Major Endocrine Glands

Why This Matters

The endocrine system is your body's chemical messaging network, and understanding how each gland contributes to homeostasis is central to success in Anatomy and Physiology II. You're being tested on more than gland locations and hormone names—exams focus on feedback loops, hormone interactions, and what happens when regulation fails. These concepts connect directly to clinical conditions like diabetes, thyroid disorders, and adrenal insufficiency that appear repeatedly on assessments.

Think of endocrine glands as falling into functional categories: some control other glands (hierarchical control), some regulate metabolism and energy, some maintain mineral balance, and others govern reproduction and development. Don't just memorize that the pituitary releases growth hormone—know why it's called the master gland and how its relationship with the hypothalamus exemplifies neuroendocrine integration. This conceptual approach will serve you well on both multiple choice and FRQ-style questions.

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Hierarchical Control Centers

The nervous and endocrine systems converge at key control centers that coordinate body-wide responses. These glands don't just release hormones—they regulate other glands through tropic hormones and feedback mechanisms.

Pituitary Gland

• "Master gland" controlling other endocrine organs—releases tropic hormones like ACTH, TSH, FSH, and LH that stimulate target glands
• Two distinct lobes with different embryonic origins—anterior lobe (adenohypophysis) synthesizes hormones; posterior lobe (neurohypophysis) stores and releases hypothalamic hormones
• Connected to hypothalamus via infundibulum—this anatomical link enables neuroendocrine integration and explains how stress, sleep, and environmental cues influence hormone release

Pineal Gland

• Produces melatonin to regulate circadian rhythms—secretion increases in darkness and decreases with light exposure, synchronizing the sleep-wake cycle
• Functions as a neuroendocrine transducer—converts neural signals about light into hormonal output, linking the environment to internal physiology
• Influences reproductive hormone timing—plays a role in seasonal breeding patterns in animals and may affect puberty onset in humans

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Metabolic Regulators

These glands control how your body uses energy, responds to stress, and maintains blood glucose levels. Their hormones affect virtually every cell, making them high-yield targets for exam questions about systemic effects.

Thyroid Gland

• Produces $T_3$ and $T_4$ to set basal metabolic rate—these iodine-containing hormones increase oxygen consumption and heat production in most tissues
• Regulated by TSH in a classic negative feedback loop—low thyroid hormone triggers TSH release; high levels suppress it, a mechanism frequently tested
• Also secretes calcitonin for calcium regulation—lowers blood $Ca^{2+}$ by inhibiting osteoclast activity, working opposite to parathyroid hormone

Adrenal Glands

• Two glands in one: cortex and medulla with distinct functions—cortex produces steroid hormones (cortisol, aldosterone, androgens); medulla produces catecholamines (epinephrine, norepinephrine)
• Cortisol mediates the stress response and metabolism—raises blood glucose, suppresses inflammation, and is regulated by the hypothalamic-pituitary-adrenal (HPA) axis
• Aldosterone maintains blood pressure via $Na^+$ retention—part of the renin-angiotensin-aldosterone system (RAAS), a key concept linking endocrine and cardiovascular physiology

Pancreas

• Dual function as endocrine and exocrine organ—islets of Langerhans contain alpha cells (glucagon) and beta cells (insulin), while acinar cells produce digestive enzymes
• Insulin and glucagon maintain blood glucose homeostasis—insulin lowers glucose by promoting cellular uptake and glycogen synthesis; glucagon raises it by stimulating glycogenolysis
• Dysfunction causes diabetes mellitus—Type 1 involves autoimmune destruction of beta cells; Type 2 involves insulin resistance, both high-yield clinical correlations

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Calcium and Mineral Homeostasis

Precise regulation of blood calcium is essential for nerve function, muscle contraction, and bone health. The interplay between these glands demonstrates antagonistic hormone action—a concept that appears frequently on exams.

Parathyroid Glands

• Four small glands secreting PTH to raise blood $Ca^{2+}$—located on the posterior surface of the thyroid, they respond directly to low calcium levels
• PTH acts on three target sites—increases bone resorption (osteoclast activation), enhances renal $Ca^{2+}$ reabsorption, and stimulates vitamin D activation for intestinal absorption
• Antagonistic relationship with calcitonin—PTH raises calcium while thyroid calcitonin lowers it, a classic example of hormonal opposition maintaining homeostasis

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Reproductive and Developmental Regulators

These glands control sexual development, fertility, and immune system maturation. Their activity changes dramatically across the lifespan, making developmental timing a key testable concept.

Gonads (Ovaries and Testes)

• Produce sex steroids under pituitary control—ovaries secrete estrogen and progesterone; testes secrete testosterone, all regulated by FSH and LH from the anterior pituitary
• Drive puberty and secondary sexual characteristics—estrogen promotes female fat distribution and breast development; testosterone promotes male muscle mass and facial hair
• Essential for gametogenesis and fertility—testosterone supports spermatogenesis; estrogen and progesterone regulate the menstrual cycle and prepare the uterus for implantation

Thymus

• Produces thymosin for T-lymphocyte maturation—essential for developing adaptive immunity by "training" T-cells to recognize self vs. non-self
• Most active during childhood, involutes after puberty—this age-related shrinkage explains why immune function changes across the lifespan
• Critical bridge between endocrine and immune systems—demonstrates that hormones regulate more than metabolism; they shape immune competence

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Quick Reference Table

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Self-Check Questions

1. Which two glands work antagonistically to maintain blood calcium levels, and what happens to bone when each hormone dominates?

2. Compare the anterior and posterior pituitary: How do their embryonic origins explain their different mechanisms of hormone release?

3. A patient presents with high blood glucose despite normal insulin production. Which gland is affected, and what type of diabetes does this suggest?

4. Both the adrenal medulla and adrenal cortex respond to stress. Compare their hormones, timing of response, and mechanisms of action.

5. If an FRQ asks you to trace the hormonal pathway from low thyroid hormone to restored levels, which glands and hormones would you include, and where does negative feedback occur?