The thyroid gland is a vital endocrine organ responsible for regulating metabolism, energy production, growth, and body temperature.

It secretes three main hormones:

• TSH (Thyroid Stimulating Hormone) : the controller
• T4 (Thyroxine) : storage hormone
• T3 (Triiodothyronine) : active hormone

Memory Tip: “TSH orders, T4 stores, T3 works.”

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Anatomy and Structure of the Thyroid Gland

Location

• Situated in the anterior neck, below the larynx, in front of the trachea

Shape

• Butterfly shaped with:
  • Right lobe
  • Left lobe
  • Isthmus connecting the lobes

Microscopic Structure

1. Thyroid Follicles : spherical units storing hormones in colloid
2. Follicular Cells : produce T3 and T4
3. Parafollicular Cells (C Cells) : produce calcitonin, regulating calcium

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Functions of the Thyroid Gland

The thyroid regulates multiple body systems:

• Metabolism: Increases basal metabolic rate (BMR)
• Cardiovascular System: Enhances heart rate and cardiac output
• Nervous System: Supports brain function and cognitive development
• Thermoregulation: Maintains body temperature
• Growth and Development: Critical for childhood growth and tissue maturation

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Thyroid Hormones Overview (TSH, T3, T4)

TSH (Thyroid Stimulating Hormone)

• Produced by the pituitary gland
• Regulates thyroid hormone production
• Acts as the thermostat of the thyroid system

T4 (Thyroxine)

• Major hormone produced by the thyroid
• Inactive prohormone, converted to T3 in tissues
• Functions as storage hormone

T3 (Triiodothyronine)

• Mostly formed from T4 conversion in liver and kidneys
• Active hormone, 3–4× more potent than T4
• Controls metabolism, energy, heart rate, and brain function

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How Thyroid Hormones Are Produced

Step by Step Synthesis

1. Iodine Intake: Absorbed from diet
2. Iodine Trapping: Follicular cells uptake iodine
3. Thyroglobulin Production: Protein scaffold for hormones
4. Iodination: Iodine binds tyrosine → MIT & DIT
5. Coupling:
   • MIT + DIT → T3
   • DIT + DIT → T4
6. Storage: Hormones stored in colloid

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How Hormones Are Secreted

1. TSH stimulates the thyroid
2. Colloid is reabsorbed by follicular cells
3. T3 and T4 released into blood
4. T4 converts to T3 in peripheral tissues

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Thyroid Hormone Regulation (Feedback Loop)

• Hypothalamus → TRH
• Pituitary → TSH
• Thyroid → T3 & T4

Negative Feedback System:

Hormone Level	TSH Response
High T3/T4	↓ TSH
Low T3/T4	↑ TSH

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How Thyroid Hormones Work

• T3 enters cells, binds to nuclear receptors
• Activates gene transcription
• Increases protein synthesis and metabolism
• Enhances ATP production, heat generation, and organ function

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Clinical Relevance

Hypothyroidism

• High TSH, low T3/T4
• Slowed metabolism, fatigue, weight gain, cold intolerance

Hyperthyroidism

• Low TSH, high T3/T4
• Increased metabolism, heat intolerance, weight loss, tremors

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Copyright

© 2026 Amina Rehman. All Rights Reserved.

This content is original intellectual property of Amina Rehman, published exclusively on aminarehman.com.

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