side effects and they can be mild, temporary, severe or permanent. Synthroid also causes side effects and not everyone who uses it will experience the same side effects. Some people may not even experience these side effects, therefore, if you have any questions or concerns about side effects ensure to consult with your doctor. Common side effects include:[3, 5, 6, 8, 9]

  • Hot flashes
  • Sweating
  • Nausea
  • Fever
  • Temporary hair loss
  • Menstrual periods changes
  • Sensitivity to heat
  • Headache
  • Sleep problems
  • Nervousness
  • Appetite changes
  • Weight changes
  • Irritability
  • Fatigue
  • Vomiting
  • Reduced urine output
  • Difficulty in swallowing
  • Dilated neck veins
  • Eye pain
  • Crying
  • False or unusual sense of well-being

A very serious allergic reaction to this drug is quite uncommon. However, you should get medical help immediately if you notice symptoms of serious allergic reaction such asitching, rash, swelling of the face, throat, and tongue, severe dizziness and difficulties in breathing.Serious side effects include:

  • Chest pain
  • Rapid heartbeat
  • Fluttering in your chest
  • Anxiety
  • Hyperactivity
  • Emotional lability
  • Tremors
  • Muscle weakness
  • Diarrhea
  • Abdominal cramps
  • Irregular heartbeat
  • Seizures
  • Shaking or tremor
  • Shortness of breath
  • Mood swings
  • Memory problems

Overdose of Synthroid Side effects

It’s possible for a person to overdose on drugs and it could be unintentional or intentional. Most overdose cases are unintentional but in the case of an overdose ensure to immediately inform your doctor. If a person takes more than prescribed dose of Synthroid, more severe symptoms are likely to occur. Overdose synthyroid side effects include:

  • Cold, clammy skin
  • Disorientation
  • Sudden headache
  • Change in consciousness
  • Confusion
  • Fast or weak pulse
  • Sudden slurring of speech
  • Lightheadedness
  • Sudden loss of coordination
  • Muscle weakness
  • Irritability
  • Insomnia
  • High blood pressure
  • Headaches
  • Fever
  • Hair loss
  • Diarrhea
  • Chest pain
  • Concentration difficulties
  • Nervousness
  • Profuse sweating
  • Shortness of breath
  • Temperature sensitivity
  • Tremors

Withdrawal Side effects

If an individual abruptly discontinues using Synthroid, their body goes through withdrawal. Do not stop taking this drug unless your doctor advisesyou to do so. Withdrawal side effects include:

Dry skin

Synthroid withdrawal can cause dry or pale skin due to the insufficient amount of hormone which affects the moisture content of the skin.


Withdrawal from Synthroid may cause underactive thyroid which may cause fatigue, depression, sensitivity to cold and weight gain due to insufficient thyroid hormone. Acute hypothyroidism may occur in patients with thyroid cancer due to withdrawal.

Depression and anxiety

A study published in the European Journal of Endocrinology in December 2005 showed that stopping or decreasing your dosage can cause increased anxiety and depression. Many people have experienced increased anxiety to the point of causing panic attacks.

Insulin resistance

A study about the issue of Thyroid published in June 2009 showed that Synthroid withdrawal can cause a state of insulin resistance. When a test was carried out in four patients after six weeks of withdrawal from Synthroid, they were found to have a significant amount of insulin resistance which led to decreased glucose disposal.

Long Term side effects

Long term Synthroid Side Effects – The thyroid gland is responsible for producing thyroid hormone which controls the body’s metabolism. People with low levels of thyroid hormone take Synthroid which works to replace it. This causes long-term synthyroid side effects since it is used for a long-term. These side effects include:

  • Long-term use of Synthroid can lead to osteoporosis. This drug can cause the minerals in the bones to break down leading to a loss in bone density.
  • Weight loss is also associated with long-term use of Synthroid. Since the thyroid gland produces a hormone that regulates metabolism and Synthroid works to replace it, long-term use can cause some of the features of an overactive metabolism.Therefore, over time people taking this medication can lose weight and have decreased in appetite.
  • Synthroid may also cause an increase in weight in some people this is because it can cause an increase in appetite in some people; therefore, they’ll consume more food and are more likely to gain weight.
  • Individuals using Synthroid may be advised not to take certain minerals or vitamins including magnesium, calcium, and iron because they can affect absorption of Synthroid, therefore, they may suffer from deficiencies of these minerals which can cause anemia.
  • Taking Synthroid for a prolonged period of time can cause interaction with other medications. For example, if a person uses certain antidepressant drugs or lithium together with Synthroid it can lead to hypothyroidism.Antidepressants can also increase its toxicity.
  • Some substances may also cause adverse effects that may be very serious. If Synthroid is taken in combination with Ketamine it may cause tachycardia and hypertension.

Reference List

  1. Synthroid Side Effects –
  8. Synthroid Side effects Available at

Leave a Reply

Your email address will not be published. Required fields are marked *