What Are the Thyroid Hormones?

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Question: What Are the Thyroid Hormones?


The Thyroid Gland

The thyroid gland is a small gland, consisting of two connected lobes which are located in the front of the throat, directly on the windpipe. It produces hormones, called thyroxine (or T4) and triiodothyronine (or T3), which are crucial for normal metabolism and everyday functioning. The thyroid gland is stimulated to produce those hormones when exposed to a different hormone called thyroid stimulating hormone (TSH).

Thyroid Stimulating Hormone and Thyrotropin Releasing Hormone

Thyroid stimulating hormone is produced by the pituitary gland, in response to exposure to thyrotropin releasing hormone (TRH) which is secreted by the hypothalamus. TRH is produced during times of stress, cold and decreased T4 levels. When released, TSH travels in the bloodstream to the thyroid gland and causes it to produce T3 and T4. High levels of T4 then cause the hypothalamus to shut down production of TRH.

Thyroid Hormones

The precursor to thyroid hormone is produced by special cells in the thyroid, called follicular cells. This precursor is called thyroglobulin. It then moves into a different part of the thyroid where it combines with iodine. If thyroglobulin attaches to three iodine molecules, the result is T3. If attached to four iodine molecules, T4 results. Most thyroid hormone is in the form of T4.

Once completed, thyroid hormone moves into the bloodstream, where it circulates either by itself, or bound to a special protein.

The unbound thyroid hormone is free to work on the body cells, where the bound thyroid hormone is stored for future use. Thyroid hormones have many important functions, including:

  • Regulation of body heat production
  • Metabolic rate regulation
  • Muscle tone and rigor
  • The breakdown of protein, fat and carbohydrates throughout the body
  • Regulates heart and respiratory rate

Understanding Your Thyroid Function Tests

If your doctor suspects thyroid disease, s/he may order blood testing to determine the cause. These may include TSH, T3 and T4 levels, and tests to look for antibodies to the thyroid gland. Hyperthryroid states, where too much thyroid hormone is produced, are generally characterized by elevated T3 and T4 levels, with a decrease in levels of circulating TSH. Hypothyroid states, where too little thyroid hormone is present, are characterized by lower levels of T3 and T4, with high levels of TSH. If there are elevated levels of anti-thyroid antibodies, it can indicate an autoimmune disorder, like Grave’s disease or Hashimoto’s thyroiditis.

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