What is the Connection Between Synthroid&Reg; and Cytomel&Reg;?

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  • Written By: H. Colledge
  • Edited By: Heather Bailey
  • Last Modified Date: 20 October 2019
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    Conjecture Corporation
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Synthroid® and Cytomel® are synthetic copies of two thyroid hormones known as T4 and T3, respectively. T4 and T3 are made by the thyroid gland which is located in the neck. In a condition known as hypothyroidism, the thyroid gland stops functioning and the body does not receive enough thyroid hormones. As thyroid hormones are involved in regulating metabolism and energy, a deficiency can cause symptoms such as weight gain, fatigue, dry skin and constipation. Thyroid medications such as Synthroid® and Cytomel® may be used to treat hypothyroidism.

The drug Synthroid® is an artificial copy of the thyroid hormone T4, also known as tetraiodothyronine. T4 is produced by the thyroid gland in response to thyroid stimulating hormone, or TSH, which is released by the pituitary gland in the brain, together with a small amount of T3, or triiodothyronine. T3 is the active thyroid hormone, and T4 is converted into T3 in body tissues where it acts on cells. Cytomel® is a synthetic form of T3. While both Synthroid® and Cytomel® may be used in the management of thyroid disease, synthetic versions of T4 are the most commonly used treatment for hypothyroidism.


Synthroid® and Cytomel® can replace the thyroid hormones which are needed to regulate the chemical processes going on inside living cells. In the condition known as hypothyroidism, a lack of sufficient thyroid hormones leads to symptoms which indicate a slowing down of the body, both mentally and physically. Since the body is normally able to convert T4 into T3, a T4 copy such as Synthroid® may be used on its own to treat hypothyroidism in many cases.

As T4 remains in the blood for a long time, creating an effect over a number of days, T3 medication such as Cytomel® is sometimes used instead when sudden changes in treatment are required. An example of this might be in preparation for radioisotope imaging studies of the thyroid gland, when it is necessary to stop medication before the test is run. T3 only remains in the body for up to 24 hours, so an advantage of Cytomel® vs Synthroid® is that doses can be adjusted quite quickly. The advantage of using Synthroid® vs Cytomel® is that the dose is likely to remain at a more constant level, as the drug remains in the body for longer. Synthroid® is also thought to carry a lower risk of side effects affecting the heart.


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