Answer Key to Short Answer Questions for
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 hypothalamus also detects low thyroid hormone and increases the secretion of thyrotropin-releasing hormone (TRH), which then stimulates the anterior pituitary to increase secretion of TSH. High levels of thyroid hormone will have the opposite effects on the hypothalamus and anterior pituitary gland in order to lower the levels of TSH.
5. In Graves’ disease, the immune system makes antibodies that stimulate TSH receptors on the thyroid gland. What effect does this have on Krista’s thyroid?
The antibodies mimic TSH, which stimulates the thyroid gland to secrete thyroid hormone, even in the absence of TSH.
6. Why is Krista’s TSH level low instead of high?
The level of TSH is determined by negative feedback from levels of circulating thyroid hormone. Low levels of thyroid hormone are a signal to the pituitary gland to secrete more TSH in order to increase thyroid hormone. Krista’s level of thyroid hormone is high, which inhibits the secretion of TSH in an attempt to reduce thyroid hormone secretion. However, the antibodies stimulate the release of excess thyroid hormone regardless of how much TSH is present.
7. If Krista had been diagnosed with hypothyroidism, what would happen to her levels of TSH and thyroid hormone?
In hypothyroidism, the thyroid gland fails to produce sufficient amounts of thyroid hormone. The low levels of thyroid hormone produce an increase in TSH secretion, which should then stimulate the thyroid gland. However, the thyroid gland fails to respond to the TSH so TSH levels remain elevated and thyroid hormone levels are low.
8. A common treatment for Graves’ disease involves ingesting a dose of radioactive iodine, which slowly destroys the thyroid gland. Why are other tissues unaffected?
The thyroid gland readily takes up iodine in order to produce its hormones. Other glands and tissues do not take up iodine and are therefore unaffected by the radioactivity.
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