T4 and T3, what is it and where does it come from?
Welcome back! In a previous post, we discussed the role of TSH and how the pituitary gland acts as your thyroid's manager. Now, let’s dig deeper into the hormones themselves: T4 and T3. Understanding the difference between these two is key to truly grasping how your body works.
The Thyroid's Job: A Tale of Two Hormones
Your thyroid gland produces two main hormones: T4 (thyroxine) and T3 (triiodothyronine). But here’s the crucial part: it produces them in vastly different amounts. The thyroid makes about 70% T4 and only about 30% T3.
Think of it this way: T4 is a crude oil, your cells can't use it directly. T3 is the active hormone, gasoline your body can burn. T3 is what fuels your metabolism, regulates your body temperature, and keeps your brain, heart, and digestive system running smoothly.
How Your Thyroid Medication Works
If you're taking a thyroid medication like Synthroid, Levothyroxine, or Levoxyl, you are taking a synthetic version of T4. These medications are designed to provide your body with the raw material it's no longer producing on its own.
This is why, after you start your medication, your doctor primarily checks your TSH and your T4 levels. The goal is to get your T4 levels back into a healthy range, so your body has the building blocks it needs.
The Conversion Process: Where the Magic Happens
Once your body has T4, the real work begins. The T4 hormone travels through your bloodstream, and must be converted into the active T3 hormone that your cells can use. Crude oil going to the refinary to be refined into gasoline. This conversion happens primarily in your liver, kidneys, and your gut the bodies refinery.
For most people, this conversion process works perfectly, and their symptoms improve. They have enough T4 from their thyroid gland or thier medication, and their body efficiently creates all the T3 that is needed.
However, for a significant number of people, this conversion is a problem. If your body isn't efficiently converting T4 to T3, you may still experience symptoms of a sluggish thyroid even with "normal" TSH and T4 levels. This is why testing for Free T3 is so important, how much gasoline is being made. Just because there is enough crude oil if the refinery is down no gasoline is being produced.
The conversion from T4 to T3 can be impacted by many factors, including nutrient deficiencies (like selenium, zinc, and iron), inflammation, chronic stress, liver congestion, stress, and poor gut health. Understanding this process is the first step toward a resolving thyroid symptoms if you have normal TSH.
