Thyroid, Pancreas, and Other Endocrine Glands

Overview of Endocrine Glands

The endocrine system consists of glands that secrete hormones directly into the bloodstream to regulate various physiological processes. This section focuses on the thyroid gland, pancreas, parathyroid gland, pineal gland, thymus, gonads, adipose tissue, heart, and kidney.

• Thyroid gland: Secretes thyroid hormones and calcitonin.

• Pancreas: Produces insulin and glucagon for blood glucose regulation.

• Other glands: Parathyroid, pineal, thymus, gonads, adipose tissue, heart, and kidney also have endocrine functions.

Thyroid Gland

Location and Structure

The thyroid gland is located superficial to the larynx in the anterior neck. It consists of two lobes connected by an isthmus and is highly vascularized.

• Gross anatomy: Right and left lobes, isthmus, and proximity to the larynx and trachea.

• Histology: Composed of follicular cells and parafollicular cells.

Hormones Secreted by the Thyroid Gland

• Thyroid hormones: Triiodothyronine (T3) and Thyroxine (T4) regulate growth and metabolism.

• Calcitonin: Regulates calcium (Ca2+) metabolism.

Functions of Thyroid Hormones

Thyroid hormones are essential for normal development and metabolic regulation.

• Metabolic rate: Increase basal metabolic rate and thermoregulation.

• Growth and development: Promote growth, especially of the nervous and skeletal systems.

• Synergism: Enhance effects of the sympathetic nervous system.

Thyroid Hormone Synthesis, Storage, and Transport

• Synthesis: Occurs in follicular cells; involves iodination of thyroglobulin in the colloid.

• Storage: Iodinated thyroglobulin stored in colloid until needed.

• Transport: T3 and T4 released into blood, mostly bound to plasma proteins; only free hormones are biologically active.

Equation:

Regulation of Thyroid Hormone Production

Thyroid hormone production is controlled by a multi-tiered negative feedback loop involving the hypothalamus and pituitary gland.

• TRH (Thyrotropin-releasing hormone) from hypothalamus stimulates TSH (Thyroid-stimulating hormone) release from anterior pituitary.

• TSH stimulates thyroid hormone synthesis and release.

• Negative feedback: High levels of T3 and T4 inhibit TRH and TSH secretion.

Thyroid Disorders

• Hyperthyroidism: Excess thyroid hormone (e.g., Grave's disease). Symptoms: weight loss, heat intolerance, increased blood pressure, goiter.

• Hypothyroidism: Deficient thyroid hormone (e.g., Hashimoto's thyroiditis, iodine deficiency). Symptoms: weight gain, cold intolerance, bradycardia, goiter.

Calcitonin

Calcitonin is secreted by parafollicular cells in response to high blood calcium levels (hypercalcemia).

• Action: Inhibits osteoclast activity, promoting calcium deposition in bone.

• Result: Lowers blood calcium concentration.

Parathyroid Glands

Structure and Function

Four small glands located posterior to the thyroid gland. Chief cells secrete parathyroid hormone (PTH) in response to low blood calcium (hypocalcemia).

• PTH actions:

  • Stimulates osteoclasts to release Ca2+ from bone.

  • Promotes calcitriol synthesis for increased intestinal Ca2+ absorption.

  • Increases renal tubular reabsorption of Ca2+.

Pancreas

Structure and Cell Types

The pancreas is a tub-shaped organ located posterior to the stomach. It has both endocrine and exocrine functions.

• Islets of Langerhans (endocrine):

  • Alpha (α) cells: secrete glucagon

  • Beta (β) cells: secrete insulin

  • Delta (δ) cells: secrete somatostatin

• Acinar cells (exocrine): secrete digestive enzymes

Hormones of the Pancreas

• Glucagon: Raises blood glucose by stimulating glycogenolysis, gluconeogenesis, and lipolysis.

• Insulin: Lowers blood glucose by promoting uptake and storage of nutrients.

• Somatostatin: Inhibits secretion of both insulin and glucagon.

Blood Glucose Regulation

Insulin and glucagon act as antagonists in a negative feedback loop to maintain blood glucose homeostasis.

• Hyperglycemia: High blood glucose (>100 mg/dL), often due to diabetes mellitus.

• Diabetes Mellitus Type 1: Autoimmune destruction of β cells; insulin-dependent.

• Diabetes Mellitus Type 2: Insulin resistance; non-insulin-dependent.

Other Hormone-Secreting Tissues

Pineal Gland

• Melatonin: Regulates sleep-wake cycles; secretion peaks at night.

Thymus

• Thymosin and thymopoietin: Promote T lymphocyte maturation; most active in children.

Gonads

• Testes: Produce testosterone (anabolic and androgenic effects).

• Ovaries: Produce estrogens and progesterone (regulate menstrual cycle, secondary sex characteristics, pregnancy).

Adipose Tissue

• Leptin: Protein hormone that reduces appetite and prevents overeating.

Heart

• Atrial Natriuretic Peptide (ANP): Released in response to increased blood volume; causes vasodilation and promotes excretion of sodium and water to lower blood pressure.

Kidneys

• Erythropoietin (EPO): Stimulates red blood cell production in response to low blood oxygen.