TSH Test Explained: What a High or Low Thyroid Result Means

TSH is made by the pituitary gland at the base of the brain. It tells the thyroid gland in your neck to make thyroid hormones, mainly T4 and some T3, which set the pace of your metabolism. When thyroid hormone levels drop, the pituitary releases more TSH to push the thyroid harder; when they rise, TSH falls.

A high TSH usually means the thyroid is not making enough hormone, called hypothyroidism or an underactive thyroid. In many countries the most common cause is Hashimoto’s thyroiditis, an autoimmune condition. Symptoms can include tiredness, feeling cold, weight gain, constipation, dry skin and low mood, though many people with a mildly high TSH feel well.

A low TSH often means the thyroid is making too much hormone, called hyperthyroidism or an overactive thyroid, for example from Graves’ disease or thyroid nodules. It can also be low if you take more thyroid medication than you need. Symptoms can include a fast heartbeat, anxiety, weight loss, tremor, feeling hot and trouble sleeping.

If TSH is outside the range, the next test is usually free T4, and sometimes T3. A high TSH with a normal free T4 is called subclinical hypothyroidism; a high TSH with a low free T4 is overt hypothyroidism. Thyroid antibodies, such as TPO antibodies, can show whether an autoimmune cause is likely.

TSH can shift with illness, recovery from illness, pregnancy, some medicines (including steroids, amiodarone and lithium) and the time of day; it tends to be higher in the early morning. High-dose biotin supplements can interfere with some lab methods, so mention them before the test. A single slightly abnormal result is often repeated in six to twelve weeks.

People who take levothyroxine are usually rechecked six to eight weeks after a dose change, then about once a year when stable. Taking the tablet the same way each day, on an empty stomach and apart from calcium, iron and coffee, keeps levels steadier. Any dose change is a decision for you and your clinician.