The Thyroid Gland

• Located in the neck region.
• Produces two main thyroid hormones: T₄ (Thyroxine) and T₃ (Triiodothyronine).

a) Function of Thyroid Hormones:

• • Regulate metabolic rate.
  • Control the basal metabolic rate, which is the rate of reactions throughout the body.
  • Play a key role in thermoregulation (body heat regulation).
  • Influence cellular function by dictating enzymatic reaction rates.

Importance in Development

• Essential during childhood for growth and nervous system development.
• Regulating cellular reactions helps prevent large issues in growth and function.

b) Calcitonin

• Secreted by the thyroid gland.
• Function:
  • Regulates calcium ion levels in the blood.
  • When blood calcium is high: Calcitonin is released to lower calcium levels by inhibiting osteoclast activity.
  • Critical for bone health, heart function, nervous system, and skeletal muscle activity.

c) Parathyroid Glands

• Four small glands located on top of the thyroid gland.
• Secrete parathyroid hormone (PTH).
• Function:
  • Opposes calcitonin.
  • When blood calcium is low:
    1. PTH is released to increase calcium levels by stimulating osteoblasts to release RANKL which increases osteoclast activity.
    2. PTH stimulates an increase in calcitriol release by the kidneys.
       • Calcitriol is a hormone that increase the absorption of dietary calcium by intestinal cells.
    3. PTH stimulates kidneys to reabsorb more calcium, resulting in less calcium being excreted in urine.
  • Vital for bone, nerve, and muscle function.
  • Maintains serum calcium homeostasis.

The Adrenal Glands

• Located on top of the kidneys.
• Consist of two main parts:

a) The Adrenal Medulla (Inner part)

• Secretes epinephrine (adrenaline) and norepinephrine.
• These act as both neurotransmitters and hormones.
• Function during “Fight or Flight”:
  • When in SNS mode, these hormones are released into the bloodstream.
  • Prepare the body for rapid activity (increased heart rate, energy mobilization).

b) The Adrenal Cortex (Outer part)

• Produces cortisol, aldosterone, and androgens.

Functions of Adrenal Cortex Hormones

Cortisol (a glucocorticoid)

• Anti-inflammatory.
• Suppresses immune response.
• Catabolic: Breaks down proteins into glucose for energy.
• Involved in metabolic control and resistance to stress (e.g., during starvation).
• Categories: Cortisone and cortisol.

Aldosterone

• Regulates salt-water balance in blood.
• Works alongside antidiuretic hormone and cortisol.
• Supports metabolic processes.

Androgens

• Include hormones like testosterone.
• Secreted by the adrenal cortex.
• In females, provide a small amount of testosterone.
• Important for skeletal muscle maintenance and growth.

The Adrenal Gland – Stress Response and Catabolism

• During biological stress (such as starvation), glucocorticoids promote catabolism, generating glucose from proteins.
• This process helps survive stressful conditions.

The Adrenal Medulla and Fight or Flight

• During SNS activation, the medulla secretes epinephrine and norepinephrine.
• These hormones prepare the body for rapid response during fight or flight situations.