What is synthroid?

Synthroid is a prescription only man-made synthetic hormone similar to the one that is naturally produced by the thyroid, called thyroxine. Thyroxine (T4) plays an essential role in the body by regulating many of its processes such as digestion, temperature, and metabolism. Low thyroid hormone production can occur naturally for some, but this can also be a result of injury to the thyroid gland after radiation/medication or surgical removal of the gland altogether.

Synthroid is used to treat hypothyroidism (underactive thyroid) and other types of thyroid disorders in adults and children, including infants. It is important to have enough thyroid hormone in order to support normal physical and mental activity, and even more important for children since they need it to achieve normal physical and mental development.

Brand names and generics:

Synthroid is one of several brand names for a medication containing the active ingredient levothyroxine, a synthetic thyroid hormone. Other brand names for levothyroxine include Tirosint, Levothroid, Unithroid, Levo-T, and Levoxyl, and the generic name for Synthroid is levothyroxine sodium. While these medications all share the same active ingredients, their inactive ingredients and preparations may vary. For example, while Synthroid is available as a capsule, Tirosint is available as a gel capsule and liquid solution as well.


Synthroid comes as a tablet that you swallow in the following dosages: 25 micrograms (mcg), 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, and 300 mcg.

The recommended dosage for Synthroid depends on the cause of the hypothyroidism and how long the hypothyroidism has been diagnosed. A healthcare professional will assist in guiding you to the correct dosage of this medication.


Side effects:

Synthroid has some common side effects which include:

  • Shortness of breath
  • Nervousness/irritability
  • Rapid or irregular heartbeat
  • Hair loss
  • Excessive sweating
  • Change in appetite/weight loss
  • Vomiting/diarrhea
  • Change in menstrual periods
  • Impaired fertility
  • Muscle weakness/spasms
  • Decrease bone density
  • Intolerance to heat

More serious side effects may occur also:

  • Fast or irregular heartbeat
  • High blood pressure
  • Heart failure
  • Chest pain
  • Heart attack
  • Cardiac arrest

Synthroid Cost:

Like many other medications, the cost of Synthroid will vary based on insurance coverage and other factors. Synthroid is available in a generic format as levothyroxine sodium which could be a cheaper alternative for some patients. The FDA assures consumers that the generic medications it approves are just as safe and effective as the brand names, but in this case the small discrepancies that the FDA allows can sometimes have significant effects. Check out eNavvi to see what synthroid might cost you.

Brand vs Generic:

The FDA has stated that generic and brand name drugs should be able to be used interchangeably without negative effects because of the strict rules it enforces in maintaining the quality, strength, and purity of the active ingredient in both generic and brand name drugs. A 2020 study ( showed that generic and brand name levothyroxine achieved similar success rates in patients.

However, healthcare professionals have pointed out that the variations in the inactive ingredients in these drug preparations can have a significant effect on how certain people absorb and metabolize the medication. It is also important to note that even though the FDA requires levothyroxine by law to fall within 5% of its stated potency, it can range anywhere from 95% to 105% between manufacturers. These small discrepancies can lead to major differences over time, which is important to consider when dealing with a condition like hypothyroidism that often requires long term treatment.

Let’s use a 100 mcg tab of levothyroxine as an example, with drugmaker A at 96% potency and drugmaker B at 105%. If a patient has been taking effectively 96 mcg of levothyroxine daily from drugmaker A and then starts taking 105 mcg daily after picking up a new prescription from drugmaker B, after one week that is now a difference of + 65 mcg, which is almost like taking an extra pill each week. Conversely, if the patient was to make the switch from B to A, this would be like taking half a pill less each week, which could also have a negative effect.

Because of these discrepancies, the American Thyroid Association recommends sticking to the brand, dosage, and manufacturer that works best for you individually. Hypothyroidism is a condition that can require lifelong therapy and consistent, precise treatment is the best way to maximize success.

Make sure to ask your doctor to write their specific preference with “DAW” for “dispense as written” or “no generic substitution” on your prescription to prevent unexpected changes and ensure that your medication is filled correctly and consistently by the pharmacist.


The information included in this page is not a substitute for medical advice. The list of side effects may not include all side effects as drugs can affect all people differently. Any dosage information may not include all dosage information. It is important to discuss with a medical professional that knows your medical history well which dosage is right for you and if there are any potential risks or possible side effects. Our goal is to provide relevant information but this page may not include all up-to-date information.

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