Written by &you Labs Team
Updated September 29, 2026
Your TSH came back normal, but you still feel cold, tired, and slow. This is one of the most common thyroid stories we hear.
The missing piece is often Free T3, the active thyroid hormone your cells actually use. Optimal free T3 sits in the upper third of your lab's range, not just anywhere inside it, and Free T4 has a target of its own.
This guide gives you both target numbers, explains why a normal TSH can hide a low Free T3, and shows you where reverse T3 fits in.
Optimal Free T3 sits in the upper third of your lab's range, about 5.0 to 6.8 pmol/L (3.2 to 4.2 pg/mL) on many reports. Optimal Free T4 sits mid range to upper half, about 15 to 19 pmol/L (1.1 to 1.5 ng/dL). Ranges differ by lab, so compare your number to the range printed on your own report. A normal TSH with a low Free T3 can still cause real symptoms.
Your thyroid is a small gland in your neck that sets the speed of your body, like the throttle on an engine.
It makes two main hormones. T4 is the storage form, and T3 is the active form that does the real work inside your cells.
Most thyroid hormone in your blood is stuck to carrier proteins, like cargo locked inside a truck. Only the free part can get out and do a job, and that free part is what a Free T3 or Free T4 test measures.
Your thyroid mostly makes T4, and your body then converts that T4 into T3, mainly in your liver, kidneys, and gut. About 80% of your T3 is made this way, outside the thyroid itself.
This conversion step is where things go wrong for many people, because your thyroid can work perfectly while conversion lags behind.
Philippine lab reports usually print Free T3 as FT3 and Free T4 as FT4. Free T4 is also called free thyroxine. Older panels may show Total T3 and Total T4 instead, which include the bound hormone and answer a different question.
TSH is a message from your brain to your thyroid, so it is not a thyroid hormone at all.
Your brain reads mostly T4, and it makes its own T3 locally. So your brain can be satisfied while your muscles, gut, and heart run short.
That is how TSH can look normal while Free T3 sits at the very bottom of its range, and your report still prints "normal" beside it.
Symptoms that often show up in this pattern:
None of these symptoms prove a thyroid problem on their own, because iron, sleep, and stress cause them too. But several of them together, alongside a low Free T3, is worth a closer look. The optimal TSH range is tighter than the standard one, which matters here as well.
Marker | Usual lab range | Healthy target range |
|---|---|---|
Free T3 (FT3) | 3.1 to 6.8 pmol/L | 5.0 to 6.8 pmol/L |
Free T3, other units | 2.0 to 4.4 pg/mL | 3.2 to 4.2 pg/mL |
Free T4 (FT4) | 12 to 22 pmol/L | 15 to 19 pmol/L |
Free T4, other units | 0.9 to 1.7 ng/dL | 1.1 to 1.5 ng/dL |
Reverse T3 | 10 to 24 ng/dL | Lower half of the range |
TSH | 0.4 to 4.5 mIU/L | 0.5 to 2.5 mIU/L |
Lab ranges differ. Where your number sits inside your own lab's range matters more than the raw figure.
These targets are what symptom research and everyday functional medicine practice point toward. They are not official guideline cutoffs, and no single number decides anything. Read them with your doctor, next to how you actually feel.
People whose Free T3 sits in the top third of the range tend to report fewer low thyroid symptoms. Energy, mood, and body temperature track T3 much more closely than they track T4.
The bottom of the range is still normal on paper, but it sits closer to the level where symptoms begin.
Free T4 does not need to be high, because mid range is common in healthy people and perfectly fine.
The gap between the two numbers tells you more. A high normal Free T4 with a low normal Free T3 is a classic sign that the conversion step is slow.
Your body can turn T4 into reverse T3 instead of T3. Reverse T3 fits the same locks but does not turn them. Think of a key that slides into the door and will not open it.
This is a safety feature, not a fault, because your body slows itself on purpose whenever it is under real strain.
Reverse T3 is not a routine test. It earns its place when your TSH and Free T4 look fine, your Free T3 runs low, and symptoms persist. In that case it can explain a lot of stubborn fatigue.
Some doctors compare Free T3 to reverse T3 as a ratio. Units differ between labs, so this only works when both results come from the same lab and your doctor does the math.
If you want the full thyroid picture from one blood draw, a &you Labs panel reads Free T3 and Free T4 next to TSH.
Selenium, zinc, and iron all help the conversion step. Low ferritin, your iron storage marker, is a common cause in women who still have monthly periods.
Very low calorie diets drop T3 within days, because your body reads a food shortage and slows down to save energy.
High cortisol blocks conversion and pushes reverse T3 up at the same time.
Any serious illness lowers T3 for a while. This is a sick thyroid pattern, and it usually settles as you recover, so retest afterward rather than during.
A few common medicines lower T3 or slow conversion. Never start or stop a prescription on your own, and ask your doctor to read your thyroid results next to your medicine list.
Much of your conversion happens in the liver and gut, so fatty liver and poor gut health can both slow it down.
Yes, that is a reasonable next step. TSH on its own cannot show a slow conversion step. Ask for Free T3, Free T4, and thyroid antibodies together, then read all four numbers side by side.
Aim for the upper third of the range printed on your report. On many reports that is about 5.0 to 6.8 pmol/L, or 3.2 to 4.2 pg/mL. Ranges vary, so always check your own.
No. Thyroid tests do not need fasting. If the same panel also checks blood sugar or cholesterol, you will be asked to skip food for 8 to 12 hours. Tell your doctor if you take a biotin supplement, since it can skew thyroid results.
Yes. A Free T3 above range, paired with a low TSH, points to an overactive thyroid. Racing heart, weight loss, shaky hands, and trouble sleeping are common signs. That pattern needs a proper workup with a doctor.
TSH is the standard first screen, and it catches most clear thyroid disease well. It simply cannot show you a conversion problem.
Retest 6 to 8 weeks after any real change, such as fixing a nutrient gap or a dose change made by your doctor. Once your numbers are stable and you feel well, once a year is enough.
The &you Labs Team combines medical science with personal care. We help Filipinos understand their health markers with blood work analysis that is easy to follow, whole person support, and health expertise built for Filipino life.
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