Anatomy and Physiology: Thyroid

Thyroid

Since I talked about the hypophysis and its secretion of TSH (thyroid-stimulating hormone), it seems only fitting that I talk about what TSH actually does (see Figure 18.4)! First of all, the thyroid gland is anterior and inferior to the thyroid cartilage, which itself is anterior to the larynx. The thyroid itself has left and right lateral lobes, connected by a thinner isthmus, which, in turn, sometimes has a small, vertical, pyramidal lobe extending upward. Posterior to the thyroid, next to the trachea, are four small glands called the parathyroid (para = next to).

On a microscopic scale you will see thyroid follicles. When inactive, the cells are squamous, but that good ol' TSH turns 'em into cuboidal and columnar cells that are ready to churn our hormones! In the middle of this follicle is a colloid (think egg white) filled with a protein called thyroglobulin, which contains many tyrosine amino acid molecules. Remember how some hormones are biogenic amines (altered amino acids)? Well, you're about to meet one.

Figure 18.4The feedback loop between the thyroid and the pituitary. (© Michael J. Vieira Lazaroff)

Next I want you to think about how salt in the grocery store is almost always “iodized.” This addition of iodine is important, because we need it for our thyroid glands to make two of its hormones: T₃ and T₄ (formerly thyroxine). A good source of dietary iodine is fish, but people in certain areas of the world rarely get fish. Without enough iodine the thyroid gland enlarges, producing a goiter in one's neck. Iodized salt was the simplest solution to this problem.

The first step in making these hormones is to bind iodine ions to tyrosine molecules. A pair of these, when joined, becomes T₂ (a reference to the pair of iodized tyrosines). Joining a pair of this, of course, makes a T₄ (also called tetraiodothyronine), or a T₂ and a single iodized tyrosine makes T₃ (also called triiodothyronine). Most of the T₃ and T₄ releases are bound to various plasma proteins, thus keeping it in reserve for release into peripheral tissues.

T₃ and T₄ are lipid soluble, so they enter the cell directly. Much of the activity involves energy usage. These hormones attach to the mitochondria, thus increasing the amount of ATP produced. These hormones also trigger (via operons) the production of enzymes for glycolysis and ATP production. The overall effect is to increase the metabolic rate, as well as the production of body heat.

Young children, due to their small size and large surface area to volume ratio, risk losing heat in cold weather, so TSH production rises, thus raising the child's body heat! Pretty cool!

Parathyroid

There is one other thyroid hormone, calcitonin (CT), which works in conjunction with parathyroid hormone (PTH). In response to high blood Ca²⁺ and phosphate (PO₄^3- and HPO₄^2-) levels, CT, which is produced by the parafollicular cells (next to the follicles, of course) of the thyroid gland, reduces blood Ca²⁺ and phosphate levels by stimulating osteoblasts to build bone (see The Bones). PTH, on the other hand, stimulates osteoclasts to break bone (once again, The Bones) down, thus releasing it into the blood and raising blood Ca²⁺ and phosphate levels. PTH, from the parathyroid, also increases reabsorption (see The Excretory System) of Ca²⁺ and phosphate in the kidneys (thus reducing Ca²⁺ and phosphate levels in the urine). The kidneys, in addition, release calcitrol, which increases the absorption of Ca²⁺ in the GI tract. As such, calcitrol, PTH, and calcitonin are all regulated by blood Ca²⁺ and phosphate levels via negative feedback loops.

Thy Must? Thymus!

When we dissect the cats in my anatomy and physiology class, I always love the day we open the thoracic cavity. There, above the heart, attached to the mediastinum (see The Heart and The Respitory System), is the unassuming thymus. I always get the students to compare the size of their thymuses (thymi?). This strange little organ grows, of course, as the animal does, reaching maximum size at puberty, and then this bizarre little organ actually gets smaller after puberty; so you can, in relative terms, gauge the age of an animal by its thymus.

You can learn a lot from this little tidbit, because the thymus is involved in immunity, as it is also an organ of the lymphatic system. The seven hormones of the thymus are called we thymosins, but they used to be considered a single hormone (thymosin). The target cells are lymphocytes, and the hormones are involved in the development and maturation of lymphocytes. When you are very young, your body is exposed to many pathogens, as well as just typical bacterial flora, for the first time. The large size is representative of the busy job the organ has at that age.

As your immune system develops, in general it has fewer new challenges as you age. In the long term, however, your immunity drops as you enter your later years, along with your shrinking thymus, making you more vulnerable to colds, which often last longer. A flu that is merely debilitating can be lethal to the elderly. Lastly, the lowered immunity makes it harder for the body to seek out and destroy errant cancer cells, thus increasing one's cancer risk.

Excerpted from The Complete Idiot's Guide to Anatomy and Physiology © 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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