Endocrinology, Posterior Pituitary, and Parathyroid

Endocrine System

Specialized glands that create and release hormones to regulate electrolytes, metabolism, and reproduction

Negative Feedback Loop

All hormones have this to regulate their release, otherwise disease will occur

Negative feedback can come from…

Hormones themselves, target organ, or cellular response

General flowchart for hormone release

Gland → hormone → target → cellular response

8 Specialized Tissues

Hypothalamus, pituitary (anterior and posterior), thyroid, parathyroid, adrenals, pancreas, ovaries, testes

Hormones

Chemicals released into the blood to convey regulatory information to target organs

Receptors

Molecules that bind hormones at target tissues to initiate cellular responses

Hormones will ONLY elicit a response when…

The target expresses the specific hormone receptors

Steroid Hormones (lipid-soluble)

Can pass through the membrane, SLOW response because they rely on gene transcription, but LONG-lasting

Peptide Hormones (water-soluble)

Use a surface receptor to transduce signal, VERY FAST but SHORT-lasting because easily degraded

Clinical relevance of steroid hormones

Can take longer than a day for effect of a blocking drug to wear off

Patterns of Hormone Secretion

• Diurnal (24 hr cycle)

• Pulsatile (burst then lag)

• Infradian (> 1 day)

Diagnostic Tools

• Blood and urine tests

• Dynamic “stress tests”

• Ultrasounds/scans of glands

• Genetic testing

Endocrinology

Study of hormones, receptors, and signaling pathways

Endocrine and nervous system

Nervous system connects internal to external environment via the hypothalamus and pituitary

Hypothalamus (internal) → Pituitary → Body (external)

Hypothalamus

Brain region directly coupled to endocrine via the pituitary

Location of pituitary

Inferior to the optic nerve chiasm: swelling can cause vision problems

How does the posterior pituitary release hormones?

The HYPOTHALAMUS produces the hormones with neurosecretory neurons, and sends them down to be stored in terminals and released after signaling

Posterior Pituitary Hormone

Antidiuretic Hormone (ADH) aka vasopressin

What kind of hormone is ADH?

Peptide hormone (water-soluble, fast-acting, short duration)

Anterior Pituitary signaling pathway

Hypothalamus creates hormones that signal the endocrine cells of this gland to create ITS OWN hormones to be released

What are the hormones that signal the anterior pituitary called?

Tropic hormones

Anterior Pituitary Hormones

• Growth Hormone (GH)

• Adrenocorticotropin (ACTH)

• Thyroid Stimulating Hormone (TSH)

• Luteinizing Hormone (LH)

• Follicle-Stimulating Hormone (FSH)

Role of ADH

Promotes reabsorption of water from kidney’s collecting ducts

Stimulus that induces ADH release

High sodium concentration (osmolality) sensed by osmoreceptors in hypothalamus

ADH affect on target

Kidneys retain water by reabsorption to restore balance (lower Na)

ADH target

Kidneys

Negative Feedback of ADH

As sodium levels decrease, stimulus is gone and ADH is not released

Pathway of ADH secretion

High sodium → Osmoreceptors in hypothalamus → Posterior Pituitary → Kidney (Reabsorption) → Low sodium → No release

Syndrome of inappropriate ADH secretion (SIADH)

Unsuppressed release of ADH, leading to electrolyte imbalance (water retention) of sodium

Who is more at risk for SIADH?

People with hyponatremia (low sodium), medications like narcotics and NSAIDs, elderly population

Symptoms of SIADH

Nausea, vomiting, confusion, fatigue, seizures

Pathology of SIADH

Negative feedback regulation is not working, so there are high levels of ADH on receptors leading to water retention and electrolyte imbalance

Causes of SIADH

• Hyperactive stimulus-sensing by hypothalamus

• Tumor cell secretion of ADH in pituitary or lung

• Increased ADH sensitivity in kidney

Iatrogenic

Illness caused by medical examination or treatment

Iatrogenic causes of SIADH

Medications like NSAIDs, for cancer, anti-seizure, antidepressant, heart, diabetes, and blood pressure

Diagnosis of SIADH

Must measure BOTH blood and urine sodium levels

LOW blood Na, HIGH urine Na

Treatment of SIADH

• Fluid restriction if non-acute

• IV hypertonic saline if extreme

• Adjust medication if iatrogenic

Parathyroid Gland Hormone

Parathyroid Hormone (PTH)

PTH stimulus

Low blood calcium levels

PTH Function

Control blood calcium levels by stimulating osteoclasts to release calcium via bone resorption

What do parathyroid glands look like?

4 small structure on the BACK of the thyroid gland

PTH Pathway

Low Ca → Parathyroid → Bones (Osteoclasts) → Resorption → Increase Ca → Stop secreting

Function of osteoclasts

Bone resorption (break down bone & release reservoir of Ca) when Ca is LOW

Function of osteoblasts

Bone deposition (sequester Ca into bones) when Ca is HIGH

Why must Ca be in a narrow range?

It is very important to the heart, brain, muscle functions, in blood clotting and cellular metabolism

Hypercalcemia

Leads to kidney stones, constipation, confusion, fatigue; often with overactive PTH

Gut and kidney function with Calcium

Gut and kidney function is important to maintaining levels, both have receptors that regulate calcium

Calcium in the kidney

Receptors respond to PTH to increase Ca by reabsorption and produce Vitamin D needed for calcium absorption in small intestine

Calcium in Small Intestine

Use vitamin D from kidney for calcium absorption

PTH targets

Kidney and bone

Osteoporosis

Metabolic bone disorder developed progressively, causing bones to be fragile

Symptoms of Osteoporosis

Back pain, stooping, loss of height, bone fractures

Fractures are PATHOLOGICAL, not due to force

Risk for Osteoporosis

• Hyperparathyroidism and hyperthyroidism

• Malabsorptive or eating disorders

• Vitamin D deficiency

• Diabetes

• Breast cancer

Pathology of Osteoporosis

Balance between bone resorption and deposition is disrupted, leading to excess resorption

Diagnosis of osteoporosis

Bone mineral density test via DXA scan to verify low bone mineral density

Treatment of Osteoporosis

• Dietary supplements, limit caffeine and alcohol (lifestyle)

• Bisphosphonates to slow resorption

Preventing osteoporosis

• Exercise regularly, especially weight-bearing/resistance

• Get enough calcium and vitamin D

  • Supplements for vitamin D usually required, not as common for calcium