TSH, free T4 and free T3, in plain English

The three main tests

A standard thyroid panel usually includes two or three values. It helps to understand what each one is actually detecting.

TSH - the pituitary's signal

TSH (thyroid-stimulating hormone) is not a thyroid hormone at all. It is produced by the pituitary gland - a small gland at the base of the brain - and its job is to tell the thyroid how much T4 and T3 to make. Think of it as a thermostat signal rather than the temperature itself.

When thyroid hormone levels fall, the pituitary turns the signal up (TSH rises). When levels are high, the signal drops (TSH falls). This is why TSH is often the most sensitive early indicator that something is changing - the pituitary picks up shifts before they become large enough to move T4 or T3 outside their ranges.

Free T4 - the thyroid's main output

Free T4 (free thyroxine) is the main hormone the thyroid secretes. Most T4 in the blood is bound to carrier proteins and cannot enter cells; "free" T4 is the small, biologically active fraction that tissues can actually use.

Free T4 is essentially a measure of the thyroid's production. It is checked alongside TSH to give a clearer picture of what the thyroid is actually doing, not just what the pituitary is asking for.

Free T3 - the active hormone

Free T3 (free triiodothyronine) is the most potent thyroid hormone. Most T3 is not made directly by the thyroid - the body converts T4 into T3 in tissues such as the liver and kidneys as it is needed.

Free T3 is not always included in routine tests. It tends to be measured when hyperthyroidism is suspected but free T4 is normal, or in certain specialist monitoring situations. Some clinicians and patients advocate for broader T3 testing; this remains a debated area. If you have questions about whether T3 testing is relevant to you, it is worth raising with your clinician.

Why TSH and T4 move in opposite directions

The inverse relationship between TSH and free T4 confuses many people when they first see their results. If the thyroid is underactive and free T4 is low, the pituitary responds by raising TSH - pushing harder to stimulate production. If the thyroid is overactive, free T4 is high and TSH drops - the pituitary backs off.

This is normal feedback biology. It means a high TSH paired with a low free T4 suggests an underactive thyroid, while a low TSH paired with a high free T4 suggests an overactive one. Your own report's reference range is the benchmark - not numbers you find online.

Why one result rarely tells the whole story

Each test has limitations when read in isolation:

• TSH can be suppressed for reasons unrelated to thyroid overactivity (recent illness, some medications, pituitary problems)
• Free T4 alone does not capture how much T3 the body is actually producing from it
• Free T3 fluctuates more over the day than TSH or free T4
• Reference ranges vary between laboratories - a result just outside one lab's range may sit inside another's

The pattern across all three values, alongside your symptoms and history, is what helps your clinician form a picture. No single number should be read as a verdict.

What to do with your results

The lab results tool lets you log and track your values over time, always compared against your own report's reference range - not a generic internet range. Seeing the trend is often more informative than any single result.

If a result looks unexpected, write down what you noticed and bring it to your next appointment. A good starting point is the questions tool, which can help you frame what you want to ask.

Questions worth raising with your clinician

• Which tests are routinely included in my monitoring, and why?
• Should free T3 be measured in my case?
• If my TSH is outside the range, does my free T4 also need to be checked?
• Are my results trending in a particular direction over time?