TSH (thyroid-stimulating hormone) is made by the pituitary gland and tells the thyroid how much hormone to produce. It's the first and most useful thyroid blood test.
The result can feel backwards: a high TSH usually means an underactive thyroid (the body is shouting at a sluggish gland), while a low TSH usually means an overactive thyroid. Results are always interpreted alongside symptoms and sometimes free T4, not in isolation.