Free T3

What it actually measures

Your thyroid mostly makes T4. That is not the hormone that does anything.

T4 is storage. It circulates, it waits, and when a tissue needs thyroid activity an enzyme strips one iodine atom off and turns it into T3. That conversion happens mostly in the liver, the kidneys and other peripheral tissues rather than in the thyroid gland at all.

T3 is what binds the receptors. T3 is what sets your metabolic pace, your body temperature, how fast your heart beats at rest, how quickly your gut moves. When people talk about thyroid function they are usually describing the effects of T3 without knowing it.

So a thyroid panel measuring only TSH and T4 is describing the signal and the reservoir. Free T3 is the water actually coming out of the tap, and free means the unbound portion that can get into cells rather than the fraction stuck to carrier proteins.

Why it can be low when everything else reads fine

Here is the tension though.

TSH is the pituitary's report on thyroid hormone availability, and it responds primarily to T4. If your thyroid is producing plenty of T4, the pituitary is satisfied and TSH sits comfortably in range.

But conversion happens downstream of that. If T4 is not being converted efficiently into T3, the reservoir stays full, the pituitary stays satisfied, TSH stays normal, and the active hormone reaching your tissues is still low.

That is the case for measuring free T3 rather than inferring it. TSH cannot see a conversion problem, because conversion happens after the point TSH is reporting on.

Which does not mean every tired person has one. It means the question is unanswerable from a TSH alone, and answering it costs one more line on a lab order.

The reverse T3 question, honestly

There is a second pathway worth knowing about. T4 can also be converted into reverse T3, an inactive form that occupies the space without doing the work.

That happens more during illness, starvation, and periods of significant physiological stress. It is an energy-conserving response: the body deliberately dials down metabolic pace when resources are scarce or it is busy fighting something.

This pattern is well described in the literature as the low T3 syndrome of systemic illness, where T3 falls, reverse T3 rises, and TSH stays normal or low. That is a real and documented phenomenon.

Now the honest part, because reverse T3 gets used with more confidence in wellness circles than the evidence supports. A 1995 study in Thyroid examined whether reverse T3 could distinguish a genuinely hypothyroid sick patient from a euthyroid sick one, and found that it did not do so reliably.

So reverse T3 is worth understanding as a mechanism and worth treating carefully as a test. It tells you something about the state your body is in. It is not a clean readout of whether your thyroid needs help, and using it that way goes further than the data.

What moves it

Down: any significant systemic illness, which is the best documented cause. Prolonged calorie restriction, since the body lowers metabolic pace when energy is scarce, and this is an adaptation rather than a fault. Very heavy training loads without adequate fuelling. Shortfalls in the nutrients conversion depends on, particularly selenium, zinc and iron. Liver and kidney disease, since much of the conversion happens there. Some medications, including certain beta blockers and amiodarone.

Up: treating whatever is suppressing it. Eating enough. Correcting a genuine nutrient shortfall. And, where a diagnosed thyroid condition exists, prescribed thyroid hormone, which is a decision for a physician rather than something to approach through supplements.

The honest hedge

I am not a doctor and I am not diagnosing anyone.

Free T3 is genuinely useful and it is also the marker most often over-interpreted in wellness writing. A low free T3 in someone who is unwell, under-eating, or training hard is frequently the body doing something sensible rather than something broken, and treating the number without addressing the reason misses the point entirely.

The other thing I would say plainly: if you are on thyroid medication, dose decisions belong with your prescriber. Adding T3 to T4 therapy is a real clinical conversation with genuine trade-offs, and it is not something to pursue on the basis of a number and a website.

What free T3 is good for is completing the picture. It is a variable, and it is one a TSH alone cannot see.

So. What to actually do.

Tonight, at no cost. Look at your last thyroid result. If it says TSH and nothing else, you have measured the signal and not the hormone.

When you test, run the panel rather than the screen. A thyroid panel covering TSH, free T4 and free T3, with antibodies alongside. Four numbers, one draw, and each makes the others readable.

Before chasing a low result, check the obvious. Are you eating enough, training sensibly, and recovering? Those move this marker more than most people expect, and they are free to address.

Check the nutrients conversion depends on. Selenium, zinc and iron. A shortfall in any of them is a plausible and correctable contributor.

Take the whole panel to someone who reads it as a system. A good functional medicine doctor, or an endocrinologist if a genuine thyroid condition is on the table.

None of this has to happen this week. But tonight you can check whether the active hormone has ever been measured at all.

You are not slow because your thyroid is lazy. You are a conversion process with several steps, running on nutrients and enough food, measured almost everywhere by a test that stops one step too early.

Your body is not broken. It is blocked. And sometimes the block is a conversion nobody has looked at, downstream of every number on the report.

Go see whether anyone measured the active one.