Thyroid Replacement Hormones

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What are thyroid replacement hormones and how do they work?

Thyroid replacement hormones are medications used to treat hypothyroidism, a condition in which the production of thyroid hormone in the body is abnormally low. Thyroid hormones increase cellular metabolism (activity of cells) that is responsible for growth, development of tissues, maintenance of brain function, body temperature regulation and several other cellular processes. Low levels of thyroid hormones in the body can result in many problems given the numerous activities that they mediate. The thyroid gland, a gland found in the lower neck is responsible for the production of thyroid hormones. It produces two main hormones, thyroxine (T4) and triiodothyronine (T3). The hormone responsible for most of the biological effects in the body is T3. When T4 is released into the blood by the thyroid gland, most of it is converted to T3 which is responsible for the cellular metabolic processes. Commercially available thyroid hormones are either natural or synthetic (man-made). Desiccated thyroid or thyroid extract (Armor Thyroid, Nature-Throid), a natural thyroid hormone is derived from beef or pork. Levothyroxine sodium (for example, Synthroid, Levoxyl and Levothroid), is the synthetic version of thyroxine (T4), liothyronine sodium (Cytomel, Triostat), is the synthetic version of tT3 and liotrix (Thyrolar) is a synthetic thyroid hormone containing a mixture of T4and T3.

What are some examples of thyroid replacement hormones?

The following is a list of the thyroid replacement hormones that are available in the United States:

  • thyroid extract (Armour Thyroid, Nature-Throid, NP Thyroid, Westhroid,
  • Westhroid-P [DSC], WP Thyroid)
  • levothyroxine sodium (Levothroid [DSC], Levoxyl, Synthroid, Tirosint, Unithroid, Unithroid Direct)
  • liothyronine sodium (Cytomel, Triostat)
  • liotrix (Thyrolar)

For what conditions are thyroid replacement hormones used?

Thyroid replacement hormones are used to treat hypothyroidism (low production of thyroid hormone) and myxedema, a condition that is caused by prolonged hypothyroidism. Thyroid replacement hormones prevent thyroid hormone release from cancerous thyroid nodules and are used therefore to treat thyroid cancers. They also are used to manage thyrotoxicosis, a condition in which there are high levels of thyroid hormones resulting from over-active thyroid glands and too much thyroid hormone. Thyrotoxicosis may progress to hypothyroidism or cause the growth of goiters necessitating the use of thyroid replacement hormones.

Are there any differences among the different types of thyroid replacement hormones?

There is conflicting evidence regarding which hormone replacement therapy should be preferred. The American Association of Clinical Endocrinologists recommends that clinical hypothyroidism is best treated with synthetic T4 levothyroxine (for example levothyroxine and sodium [Synthroid, Levoxyl and Levothroid]). There is variability between the absorption and distribution of generic T4 compared to brand name preparations. Hence it is recommended that patients remain with specific brand names during treatment. There is also variability between generic formulations and brand names of pure T3 (liothyronine [Cytomel, Triostat]), combined T4/T3 formulations (liotrix [Thyrolar]) and thyroid extracts from animal sources (Armour Thyroid, Nature-Throid etc.). Emerging information shows that combination of T4/T3 therapy may have some advantages over T4 in cognitive performance and mood but studies are not conclusive.

Medically Reviewed by a Doctor on 5/30/2014



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