Free T4

Your thyroid makes two hormones: mainly T4, with four iodine atoms, and a little T3, with three. T4 is the storage form. It travels through your blood clamped to proteins and is converted in your liver, kidneys and muscles into T3, the hormone that really sets your cells to burning. But a small part of the T4, less than a percent, is not bound to protein, and only that free part can enter the cells. That is what free T4 measures: not how much hormone you carry, but how much is available right now. That free part is tightly guarded. If it falls, your pituitary sends more TSH and the thyroid makes more; if it rises, the pituitary goes quiet. So free T4 stays within the band for a long time while TSH is already moving: in a thyroid that is slowly declining, as in Hashimoto's, a rising TSH keeps free T4 up for years. Only when the thyroid can no longer keep up does free T4 fall below the band, and then there are usually symptoms. The other way round, a high free T4 with a suppressed TSH means the thyroid is making too much on its own. Your report shows free T4 in pmol/L. Normal between 30 and 40 years is about 12 to 20.5 pmol/L, where the labs overlap; one of the labs splits the band by decade, so the overlap shifts with your age. Above 50 pmol/L the requester is phoned the same day.

Free T4 is the value that says whether a deviating TSH already has consequences. A high TSH with a normal free T4 is a thyroid that is just keeping up; a high TSH with a low free T4 is a thyroid that no longer copes, and then the symptoms are usually there too: tired, cold, slow, gaining weight, a dry skin, constipation, a slow heartbeat. A low TSH with a high free T4 is a thyroid working too hard: restless, warm, losing weight, palpitations, trembling hands, poor sleep. The difference between keeping up and no longer keeping up decides whether you are treated or followed. For anyone taking levothyroxine, free T4 is the second value beside TSH to judge the dose. A free T4 at the top of the band with a suppressed TSH means too much; a free T4 at the bottom with a high TSH too little. And because a levothyroxine tablet temporarily lifts free T4 for a few hours after taking it, you have blood drawn before you take the tablet. The pitfalls are the same as with TSH, but the other way round. Biotin from supplements gives a falsely high free T4, iodine from medicines or contrast fluid genuinely lifts it, and a serious illness or a hospital stay lowers it temporarily without anything wrong with the thyroid. And the rare case in which TSH and free T4 point the same way, both low or both high, points not to the thyroid but to the pituitary.

A low free T4 means the thyroid delivers too little, or something is in between. Usually it is the first and rarely the last: an underactive thyroid from Hashimoto's, in which TSH is high and TPO antibodies confirm the diagnosis; too low a levothyroxine dose, or tablets not absorbed well because of coffee, calcium, iron or an acid-reducing medicine; the period after a thyroid treatment with radioactive iodine or an operation; and medicines such as lithium, amiodarone and some drugs for epilepsy. Rare are a serious illness, which lowers free T4 temporarily, and a problem in the pituitary, recognisable by a TSH that does not rise while free T4 is low. A high free T4 means too much hormone is available. Too high a levothyroxine dose is the most common reason in anyone taking tablets, and Graves' disease, in which antibodies drive the thyroid, in anyone who does not. After that come an overactive nodule, an inflammation of the thyroid that temporarily leaks hormone, iodine from medicines or contrast fluid, and biotin, which falsifies the measurement. Rare is a growth in the pituitary that keeps making TSH; then TSH and free T4 are both high. Above 50 pmol/L the value is assessed the same day, because the heart can then lose its rhythm. Free T4 belongs beside TSH, because only together do they say whether it is about the thyroid or about the pituitary and whether a deviation already works through. With a low free T4 with a high TSH, TPO antibodies belong with it; with a high free T4 with a low TSH a free T3, because sometimes the thyroid mainly makes too much T3.