TSH

Your thyroid makes the hormones that set the pace of your metabolism: how fast your cells burn energy, how warm you feel, how fast your heart beats and how sharp you think. How much it makes is decided by your pituitary, a gland under your brain, with TSH. If the thyroid delivers too little, the pituitary sends more TSH to drive it; if it delivers too much, the pituitary stops sending. TSH therefore moves the opposite way to the thyroid, and that is the first thing to know to read the value. The second is that the pituitary is oversensitive. A small fall in thyroid hormone gives a large rise in TSH, and so TSH responds long before free T4 leaves the band. That makes TSH the more sensitive value of the two, but also the value that often deviates without anything being wrong: after an illness, in recovery, with age, with a lack of sleep, and with biotin from supplements, which disturbs the measurement itself. A TSH between the upper limit and 10 mIU/L with a normal free T4 occurs in one in twenty adults, and in most of them it returns to normal by itself. Your report shows TSH in mIU/L. Healthy for most adults sits roughly between 0.4 and 4, though every lab sets the limits differently and they shift up with age. Above 10 mIU/L TSH returns to normal by itself far less often, below 0.1 mIU/L the thyroid is overactive or the dose of thyroid hormone too high.

A thyroid that goes out of balance does so insidiously, and the symptoms look like ordinary tiredness. An underactive thyroid makes you tired, cold, slow in thought, makes you gain weight, gives a dry skin, constipation and a slow heartbeat; an overactive thyroid makes you restless, warm, makes you lose weight, gives palpitations, trembling hands and poor sleep. Both are common, especially in women, and both are very treatable. TSH is the value that makes the difference between a suspicion and a diagnosis, and it is the reason TSH goes with almost every blood test. For anyone taking thyroid hormone, TSH is the value the dose is tuned on. A TSH that stays too high means the dose is too low or the tablets are not absorbed well, which happens if you take them with coffee, calcium, iron or an acid-reducing medicine. A TSH that becomes suppressed means a dose that is too high, and in the long run that costs bone density and lifts the risk of atrial fibrillation. The goal is a TSH in the band, measured at least six weeks after every adjustment. For anyone who is healthy, TSH is mainly a value not to read too much into. A slightly raised TSH with a normal free T4 and without symptoms is usually not a disease but a snapshot, and in older people a higher TSH is simply age. What does count: the trend over the years, whether there are antibodies, and whether you have symptoms that fit.

A low TSH means the pituitary is slowing the thyroid because too much hormone is circulating. The most common explanation first: an overactive thyroid, usually Graves' disease or an overactive nodule; too high a dose of thyroid hormone, the most common reason in anyone taking tablets; an inflammation of the thyroid, which temporarily leaks hormone and afterwards often swings to underactive; and biotin from supplements, which disturbs the measurement without the thyroid doing anything. Below 0.1 mIU/L with a high free T4 the thyroid is overactive, and then treatment belongs with it. A low TSH with a normal free T4 is repeated after three months. A high TSH means the pituitary is driving the thyroid because it delivers too little. An underactive thyroid, usually Hashimoto's, in which the immune system slowly breaks the thyroid down, is the most common reason; with a low free T4 alongside that is the diagnosis, and the treatment is thyroid hormone in tablet form. Age lifts TSH without disease: above seventy the upper limit sits a good deal higher at some of the labs. Recovery after an illness or an operation lets TSH shoot up for a few weeks. Rare are too little or too much iodine. With a normal free T4 the thyroid still delivers enough hormone, and TSH usually returns to normal by itself; above 10 mIU/L that happens far less often. Read TSH beside free T4, because TSH says what the brain asks and free T4 what the thyroid delivers, and together they say whether it is about the thyroid or about the pituitary. With a raised TSH, TPO antibodies belong with it, which say whether it is Hashimoto's, and CRP if you have just been ill. An underactive thyroid also lifts LDL.