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Canadian Med Center

What is Synthroid Used For?

Thursday 13 June 2024
5 minute(s) read

Table of Contents

I. What is Synthroid?

II. How Synthroid Works

III. Synthroid Dosage

IV. Synthroid Side Effects

V. Synthroid Generic

VI. Levothyroxine Weight Loss

VII. Conclusion

If you've been experiencing fatigue, weight gain, and dry skin, your doctor may test your thyroid levels to check for hypothyroidism. Hypothyroidism means your thyroid gland is underactive and not producing enough important hormones. [1] The good news is that hypothyroidism is treatable.

One of the most prescribed medications for hypothyroidism is Synthroid. This drug helps supplement the hormones your thyroid isn't making enough of. In this article, we'll explore how Synthroid works, proper dosage, potential side effects, and whether it may aid weight loss. 

What is Synthroid?

Synthroid is an oral medication that was approved by the FDA in 2002 [2] to treat:

  • Hypothyroidism: Synthroid is used to treat several different types of hypothyroidism, including thyroidal, pituitary, and hypothalamic.
  • Thyrotropin-dependent thyroid cancer: Synthroid may be used in conjunction with surgery and radioiodine therapy to help suppress pituitary thyrotropin in patients with thyrotropin-dependent thyroid cancer. [2]

The thyroid gland produces and releases two essential hormones

  • Thyroxine
  • Liothyronine [3]

Thyroid hormones are responsible for maintaining a normal rate of metabolism in the body. However, when the thyroid gland is unable to produce adequate amounts of thyroxine, the levels of thyroid hormones in the blood decrease, leading to hypothyroidism. [3] This disorder can:

  • Lead to a decreased metabolism
  • Interfere with the proper functioning of many organs [3]

Synthroid restores thyroid hormone levels in the body by treating low thyroxine. Its active ingredient, levothyroxine sodium, is a man-made hormone that works similarly to thyroxine (T4). By replenishing the T4 typically produced by the thyroid gland, Synthroid helps patients regain normal thyroid levels. It is important to note that thyroid replacement treatment is typically a lifelong commitment. [2]

How Synthroid Works

anatomical illustration of the thyroid gland

Your thyroid gland produces thyroid hormones that regulate your metabolism and energy levels. When you have hypothyroidism, your thyroid is underactive and doesn’t produce enough of these critical hormones. As a result, your body’s functions slow down. [4]

Synthroid contains synthetic levothyroxine, which is identical to the thyroxine (T4) hormone produced by a normally functioning thyroid. Once you take Synthroid, your body converts the T4 into the active hormone triiodothyronine (T3) as needed. The benefit of Synthroid is that it allows your body to determine how much T3 you require based on your needs. [2]

Synthroid Dosage

Synthroid (levothyroxine) comes in 12 different dosing options to help your doctor determine the optimal dose for your needs. [2] Your doctor will consider several factors when prescribing your starting Synthroid dose, including your:

  • Age
  • Body weight
  • Cardiovascular health
  • Other medications you may be taking [2]

The available Synthroid doses are:

  • 25 mcg
  • 50 mcg
  • 75 mcg
  • 88 mcg
  • 100 mcg
  • 112 mcg
  • 125 mcg
  • 137 mcg
  • 150 mcg
  • 175 mcg
  • 200 mcg
  • 300 mcg [2]

When beginning Synthroid, your doctor will monitor your thyroid stimulating hormone (TSH) through blood tests every six to eight weeks. They may need to make small dose adjustments during this time to find the precise dose that's right for you. It can take several weeks before you notice the effects of Synthroid, as it takes some time for your thyroid hormone levels to reach the optimal range. [4]

Synthroid Side Effects

Synthroid side effects are uncommon and usually only occur if the medication is taken in excess amounts. [2]

These side effects can include:

  • Menstrual irregularities
  • Chest pain
  • Irregular or fast heartbeat
  • Sweating
  • Nervousness
  • Depression
  • Temporary hair loss
  • Decrease of bone mineral density
  • Diarrhea
  • Vomiting
  • Weight loss or weight gain
  • Seizures
  • Muscle weakness [2]

If you take a dose of Synthroid and feel unwell soon after, be sure to contact your doctor. While side effects are rare, it's important to monitor how you feel and report anything out of the ordinary.

Synthroid Generic

Levothyroxine is the generic version of Synthroid. Both generic levothyroxine and brand-name Synthroid contain the same active ingredient: levothyroxine. [5]

The brand name is simply given by the pharmaceutical company that originally researched and patented the medication. However, the inactive ingredients that make up the actual pill can be different between the generic and brand-name versions. Despite this, the FDA carefully regulates both types of drugs to ensure that the active ingredient is of the proper quality, strength, and purity. [5]

Both generic levothyroxine and brand-name Synthroid should work equally well to treat hypothyroidism. One notable study evaluated 17, 598 patients taking either brand-name or generic levothyroxine over nine years. They found the percentage of patients who reached normal thyroid hormone levels within three months was nearly identical in both groups. [6]

However, it's important to communicate with your doctor if you feel that you are not responding well to a generic medication. In some cases, doctors may suggest switching to a brand-name drug. This is because medication absorption can vary due to different inactive ingredients, which may affect how it works in your body. [5]

Levothyroxine Weight Loss

woman measuring waist to monitor weight loss

Hypothyroidism is often associated with unexplained weight gain. However, effective treatment with thyroid hormone replacement doesn't usually lead to significant weight loss for most people. [7]

The weight gain seen with hypothyroidism is largely due to fluid retention. With treatment, a small amount of weight loss (typically less than 10% of body weight) can occur as the excess fluid is eliminated. However, since hypothyroidism tends to develop slowly over time, it's common not to see much weight loss after treatment, even when thyroid levels return to normal. [7]

If all hypothyroid symptoms except weight gain resolve with proper treatment, it's unlikely the extra weight is solely due to the thyroid. Once thyroid levels normalize, the ability to gain or lose weight is generally the same as someone without thyroid problems. [7]

One study by Boston University Medical Center followed hypothyroid patients taking thyroid hormone replacement over eight years. They found that 52% of patients lost some weight in the first two years of treatment. However, the average weight loss was modest at around eight pounds. [8]


Synthroid can help restore normal thyroid function and provide relief from the symptoms of hypothyroidism. If you suspect that you may have an underactive thyroid, it is important to speak to your doctor about getting your thyroid levels checked. If hypothyroidism is diagnosed, your doctor may prescribe Synthroid to help manage the condition.

However, Synthroid should not be viewed as a weight loss drug. While correcting the hormone imbalance may help some people lose excess weight gained from hypothyroidism, it does not cause weight loss directly. The medication simply returns your metabolism to normal functioning.

The information provided is for informational purposes only. We do not offer medical advice. Please consult with your physician or qualified healthcare professional for guidance on drugs, medical conditions, or treatments. Our content is not a replacement for professional medical recommendations.