TSH vs Free T4: what's the difference?
Educational — not medical advice · Ranges vary between laboratories
The short answer: TSH is the pituitary's signal telling the thyroid how hard to work. Free T4 is the active thyroid hormone actually available to your tissues. Because TSH rises when thyroid hormone runs low, the two usually move in opposite directions.
TSH works like a thermostat: when thyroid hormone runs low, TSH rises to push the gland harder. That's why TSH moves opposite to how the thyroid itself is performing.
Full TSH guideFree T4 is the portion of thyroxine, one of the two main thyroid hormones, that circulates unattached to carrier proteins and is available for the body's cells to use. It reflects how much active thyroid hormone is currently on hand.
Full Free T4 guideAt a glance
| TSH | Free T4 | |
|---|---|---|
| Panel | Thyroid panel | Thyroid panel |
| Unit | µIU/mL | ng/dL |
| Typical adult range | Normal 0.4–4 µIU/mL | Normal 0.8–1.8 ng/dL |
| Also known as | Thyroid Stimulating Hormone, Thyrotropin | free thyroxine, FT4 |
General adult reference intervals — your lab's printed range takes precedence.
Why they're often tested together
Reading them as a pair is what separates a thyroid gland issue from a pituitary-level one, since each produces a different pattern. Many evaluations start with TSH alone and add free T4 when the TSH is abnormal — a mildly raised TSH with a normal free T4 is often rechecked in 6–12 weeks rather than acted on straight away.
Questions people ask
Why does TSH move opposite to thyroid hormone?
TSH works like a thermostat. When thyroid hormone runs low, the pituitary raises TSH to push the gland harder; when hormone is plentiful, TSH falls back. So a high TSH generally signals an underactive thyroid, not an overactive one.
Is TSH on its own enough?
It's usually the first test, and a normal result is reassuring for most people. Free T4 gets added when TSH is out of range, when symptoms and TSH disagree, or when a pituitary-level cause is being considered.
More comparisons
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