🥛 Posterior Pituitary + Thyroid + Parathyroid

Posterior Pituitary – Function

Hormone Production
 Does not produce hormones

Hormone Storage and Release
 Stores two hormones produced by the hypothalamus
 Releases hormones periodically

Stored Hormones
 Antidiuretic hormone (ADH)
 Oxytocin

Posterior Pituitary – Hormone Release Mechanism

Hormones
 Antidiuretic hormone (ADH)
 Oxytocin

Storage
 Stored in nerve endings at the posterior pituitary

Release Control
 Nerve impulses from the hypothalamus signal the posterior pituitary
 Hormones are released into the bloodstream when needed

Antidiuretic Hormone (ADH) – Function

Function
 Helps the body conserve water during short supply

Mechanism
 Acts on the kidneys
 Causes reabsorption of water from urine back into the bloodstream

Oxytocin – Function

Uterus
 Induces contraction of the myometrium
 Breeding: helps spermatozoa movement to oviducts
 Parturition: aids delivery and placenta expulsion

Mammary Glands
 Facilitates milk movement to lower parts of the gland
 Causes cells around alveoli and small ducts to contract

Mechanism
 Oxytocin circulates from posterior pituitary to target tissues

Oxytocin – Mechanism of Action

Function
 Promotes myometrium contraction in the uterus

Mechanism
 Endogenous (produced by body) and exogenous (administered externally) oxytocin activate oxytocin receptors
 Acts on voltage-regulated Ca²⁺ channels
 Facilitates Ca²⁺ influx into smooth muscle cells of the myometrium
 Increased Ca²⁺ triggers muscle contraction

Thyroid Gland – Structure

Location
 Consists of two lobes on each side of the larynx

Cell Types
 Thyroid follicular epithelium – secretes thyroid hormones
 Medullary (C) cells – secrete calcitonin

Thyroid Gland – Cellular Organization

Thyroid Follicle (TF)
 Site where thyroid hormones are produced

Epithelial Cells
 Line the thyroid follicles and secrete thyroid hormones

Medullary (C) Cells
 Produce calcitonin, involved in calcium regulation

Thyroid Hormones – Production and Activation

Anterior Pituitary Gland
 Produces Thyroid-stimulating hormone (TSH)

TSH Function
 Activates the thyroid gland to produce thyroid hormones

Thyroid Hormones
 T4 – main hormone secreted by the thyroid
 T3 – most biologically active form

Conversion
 T4 is converted to T3 in peripheral tissues (mainly liver, kidney, and muscle) as needed

Thyroid Hormones (T3 and T4) – Effects

Calorigenic Effect
 Regulates cellular metabolic rate
 Helps generate heat and maintain body temperature

Protein, Carbohydrate, and Lipid Metabolism
 May induce protein anabolism (building) or catabolism (breakdown)
 Depends on energy availability in the diet

Thyroid Gland – Body Temperature and Metabolism Example

Experiment Setup
 PFTN: Thermoneutral zone (20°C)
 HS: Heat-stressed condition (32°C)

Purpose
 Demonstrates the calorigenic effect of thyroid hormones (T3 and T4)
 Shows how thyroid activity regulates metabolic rate and helps maintain body temperature under different environmental conditions

Calcitonin – Thyroid Hormone

Production
 Produced by C cells located between thyroid follicles

Function
 Helps maintain blood calcium homeostasis
 One of two key hormones for calcium balance (other: parathyroid hormone)

Parathyroid Gland – Structure and Function

Structure
 Several small, pale nodules located in, on, or near the thyroid gland

Hormone Produced
 Parathyroid hormone (parathormone)

Function
 Helps maintain blood calcium homeostasis
 Has an opposite effect to calcitonin

Calcitonin vs Parathyroid Hormone (PTH) – Calcium Homeostasis

Parathyroid Hormone (PTH)
 Secreted by parathyroid gland
 Increases renal Ca reabsorption
 Stimulates production of 1,25(OH)₂D₃ (active vitamin D)
 Enhances Ca absorption by intestinal epithelial cells
 Overall effect: raises blood calcium levels

Calcitonin
 Secreted by thyroid gland (C cells)
 Inhibits bone Ca resorption
 Increases urinary Ca loss
 Overall effect: lowers blood calcium levels

Calcium Homeostasis in Dairy Cows – Example

Condition
 Parturient paresis (Milk fever) – common after calving

Cause
 Failure of homeostatic mechanisms to regulate blood calcium (calcemia)
 Due to high calcium demand at the start of lactation

Calcium Homeostasis in Dairy Cows – Hypocalcemia Levels

High Hypocalcemia
 Severe low blood calcium, often at onset of lactation

Low Hypocalcemia
 Milder reduction in blood calcium

Purpose of Example
 Demonstrates variation in calcium levels and the importance of homeostatic regulation during lactation