TSH

What is TSH?

TSH (thyroid-stimulating hormone) is the hormone your pituitary gland releases to tell your thyroid how much thyroid hormone to make. It works as a thermostat: when circulating thyroid hormone (mainly T4 and T3) drops, the pituitary turns TSH up to push the thyroid harder; when thyroid hormone is plentiful, TSH falls. Because of this feedback loop, TSH moves in the opposite direction to actual thyroid output — a counter-intuitive but important point when you read your result. TSH is reported in milli-international units per litre (mIU/L). The exact cut-offs are lab-dependent and shift with age and pregnancy; most laboratories set and report their own interval, so always read your value against the range printed on your result. This sensitivity is exactly why TSH is the standard first-line screen for thyroid disease: small changes in thyroid hormone produce large, early swings in TSH, so it often flags a problem before T4 itself has clearly moved. TSH is rarely interpreted entirely on its own. In the Optimize Baseline, TSH is the primary thyroid marker, and free T4 is added when TSH is abnormal to clarify what is actually happening — the two together separate a genuine thyroid problem from a borderline blip.

Why is TSH relevant?

TSH matters because the thyroid sets the pace of your whole metabolism, and an over- or under-active gland is common, often silent, and very treatable once found. The symptoms people search for — fatigue, weight changes, feeling cold, low mood, hair thinning, palpitations, or racing thoughts — are exactly the picture a single TSH can start to explain, which is why it is almost always the first marker checked. The direction tells the story. A high TSH means your brain is shouting at an underperforming thyroid (the pattern in hypothyroidism), while a low or suppressed TSH means the thyroid is running ahead of demand (the pattern in hyperthyroidism). Clinicians describe an in-between zone as 'subclinical': a raised TSH with a still-normal free T4 is subclinical hypothyroidism, and a low TSH with normal free T4 is subclinical hyperthyroidism. Either way the first step is to confirm it with a repeat test a few weeks to a couple of months later, because a single off value often settles on its own. For a mildly raised TSH the chance of it normalising is good, and the risk of progressing to overt hypothyroidism only becomes meaningful as TSH climbs above roughly 10 mIU/L — so a mildly high TSH is usually watched rather than treated straight away. A low TSH is handled a little differently: because a persistently suppressed thyroid carries a real risk to heart rhythm and bone, the threshold for acting is lower, especially in older adults and when TSH falls below 0.1 mIU/L, so a confirmed low TSH is reviewed with a clinician rather than simply observed. Reference ranges vary between laboratories and life stages. In pregnancy, different, trimester-specific TSH targets apply; in older adults, a somewhat higher TSH can be normal, which is why age-adjusted cut-offs are used to avoid over-diagnosis. A markedly abnormal value — for example TSH well above 10 mIU/L, or a fully suppressed TSH below 0.1 mIU/L — is more clearly significant and should be reviewed with a clinician alongside free T4.

TSH high or low — what it means

A single TSH is a snapshot, and several everyday things move it. TSH follows a 24-hour rhythm and is highest overnight and in the early morning, then falls through the day, so the time of your blood draw matters when comparing results. Recent illness, a course of high-dose steroids, and even biotin supplements (common in hair and nail products) can distort the reading, so it is worth pausing biotin for at least a couple of days before testing — high-dose biotin typically lowers the measured TSH in many lab assays and can falsely mimic an overactive thyroid. For a trustworthy picture, a borderline or abnormal TSH is best repeated a few weeks later rather than acted on from one value. A high TSH usually points to an underactive thyroid (hypothyroidism). The most common cause is Hashimoto's, an autoimmune thyroiditis, but iodine imbalance, previous thyroid surgery or radioactive iodine, certain medications (such as lithium or amiodarone), and the recovery phase after a viral thyroiditis can all raise it. Confirming the picture typically means free T4, and often thyroid antibodies (anti-TPO) to check for an autoimmune cause. A low or suppressed TSH usually points to an overactive thyroid (hyperthyroidism), most often from Graves' disease or an autonomous thyroid nodule, and sometimes from taking too much thyroid medication. Less commonly, a low TSH reflects a pituitary problem rather than the thyroid itself — which is why free T4 is needed to tell where the fault sits. Severe non-thyroidal illness can also transiently push TSH out of range without any true thyroid disease. TSH itself is not something you optimise with lifestyle the way you might lower CRP or LDL; the levers are mostly medical. Genuine hypothyroidism is treated with thyroid hormone replacement and re-checked at least 6–8 weeks after any dose change, because the system needs that long to reach a new steady state. Adequate but not excessive iodine, treating the underlying autoimmune or nodular cause, and avoiding biotin around testing are the realistic supporting steps — and any persistently abnormal TSH should be worked up with a healthcare professional rather than self-managed.

Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.