Endocrine System: Thyroid and Parathyroid Hormones

News of the Day Recap

Spring Break Schedule

  • Discussion of the upcoming spring break and class schedule when students return.

Synthesis of Thyroid Hormone

Key Substances Required in Diet

  • Iodine

  • Tyrosine

Forms of Thyroid Hormone

  • Two main forms discussed:

    • T3 (Triiodothyronine) - Active form.

    • T4 (Thyroxine) - Inactive form.

Functions of Thyroid Hormone

  • Controls body functions, with key functions being:

    • Regulation of Basal Metabolic Rate (BMR) - Indicator of how quickly energy is used in the body.

Thyroid Hormone and TSH Relationship

  • An increase in thyroid hormone (TH) levels leads to:

    • Increased secretion of Thyroid Stimulating Hormone (TSH) - Answer to the true/false question is True.

Calcitonin

  • Function of Calcitonin:

    • Increases activity of osteoblasts targeting bones.

    • Promotes the deposition of calcium from the blood into the bone, effectively lowering blood calcium levels.

Exam Conduct and Expectations

  • Upcoming exam details discussed, emphasizing the need for students to catch up after the break.

Additional Notes on Thyroid Hormones and Functions

Basal Metabolic Rate Relation

  • Thyroid Hormones control BMR; higher levels lead to an increased BMR and vice versa.

  • Adrenergic Receptors:

    • Thyroid hormones upregulate adrenergic receptors, leading to increased effects of norepinephrine, especially in organs with adrenergic receptors, correlating with increased heart rate under sympathetic stimulation.

Health Implications of Thyroid Hormone Control

  • Conditions like hyperthyroidism can lead to excessive adrenergic stimulation, resulting in complications like thyroid storm characterized by rapid heart rates.

Calcitonin and Calcium Regulation

  • Calcitonin sourced from parafollicular cells of the thyroid decreases blood calcium levels by promoting bone mineralization.

Parathyroid Glands

Overview

  • Parathyroid glands located on the posterior side of the thyroid regulate blood calcium levels.

Parathyroid Hormone (PTH)

  • Functions of PTH:

    • Targets: Bones and kidneys.

    • Rapid Action: Releases calcium ions from extracellular fluid surrounding bone cells.

    • Slower Action: Activates osteoclasts leading to bone matrix breakdown, releasing calcium into the bloodstream and increasing blood calcium levels.

    • Kidney Action:

    • Activates vitamin D and decreases calcium excretion, retaining more calcium in the body.

  • Stimulus for Release: Low blood calcium levels signal the parathyroid glands to release PTH, correcting calcium deficits.

Implications of Constant Low Calcium Levels

  • Continuous secretion of PTH can lead to osteoporosis due to excessive calcium removal from the bones, emphasizing the need to maintain adequate dietary calcium intake.

Terminology

  • Hypocalcemia: Term for low blood calcium levels; can be lethal as it affects sodium channels crucial for action potentials.

Vitamin D

Function and Regulation

  • Source: Skin (initial synthesis), activated first in the liver and then in the kidneys under the influence of PTH.

  • Main Action: Promotes absorption of calcium in the intestines, enhancing calcium levels in the blood.

  • Addition of vitamin D to foods like milk helps in calcium absorption.

Activation of Vitamin D

  • Stages of Activation:

    • Stage 1: Occurs in the liver - conversion of inactive vitamin D to an active form.

    • Stage 2: Occurs in the kidneys, stimulated by parathyroid hormone.

Adrenal Glands

Structure Overview

  • Adrenal glands consist of two main parts:

    • Adrenal Cortex: Composed of three zones, producing different steroid hormones.

    • Adrenal Medulla: Produces catecholamines.

Zones of the Adrenal Cortex

  1. Zona Glomerulosa:

    • Produces mineralocorticoids, chiefly aldosterone, which:

      • Increases sodium reabsorption in kidneys and potassium excretion.

  2. Zona Fasciculata:

    • Secretes glucocorticoids, particularly cortisol, which:

      • Stimulates gluconeogenesis in body cells, generating glucose from non-carbohydrate sources during fasting.

      • Increases glycogen storage in liver and muscle.

      • Exhibits anti-inflammatory properties by inhibiting immune responses.

  3. Zona Reticularis:

    • Secretes adrenal androgens contributing to hormonal changes in both sexes during puberty, influencing body hair growth and sexual drive.

Control of Cortisol Secretion

  • Feedback Mechanism:

    • Starts with hypothalamus releasing Corticotropin-Releasing Hormone (CRH); stimulates anterior pituitary to release Adrenocorticotropic Hormone (ACTH) leading to cortisol secretion from the adrenal cortex.

    • Increased levels of cortisol provide negative feedback to decrease CRH and ACTH production.

Related Conditions

  • Cushing's Syndrome: Excess cortisol production, often due to medication.

  • Addison's Disease: Inability to synthesize sufficient cortisol, leading to fatigue and low blood sugar problems.

Growth Hormone Effects

  • Excessive growth hormone leads to conditions like acromegaly, characterized by enlarged bones and facial features.

Summary

  • The discussion covers essential endocrine regulation of thyroid hormones, calcium levels, and the adrenal cortex functions, touching on health implications and exam preparation.