slide 1

Continuing our investigation of the endocrine glands, we'll look at the thyroid gland. In mammals, the thyroid is composed of two lobes on the ventral, or front side of the trachea. The thyroid produces two structurally similar hormones:

triiodothyronine, or T3, and thyroxine, or T4. T3 has three iodine atoms and T4 has four. These hormones are derived from the amino acid tyrosine. T3 and T4 both have the same effect on their target cells.

slide 2

T3 and T4 production by the thyroid is controlled by the hypothalamus. The hypothalamus produces a releasing hormone that causes the production and secretion of thyroid-stimulating hormone, or TSH, by the anterior pituitary. TSH binds to receptors in the thyroid, triggering the synthesis and secretion of T3 and T4. The production of the releasing hormone in the hypothalamus is regulated by the levels of T3, T4, and TSH in the blood. Low levels of these hormones trigger the production of the releasing hormone, and high levels inhibit production.

slide 3

The proper regulation of T3 and T4 levels in our blood is important for good health. These hormones are involved in regulating such essential functions as blood pressure, heart rate, digestion, and reproduction. The hormones increase the rate of oxygen consumption and cellular metabolism. Too much of the thyroid hormones leads to a condition called hyperthyroidism, which is characterized by high body temperature, sweating, high blood pressure, and weight loss. Too little of the thyroid hormones causes hypothyroidism, and its symptoms include weight gain, fatigue, and intolerance to cold.

slide 4

The thyroid gland also helps regulate the level of calcium in our blood. Calcium homeostasis is also regulated by the four parathyroid glands, embedded in the surface of the thyroid gland. The regulation of blood calcium levels is a good example of how two antagonistic hormones work together to maintain homeostasis.

When the blood calcium level is too low, the parathyroid glands secrete parathyroid hormone, or PTH. This raises the blood calcium level by stimulating the bones to release calcium, and by increasing calcium uptake by the kidneys. When the blood calcium level is too high, the thyroid gland secretes the hormone calcitonin. This lowers the blood calcium level by stimulating calcium to be deposited in bones and by reducing calcium uptake in the kidneys.

slide 5

The regulation of blood calcium levels shows how two hormones work antagonistically to maintain homeostasis. In a similar way, blood glucose levels are regulated by two hormones produced by another endocrine gland, the pancreas.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education