Chapter 18 Case Study

Answer Key to Short Answer Questions for

“My Brother Calls Me ‘Bug Eyes’: A Case Study on the Endocrine System”

1. Name the two hormones commonly referred to as “thyroid hormone” and describe their general actions.

Triiodothyronine (T3) and thyroxine (T4) are secreted by the follicular cells of the thyroid and are collectively called “thyroid hormone.” Most thyroid hormone is secreted as T4 and converted in target cells to the more potent T3 form by removing one iodine atom. T3 binds to receptors in the nucleus to alter gene transcription, which has a variety of actions depending on the target cell. The primary action of thyroid hormone is to increase the body’s metabolic rate, which in turn increases body temperature. This requires more oxygen so the heart rate and contractility also increase. Thyroid hormone is also required for normal development and growth of bone, muscle, connective tissue and the nervous system.

2. Why would an overactive thyroid cause Krista’s weight loss, sweating, and elevated heart rate?

Excessive levels of thyroid hormone will increase Krista’s metabolic rate above normal, which will cause weight loss if her caloric intake doesn’t also increase. Her body temperature will also be elevated, causing her to sweat more profusely. The greater demand for oxygen to support the increased metabolism will result in an elevated heart rate.

3. Krista wonders if TSH is a “thyroid hormone,” and Dr. Weisman explains that it is not. State the action of thyroid-stimulating hormone (TSH) and name the gland that secretes it.

As the name implies, TSH stimulates the thyroid gland to secrete its hormones, but it is not produced by the thyroid. It is secreted by the anterior pituitary gland.

4. How is the secretion of TSH regulated under normal conditions?

The levels of TSH are regulated by negative feedback. The anterior pituitary responds to low thyroid hormone levels by increasing TSH secretion. The...