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Ch. 45 - Hormones and the Endocrine System
Chapter 45, Problem 6

Which of the following is the most likely explanation for hypothyroidism in a patient whose iodine level is normal? a. greater production of T3 than of T4 b. hyposecretion of TSH c. hypersecretion of MSH d. a decrease in the thyroid secretion of calcitonin

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Thyroid Hormones

Thyroid hormones, primarily thyroxine (T4) and triiodothyronine (T3), are crucial for regulating metabolism, growth, and development. T4 is converted to the more active T3 in tissues, and both hormones are produced by the thyroid gland. An imbalance in their production can lead to conditions such as hypothyroidism, characterized by insufficient hormone levels.
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Thyroid-Stimulating Hormone (TSH)

TSH is produced by the pituitary gland and stimulates the thyroid gland to produce T3 and T4. A hyposecretion of TSH can lead to decreased production of thyroid hormones, resulting in hypothyroidism. Understanding the feedback loop between the hypothalamus, pituitary gland, and thyroid is essential for diagnosing thyroid disorders.
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Iodine and Thyroid Function

Iodine is a critical element required for the synthesis of thyroid hormones. A normal iodine level typically indicates that the thyroid gland has the necessary resources to produce T3 and T4. However, hypothyroidism can still occur due to other factors, such as autoimmune disorders or pituitary dysfunction, even when iodine levels are adequate.
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Related Practice
Textbook Question

Growth factors are local regulators that a. are produced by the anterior pituitary. b. are modified fatty acids that stimulate bone and cartilage growth. c. are found on the surface of cancer cells and stimulate abnormal cell division. d. bind to cell-surface receptors and stimulate growth and development of target cells.

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Textbook Question

Which hormone is correctly paired with its action? (A) oxytocin—stimulates uterine contractions during childbirth (B) thyroxine—inhibits metabolic processes (C) ACTH—inhibits the release of glucocorticoids by the adrenal cortex (D) melatonin—raises blood calcium level

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Textbook Question

What do steroid and peptide hormones typically have in common? a. their solubility in cell membranes b. their requirement for travel through the bloodstream c. the location of their receptors d. their reliance on signal transduction in the cell

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Textbook Question

The relationship between the insect hormones ecdysteroid and PTTH is an example of a. an interaction of the endocrine and nervous systems. b. homeostasis achieved by positive feedback. c. homeostasis maintained by antagonistic hormones. d. competitive inhibition of a hormone receptor.

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Textbook Question

DRAW IT In mammals, the production of the hormone IGF-1 by the liver is controlled by growth hormone (GH) and GH-releasing hormone. Draw a simple sketch of this pathway, including glands, tissues, hormones, routes for hormone movement, and effects.

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