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Understanding Medical-Surgical Nursing: Endocrine System

Endocrine System: Chapter 38 Notes

Key Terms & Concepts

  • Affect: Emotional tone.

  • Antagonist: A substance that blocks or reverses the action of another substance (e.g., PTH is an antagonist to calcitonin).

  • Exophthalmos: Abnormal protrusion of the eyeball, often associated with thyroid disease.

  • Gluconeogenesis: The process of generating glucose from non-carbohydrate substrates (e.g., breakdown of stored glycogen in the liver).

  • Negative Feedback: A regulatory mechanism in which the end product of a process inhibits the process itself, maintaining homeostasis.

  • Positive Feedback: A regulatory mechanism in which the end product of a process stimulates the process itself, amplifying the effect.

  • Endocrine: Refers to glands that secrete hormones directly into the bloodstream.

  • Exocrine: Refers to glands that secrete substances through ducts to an outer surface of the body or into the digestive tract.

  • Concepts: Cellular regulation, fluids, and electrolytes are integral to understanding endocrine function.

Overview of Endocrine Glands

  • Hypothalamus

  • Pituitary Gland

  • Pineal Gland

  • Thyroid Gland

  • Parathyroid Glands

  • Thymus Gland

  • Adrenal Glands

  • Pancreas

  • Ovaries (female)

  • Testes (male)

Hypothalamus and Pituitary Gland

Hypothalamus - Secretes Releasing and Inhibiting Hormones

  • Growth hormone-releasing hormone (GHRH) and Growth hormone-inhibiting hormone (somatostatin): Regulate growth hormone release.

  • Thyrotropin-releasing hormone (TRH): Stimulates TSH release from the pituitary.

  • Corticotropin-releasing hormone (CRH): Stimulates ACTH release from the pituitary.

Pituitary Gland - Secretes Stimulating Hormones

  • Anterior Pituitary Hormones:

    • Growth Hormone (Somatotropin): Regulates the rate of growth in the body. Hypothalamus secretes GHRH, which acts on the anterior pituitary through the hypophyseal portal system to produce growth hormone.

    • Prolactin: Involved in milk production.

    • Adrenocorticotropic Hormone (ACTH): Stimulates the adrenal cortex.

    • Thyroid-stimulating hormone (TSH): Stimulates the thyroid gland.

  • Posterior Pituitary Hormones:

    • Oxytocin: Involved in uterine contractions and milk ejection.

    • Antidiuretic hormone (ADH) (also known as Vasopressin):

      • Anti-diuresis: Prevents excessive urine production.

      • Promotes water retention.

      • Called "Vasopressin" because it causes vasoconstriction, leading to an increase in blood pressure.

Negative Feedback Loop Example (Thyroid Hormones)

  1. Homeostasis Disturbed: Decreased ( ext{T}3) and ( ext{T}4) concentration in blood or low body temperature.

  2. Hypothalamus: Releases TRH.

  3. Anterior Pituitary: TRH stimulates the anterior pituitary to release TSH.

  4. Thyroid Gland: TSH stimulates thyroid follicles to release ( ext{T}3) and ( ext{T}4).

  5. Bloodstream: Increased ( ext{T}3) and ( ext{T}4) concentration in the blood.

  6. Homeostasis Restored: Normal ( ext{T}3) and ( ext{T}4) concentrations, normal body temperature. These higher levels then inhibit the hypothalamus and anterior pituitary, completing the negative feedback loop.

Thyroid Gland

  • Produces Three Hormones:

    • Triiodothyronine (( ext{T}3)) & Thyroxine (( ext{T}4) ):

      • Increase metabolism of glucose and fatty acids, thereby increasing energy and heat production.

      • Essential for normal physical growth, mental development, and reproductive maturation.

      • Require iodine for their synthesis.

      • Stimulated by TSH for production.

    • Calcitonin:

      • Inhibits the resorption of calcium and phosphorus from bone, thus decreasing their levels in the blood.

      • Maintains a strong, stable bone matrix.

      • Release is stimulated by hypercalcemia (high blood calcium levels).

Parathyroid Glands

  • Typically four parathyroid glands.

  • Produce Parathyroid Hormone (PTH).

    • Is an antagonist to calcitonin.

    • Primarily targets bone, but also affects the small intestine and kidneys.

    • Actions:

      • Raises blood calcium levels.

      • Lowers blood phosphate levels.

    • Significance: Normal calcium levels are crucial for the excitability of neurons and muscle cells, and for the process of blood clotting.

Adrenal Glands

Adrenal Medulla

  • Secretes Catecholamines:

    • Epinephrine and Norepinephrine.

    • Mimic the sympathetic nervous system, mediating the "flight or fight response" to stress.

Adrenal Cortex

  • Secretes three classes of steroid hormones:

    • Mineralocorticoids: Aldosterone

      • Part of the Renin-Angiotensin-Aldosterone System (RAAS).

      • Promotes sodium reabsorption (and subsequent water retention).

      • Mainly secreted in response to low blood pressure.

    • Glucocorticoids: Cortisol

      • Stimulates gluconeogenesis in the liver, ensuring glucose availability to the brain.

      • Released during stress, increasing energy reserves.

      • Decreases the immune response and can delay healing.

    • Gonadocorticoids: Androgens

      • Includes a small amount of estrogen for females.

      • Includes testosterone for males.

Pancreas

  • Has both endocrine and exocrine functions.

