TSH, thyrotropin, is made by the pituitary gland and tells the thyroid how much hormone to release. Because the pituitary reacts to the level of thyroid hormone in the blood, TSH moves in the opposite direction to it and is the most sensitive early signal of a shift in thyroid status. It reports on the pituitary–thyroid loop as a whole, not on the gland in isolation.
No readings for this marker yet.
A raised TSH most often means the pituitary is pushing a thyroid that is producing too little — the picture of an underactive gland, frequently autoimmune in origin. A lowered TSH most often accompanies an excess of thyroid hormone, whether from the gland itself or from a replacement dose set higher than needed. Both directions can also appear temporarily during and after unrelated illness, so an isolated abnormal value calls for a repeat and a doctor's reading rather than a conclusion.
TSH follows a daily rhythm, peaking at night and falling in the afternoon, so repeat tests are best drawn at a similar hour; the timing of a levothyroxine dose does not meaningfully affect it, while high-dose biotin supplements, pregnancy, and any acute illness can all distort it. It is interpreted together with free T4 and free T3, and with TPO and thyroglobulin antibodies when the cause of a shift is in question.
Reference ranges are general adult values and depend on lab, assay, age and sex. For orientation, not diagnosis — discuss results with your doctor.