Total T4 (thyroxine)
Also called: Total T4, T4 total, Total thyroxine, Thyroxine total, Thyroxine
All the thyroxine in blood — the small free fraction plus the much larger part bound to carrier proteins. Usually read next to TSH, not alone.
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What it measures
Thyroxine is the main hormone the thyroid produces, and it sets the pace of the body's basic metabolism. Total T4 counts all of it: the fraction travelling free in the blood, and the far larger fraction bound to carrier proteins and therefore inactive for the moment.
Only the free part enters cells and drives metabolism, and it is a very small share — our source puts it at 0.03% of all the thyroxine in blood. That is why a total T4 moves whenever the carrier proteins move, pregnancy and the contraceptive pill among the commonest reasons, without the thyroid itself having changed. Free T4 is the measurement that answers "is the thyroid working"; total T4 answers "how much hormone is present in all".
Typical reference ranges
Roughly 4.5–12.5 µg/dL in adults, equivalent to about 65–160 nmol/L, per the Israeli patient reference we cite.
⚠ Those two figures are rounded independently and do not convert into each other exactly: 12.5 µg/dL maps onto about 160 nmol/L, but 4.5 µg/dL maps onto about 58, not 65. Treat each unit's range as belonging to the unit your own report uses.
The range printed beside your value is the one that counts. Pregnancy and oestrogen-containing medication raise total T4 without the thyroid having changed.
What a high value can mean
- An overactive thyroid, which is the reading the test is most often ordered to look for.
- Acute thyroiditis, an abrupt inflammation of the gland, which can push stored hormone into the blood.
- More carrier protein than usual — pregnancy, oestrogen-containing medication — where the total rises while the free fraction, and the person, stay unchanged.
This is why the figure is read beside TSH and free T4 rather than on its own: those three together separate a thyroid that is genuinely fast from a carrier-protein effect.
What a low value can mean
- An underactive thyroid, the gland producing less hormone.
- An underactive pituitary, which stops the gland being asked to produce.
- Less carrier protein than usual, from liver or kidney disease or protein loss, where the total falls without the free fraction following.
A low total T4 with a normal TSH more often points at the carrier proteins than at the thyroid.
Symptoms to look out for
Total T4 inside its range causes nothing to be felt. Where the thyroid itself is genuinely fast or slow, the symptoms belong to that state rather than to the number: weight and heart-rate change, heat or cold intolerance, tiredness, dry skin or sweating, altered bowel habit.
A total T4 shifted only by carrier proteins produces no symptoms at all, which is exactly why it is not read alone.
Many out-of-range results cause no symptoms at all - this is context, not a diagnosis.
When to discuss with a doctor
A single out-of-range total T4 is rarely conclusive on its own — the usual next step is TSH and free T4 together, and your doctor may simply repeat the set. We'd suggest talking to your doctor sooner if the result comes with a racing or irregular heartbeat, unexplained weight change in either direction, persistent heat or cold intolerance, trembling hands, or a swelling at the front of the neck. If you are pregnant, or taking oestrogen, biotin supplements or amiodarone, mention it — each shifts this test for reasons that have nothing to do with the gland.