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10.11: Thyroid Hormone

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

Web Link

Video: Thyroid (0:37)

As mentioned in the video, the two primary forms of thyroid hormone are triiodothyronine (T3) and thyroxine (T4).

Figure 10.111 The structure of triiodothyronine (T3)1

Figure 10.112 The structure of thyroxine (T4)2

T4 is the primary circulating form, and is really a prohormone that is converted to the active T3 form.

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.