Endocrine Functions (Islets of Langerhans)

  • Insulin (from beta cells):

    • Acts as a "gateway" for glucose to enter cells.

    • Secreted in response to hyperglycemia (high blood glucose), thus lowering blood glucose levels.

  • Glucagon (from alpha cells):

    • Secreted in response to hypoglycemia (low blood glucose, when glucose is "gone").

    • Travels to the liver to initiate gluconeogenesis and glycogenolysis, thereby increasing blood glucose levels.

Exocrine Functions (Digestive Enzymes)

  • Lipase: Digests fats.

  • Amylase: Digests carbohydrates.

  • Protease: Digests proteins.

Blood Glucose Regulation (Insulin and Glucagon)

  1. After Eating: Blood glucose levels rise as glucose enters the bloodstream from the digestive tract.

  2. High Glucose (Hyperglycemia): Stimulates pancreatic beta cells to secrete insulin.

  3. Insulin's Actions:

    • Stimulates cells to take up more glucose from the blood.

    • Causes the liver to take up glucose and store it as glycogen.

    • Result: Blood glucose levels return to normal.

  4. Low Glucose (Hypoglycemia): When blood glucose drops (e.g., after skipping a meal), pancreatic alpha cells release glucagon into the blood.

  5. Glucagon's Actions:

    • Stimulates the liver to break down stored glycogen into glucose (glycogenolysis).

    • The liver then releases this glucose into the bloodstream.

    • Result: Blood glucose levels rise back to normal.

The Aging Endocrine System

  • Decrease in Growth Hormone:

    • Contributes to a decrease in muscle mass.

    • Contributes to an increase in fat storage.

  • Decrease in TSH and Thyroid Hormone:

    • Leads to a decrease in basal metabolic rate.

    • Can result in decreased energy.

  • Decrease in Insulin Secretion:

    • Results in a decrease in glucose tolerance (though not necessarily diabetes).

Physical Assessment for Endocrine Disorders (Tables 38.2 & 38.3)

  • Height, Weight, & Vital Signs

  • Inspection:

    • Mood & Affect: Assess emotional tone.

    • Neck: Check for thyroid enlargement (goiter).

    • Eyes: Observe for exophthalmos (bulging eyes).

    • Posture, Body Fat, Tremors: Note any abnormalities.

    • Skin, Hair Texture, & Moisture: Changes can indicate hormonal imbalances.

    • Specific Observations: Moon-like face and buffalo hump can indicate Cushing's syndrome (excess cortisol).

  • Palpation:

    • The LPN assists the HCP in thyroid palpation.

    • Do not palpate the thyroid in cases of hyperthyroidism due to the risk of precipitating a thyroid storm.

Critical Thinking Scenario

  • Scenario: Ms. Hackworth is constantly tired. Her TSH level is higher than normal.

  • Question: "If my thyroid level is high, then why am I so tired?"

  • Response: Explain the negative feedback loop. A high TSH level indicates that the pituitary gland is working harder to stimulate the thyroid, suggesting the thyroid itself is not producing enough thyroid hormones (( ext{T}3) and ( ext{T}4)). Low levels of ( ext{T}3) and ( ext{T}4)) lead to decreased metabolism and energy, resulting in fatigue.

  • Medication & Dosage Calculation: Ms. Hackworth is prescribed levothyroxine (Synthroid) ( ext{50 mcg}) daily. The pharmacist supplied Synthroid ( ext{0.05 mg}). Is her dose correct?

    • Conversion: ( ext{1 mg} = ext{1000 mcg}).

    • So, ( ext{0.05 mg} = ext{0.05} imes ext{1000 mcg} = ext{50 mcg}).

    • Answer: Yes, the dose is correct. ( ext{50 mcg}) is equivalent to ( ext{0.05 mg}).

Diagnostics

  • Serum Hormone Levels: Measure specific hormone concentrations in the blood.

  • Stimulation Tests:

    • A specific substance is injected to stimulate a gland.

    • Hormone levels are then measured in the blood to assess the gland's response.

    • Example: TRH stimulation test to evaluate thyroid function.

  • Suppression Tests:

    • The opposite of stimulation tests; a substance is injected to suppress hormone production.

    • Example: Dexamethasone suppression test is used to assess adrenal gland function by suppressing cortisol release.

  • 24-hour Urine Tests:

    • Used to collect all urine over a full ( ext{24-hour}) period to measure hormone metabolites or specific substances (e.g., cortisol, catecholamines).

    • Procedure: The first sample (e.g., ( ext{7:00 ext{ a.m.}})) is discarded. All subsequent urine samples are collected and poured into the provided bottle for ( ext{24 ext{ hours}}). The last sample is collected at the end of the ( ext{24-hour}) period (e.g., next day at ( ext{7:00 ext{ a.m.}}))$).

  • Other Lab Tests:

    • Calcium levels.

    • Serum blood glucose.

Imaging and Biopsy

  • Nuclear Scanning:

    • Thyroid Scan: Radioactive material (injected or oral) is used to scan the thyroid, showing activity and potentially tumors.

    • Radioactive Iodine Uptake (RAIU): Similar to a thyroid scan, with several scans done over a ( ext{24-hour})$$ period to assess how much iodine the thyroid absorbs.

    • PET Scan (Positron Emission Tomography).

  • Radiographic Tests:

    • CT (Computed Tomography) scans.

    • MRI (Magnetic Resonance Imaging) scans.

  • Ultrasounds.

  • Biopsy: Tissue sample collection for microscopic examination.