🔬 When it comes to fatigue, weight gain, and changes in mood or metabolism, one of the first places we look is your thyroid. But with so many thyroid-related labs—TSH, free T3, free T4, reverse T3—patients are often left confused about what’s actually being measured, what each test means, and how it all connects to weight control.
Let’s break it down from the top (the brain) to the bottom (your tissues) so you can understand how your thyroid functions and which test reflects each part of that process.
🧠1. The Pituitary Test: TSH (Thyroid-Stimulating Hormone)
- What it is: A hormone made by the pituitary gland a brain organ, TSH acts like a thermostat or signal switch.
- What it tests: It measures how hard your brain is pushing your thyroid gland to make thyroid hormone.
- High TSH = low thyroid hormone (hypothyroidism)
- Low TSH = high thyroid hormone (hyperthyroidism)
- What it doesn’t do: It doesn’t measure actual hormone levels, tissue use, or how your metabolism is affected—just how the pituitary feels about it.
🧠Think of TSH as the “manager” of thyroid function, not the hormone doing the work.
🔄 2. Gland Output: Free T4 and Free T3
These are the hormones your thyroid gland makes.
🧪 Free T4 (thyroxine)
- The main hormone secreted by the thyroid gland. In and of itself it doesn’t ever tell you what is going on with the cellular level of your thyroid. Oddly this and TSH are often the only lab tests some of our clients are receiving from their primary care physician.
- It has 4 iodine molecules, which is why iodine is so important to have in your body.
- Not very active on its own—it’s more like a storage hormone.
- Converted in the body to T3, the active form.
🧪 Free T3 (triiodothyronine)
- The active thyroid hormone. It works at the cellular and on mRNA our main signaling driver.
- Responsible for metabolic rate, body temperature regulation, gut motility, mood, and energy.
- Made from T4 conversion in organs like the liver and kidney.
đź’ˇ Testing both Free T4 and Free T3 gives you a picture of thyroid gland output AND your body’s ability to activate that hormone.
🧬 3. Tissue-Level Activity: Reverse T3 and T3 Uptake
Sometimes, even with normal TSH and T4, patients still feel hypothyroid—why?
🔍 Reverse T3 (rT3)
- A mirror image of active T3, but it’s inactive.
- Produced when your body is under stress, inflammation, or caloric restriction.
- Competes with T3 at receptors and slows metabolism.
🔎 High rT3 = your tissues aren’t responding properly to thyroid hormone, even if blood levels look “normal.”
🧪 T3 Uptake or Total T3
- Older tests that look at how much thyroid hormone is available to be used, though less commonly used now.
- Total T3 includes both bound and unbound hormone, but Free T3 is more relevant.
⚖️ Which Thyroid Test Reflects Weight Control Best?
While many think TSH or Free T4 are the most important for weight, Free T3 is actually the hormone most directly tied to metabolic rate and weight control.
Why Free T3 matters:
- It’s what your cells actually use.
- Low Free T3 = slow metabolism, fatigue, cold intolerance, difficulty losing weight—even if TSH is normal.
⚠️ TSH and Free T4 may appear normal, but if Free T3 is low-normal or low, many patients still feel the effects of low thyroid activity.
🧪 Suggested Lab Panel for a Full Thyroid Picture
If you’re evaluating thyroid function comprehensively, especially for metabolic health, these are the tests to ask for:
| Test | What It Tells You |
|---|---|
| TSH | Pituitary signal to thyroid |
| Free T4 | Thyroid hormone production |
| Free T3 | Active thyroid hormone |
| Reverse T3 | Tissue-level hormone resistance (optional) |
| Thyroid antibodies (TPO/TG) | Autoimmune causes (Hashimoto’s) |
đź’ˇ Summary
Understanding your thyroid labs means understanding where in the chain a dysfunction might be happening:
- TSH – how hard your brain is pushing your thyroid
- Free T4 – how much hormone your thyroid is making
- Free T3 – how much active hormone your body has to work with
- Reverse T3 – if stress or illness is interfering at the tissue level
- Free T3 = best marker of metabolic impact and weight regulation
If you’re struggling with symptoms that don’t match your labs—or if you’ve been told your thyroid is “normal” despite feeling unwell—it may be time to look beyond TSH and evaluate your whole thyroid axis. Once we have the full picture of tests, symptoms, and your individual health we can review the guidelines on who to treat.
