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10.1A: Thyroid Hormone

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  • The thyroid accumulates most absorbed iodine, keeping it for use to synthesize thyroid hormone. The following video shows the thyroid and describes its function.

    Video 10.1A.1: The thyroid gland, situated at the front of the neck and overlying the trachea, consists of two lateral lobes and a connecting isthmus. Thyroid tissues trap iodine circulating in the blood and use it to produce the hormones thyroxine and triiodothyronine. When secreted into the bloodstream, these hormones increase the metabolic rate of all body tissues. The thyroid gland also produces the hormone calcitonin, which helps regulate the calcium content of bones.

    As mentioned in the video, the two primary forms of thyroid hormone are triiodothyronine (T3) and thyroxine (T4). T4 is the primary circulating form, and is really a prohormone that is converted to the active T3 form.

    Figure 10.111 The structure of triiodothyronine (T3)1

    Figure 10.112 The structure of thyroxine (T4)2

    The enzymes that metabolize thyroid hormones are known as deiodinases. There are three deiodinases (Type I, Type II, Type III) that are selenoenzymes whose location and function are summarized in the table below.

    Table 10.11 Location and function of the three deiodinases3




    Deiodinase Type I (DI1)

    Liver, kidney, thyroid gland

    Plasma T3 production

    Deiodinase Type II (DI2)

    Brain, pituitary, brown adipose

    Local T3 production

    Deiodinase Type III (DI3)

    Brain, placenta

    T3 degradation

    Thyroid hormone regulates the basal metabolic rate and is important for growth and development. Thyroid hormone is particularly important for brain development, but hypothyroidism (low thyroid hormone) also leads to decreased muscle mass and skeletal development3.

    References & Links

    3. Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.
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