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NURS 208: Endocrine System Overview

Objectives

• Describe the control of endocrine secretion by the hypothalamic-pituitary axis.

• Describe the cellular responses elicited by some hormones.

• Describe the location, histology, hormones, and functions of the thyroid gland and parathyroid glands.

Hypothalamic-Pituitary Axis

• Components:

  • Hypothalamus: Connects nervous and endocrine systems; located below the thalamus, above the brainstem.

  • Pituitary Gland: Pea-shaped, divided into anterior (adenohypophysis) and posterior (neurohypophysis).

• Function:

  • The hypothalamus releases hormones that regulate the pituitary, which then stimulates target organs.

• Anatomy:

  • Includes hypophyseal portal system: Capillary plexus that links hypothalamus and anterior pituitary.

  • Infundibulum: Connects the pituitary gland to the hypothalamus.

Hormones of the Anterior Pituitary

1. Growth Hormone (GH): Stimulates growth and regulates metabolism.

2. Thyroid-Stimulating Hormone (TSH): Stimulates thyroid gland.

3. Adrenocorticotropic Hormone (ACTH): Stimulates adrenal cortex for glucocorticoid secretion.

4. Prolactin (PRL): Initiates milk production in mammary glands.

5. Follicle-Stimulating Hormone (FSH): Stimulates the production of eggs and sperm.

6. Luteinizing Hormone (LH): Triggers ovulation and testosterone production.

Hormones of the Posterior Pituitary

• Oxytocin: Triggers uterine contractions and milk ejection.

• Antidiuretic Hormone (ADH): Regulates water balance, increases reabsorption in kidneys.

Control of Endocrine Secretion

• Hypothalamic Control: Pituitary hormone secretion regulated by hypothalamic releasing and inhibiting hormones.

• Types of Hormones:

  • Releasing Hormones:

  • Growth hormone-releasing hormone (GHRH)

  • Thyrotropin-releasing hormone (TRH)

  • Corticotropin-releasing hormone (CRH)

  • Prolactin-releasing hormone (PRH)

  • Gonadotropin-releasing hormone (GnRH)

  • Inhibiting Hormones:

  • Growth hormone-inhibiting hormone (GHIH)

  • Prolactin-inhibiting hormone (PIH, dopamine)

Types of Anterior Pituitary Cells

1. Somatotrophs: Produce GH; regulates body growth.

2. Thyrotrophs: Produce TSH; controls thyroid gland.

3. Gonadotrophs: Produce FSH and LH; influence reproductive functions.

4. Lactotrophs: Produce PRL; initiates milk production.

5. Corticotrophs: Produce ACTH; stimulates adrenal glands.

Thyroid Gland

• Structure: Butterfly-shaped; located below the larynx, has right and left lobes connected by an isthmus.

• Hormones Produced:

  • Thyroxine (T4): Contains four iodine atoms, more abundant but less active.

  • Triiodothyronine (T3): Contains three iodine atoms, more active than T4.

  • Calcitonin (CT): Produced by parafollicular cells; significant for calcium metabolism.

Functions of Thyroid Hormones

• Thyroxine (T4) and Triiodothyronine (T3):

  • Increase metabolic rate, ATP production, and overall energy consumption.

  • Cause calorigenic effects (heat production) via increased BMR.

  • Enhance responses to catecholamines (e.g., adrenaline), increasing heart rate and blood pressure.

  • Critical for nervous system development and growth in children.

• Calcitonin: Decreases blood calcium levels by inhibiting osteoclast activity, thus promoting bone formation.

Control of Thyroid Hormones

• Feedback Mechanism: Low levels of T3 and T4 stimulate TRH release from hypothalamus → TRH prompts TSH release from anterior pituitary → TSH stimulates thyroid to secrete T3 and T4.

• Negative Feedback: High levels of T3 inhibit further TRH and TSH secretion.

Sample Questions (with Correct Answers)

1. Which structure is responsible for releasing tropic hormones affecting the hypophysis? Answer: a) Hypothalamus

2. What's the major function of gonadotropins? Answer: c) Reproduction

3. Major function of human growth hormone? Answer: d) Growth and metabolism

4. Function not associated with thyroid hormones? Answer: c) Destruction of catecholamines

5. Calcitonin's importance? Answer: a) Calcium homeostasis