Endocrine System - 1/23.

Thyroid Hormones and Calcium Regulation

The thyroid produces two key hormones: thyroxine (T4) and calcitonin.
Calcitonin plays a crucial role in regulating blood calcium levels and maintaining homeostasis.
High or low blood calcium levels can lead to significant health issues.

Function: Calcitonin primarily functions to lower blood calcium levels.
Stimulus: High blood calcium levels serve as the primary stimulus for the secretion of calcitonin.
Inhibition: Conversely, low blood calcium levels inhibit calcitonin release.
Mechanism:
- Calcitonin functions by stimulating osteoblasts, which are cells that build new bone, effectively drawing calcium from the bloodstream and depositing it into the bones.
- This process helps to reduce the concentration of calcium in the bloodstream.

There are two primary hormones, calcitonin and parathyroid hormone (PTH), which regulate blood calcium levels.
Calcitonin:
- Decreases blood calcium. When levels rise too high, calcitonin is secreted to stimulate calcium deposition into bones and inhibit calcium release from bones.
PTH:
- Increases blood calcium levels. Secreted when blood calcium levels drop too low. PTH has the following actions:
  - Stimulates osteoclasts to break down bone, releasing calcium into the bloodstream.
  - Increases calcium reabsorption in the kidneys.
  - Enhances calcium absorption from the intestines.

On average, there are 4 to 6 parathyroid glands located on the posterior aspect of the thyroid gland.
These glands contain chief cells that produce PTH.
Importance of PTH was exemplified in past medical cases. Patients without function of these glands were unable to regulate blood calcium, often leading to severe health outcomes or death.

Hypersecretion or hyposecretion of calcitonin/PTH can lead to abnormal blood calcium levels, resulting in:
- Hypercalcemia (high blood calcium): caused by low calcitonin or high PTH leading to symptoms like weakened bones and convulsions.
- Hypocalcemia (low blood calcium): caused by high calcitonin or low PTH can lead to muscle paralysis, including respiratory muscles leading to death.

The adrenal glands are situated atop the kidneys and play a significant role in the body's stress response.
- Adrenal Cortex: Produces steroid-based hormones (mineralocorticoids, glucocorticoids, gonadocorticoids).
- Adrenal Medulla: Produces catecholamines (epinephrine and norepinephrine) involved in the fight or flight response.

Mineralocorticoids (e.g., Aldosterone):
- Regulate salt and water balance; primarily influence blood pressure.
- Stimulated by low blood pressure; inhibits high blood pressure.
Glucocorticoids (e.g., Cortisol):
- Involved in glucose metabolism, stress response, and immune modulation.
- Stimulated by ACTH (adrenocorticotropic hormone), particularly under stress conditions.
- Suppress immune responses and increase glucose levels through gluconeogenesis and glycogenolysis.
Gonadocorticoids:
- Function as weak sex hormones contributing to sexual development and health.

Diabetes Mellitus: Characterized by elevated blood glucose levels due to issues with insulin production or function.
- Primary signs include polyphagia (excess hunger), polydipsia (excess thirst), and polyuria (excess urination).
Insulin: Secreted by beta cells of the pancreas to lower blood glucose by allowing glucose uptake by body cells and storing excess glucose as glycogen or fat.
Glucagon: Counteracts insulin, increasing blood glucose levels when they are low by stimulating glycogen breakdown in the liver.

Addison's Disease: Indicates hyposecretion of adrenal hormones, leading to symptoms like chronic fatigue, low blood pressure, and electrolyte imbalance.
Cushing's Syndrome: Results from hypersecretion of glucocorticoids, leading to symptoms such as obesity and decreased immune function.
- Notable characteristics include fat redistribution, muscle weakness, and skin changes.

Hormonal balance, particularly involving calcitonin, PTH, other adrenal hormones, and insulin, is vital for maintaining health and ensuring physiological stability.