Thyroid Hormone: T4 and T3

The thyroid gland’s job is to make thyroid hormones. Thyroid hormones help all your organs, muscles, and glands function properly. Every part of your body needs thyroid hormone, especially your heart, brain, liver, digestion, bones and muscles. Thyroid hormones regulate the body's metabolism and the energy needed for your cells and body functions, including:

  • Thinking & memory

  • Neurotransmitters

  • Breathing

  • Metabolism

  • Digestion

  • Elimination

  • Body weight

  • Calorie burning

  • Hair growth

  • Healing

  • Immune strength

  • Fertility

  • Sex drive

  • Overall energy

  • Muscle strength

  • Muscle recovery

  • Menstrual cycle

  • Breastfeeding

  • Mood

  • Nerve function

The two most important hormones made by your thyroid are:


Thyroxine (T4) -- the 4 stands for 4 atoms of iodine

Triiodothyronine (T3) -- the 3 stands for 3 atoms of iodine


Every day, a healthy thyroid makes about 80 to 100 micrograms of T4, and 20 to 40 micrograms of T3. That's a ratio of about 80% T4 to 20% T3. 

T4 is a STORAGE “pro-hormone.” It goes through a conversion process before it's used by the body. 


Before conversion, some T4 binds to proteins, and is unusable.


Some T4 loses a molecule of iodine, and converts into active Free T3. Free T3 is the active thyroid hormone that relieves hypothyroidism, helping oxygen and energy get in to cells to power ALL your body’s functions. 
The rest of the T4 converts into an inactive and unusable form of T3, called Reverse T3.

© 2020, 2021 by Mary Shomon. All Rights Reserved.

The Natural Thyroid Guide site, webinar, book, and newsletter are created and managed by Mary Shomon, a patient advocate, health coach, and New York Times bestselling author of 15 books on thyroid and hormonal health, weight loss, and wellness. For more information, visit

Natural Thyroid Guide and Mary Shomon do not provide medical advice, diagnosis or treatment. See additional information.

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