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.