Free T3

Your thyroid makes two hormones. By far the most is T4, thyroxine, a storage form that does little. The active hormone is T3, and you make most of it not in your thyroid but in your liver, kidneys and muscles, by removing one iodine atom from T4. T3 switches on the metabolism in almost every cell: your warmth, your heart rate, your energy use, your bowels, your mood. Most T3 travels bound to protein and is inactive for the time being. Free T3 is the small loose part that can enter the cells, and that is what this test measures. What is special is that your body regulates that conversion from T4 to T3 itself, independently of your thyroid. With illness, fever, a crash diet or eating too little for a long time, the body turns the conversion down and breaks T3 down faster: free T3 falls, to save energy, while TSH and free T4 simply stay normal. That is not thyroid disease but an adaptation, and it disappears as soon as you eat normally again or recover. Your report shows free T3 in pmol/L. Normal is about 3.5 to 6.1 pmol/L from age 20 to 50 where the labs overlap, and from 50 the upper limit drops a fraction. With a healthy thyroid the value is stable from day to day.

For the question of whether your thyroid works well, free T3 is not the place to start. TSH is the thermostat and moves first, free T4 shows what the thyroid releases, and those two together tell almost everything. Free T3 rarely adds anything new to that, which is why it is not part of the ordinary thyroid work-up. Anyone who does have it on the report reads it as a supplement. What free T3 does show well is how your body responds to your lifestyle. On a strict diet, in a long period of eating too little, with overtraining or an illness, free T3 falls first, sometimes below the band, while TSH and free T4 stay normal. That is why on a crash diet you get cold, slow down and the weight stops dropping after a few weeks: your body turns the metabolism down. It is not your thyroid faltering, and a thyroid pill is then the wrong answer; more food and rest are the right one. A low free T3 with a normal TSH in anyone who trains, diets or is ill is therefore more often a sign of too little than of an underactive thyroid. The only situation in which free T3 adds something to TSH and free T4 is an overactive thyroid that starts with T3: free T3 raised, TSH suppressed, free T4 still normal. That occurs in early Graves' disease or an overactive nodule, and it is the reason the test exists. Anyone taking a thyroid preparation with T3 in it, or a supplement with biotin, also sees free T3 go up.

A low free T3 usually means your body has turned down the conversion from T4 to T3. In order of how often it occurs: illness, a crash diet or eating too little for a long time, an underactive thyroid that already shows in TSH and free T4, too low a dose of levothyroxine in anyone taking it, and rarely medicines that suppress production. If your TSH is normal, a low free T3 is almost never a thyroid problem, and the question is how you have eaten and lived in recent weeks. A high free T3 means more active hormone in circulation. With a suppressed TSH that is an overactive thyroid, and if free T4 is still normal, one that starts with T3. With a normal TSH it is usually what you take: a thyroid preparation with T3, or biotin, which disturbs the assay and makes free T3 read falsely high. Stop biotin a few days before the draw. Always read free T3 beside TSH and free T4, and never alone. Mention what you take, and compare over time at the same lab, because the ranges differ at the edges.