TSH is a hormone, a chemical messenger that tells the thyroid gland how much hormone to make. TSH comes from the pituitary gland at the base of the brain. A blood test measures it to help assess how the thyroid is working.
A high TSH often suggests that the thyroid is not making enough hormone. A low TSH often suggests the opposite. These patterns have exceptions, including some pituitary problems. TSH alone does not show the cause.
A clinician may also measure T4, called thyroxine, a hormone made by the thyroid. A free T4 test measures the part not attached to blood proteins. Looking at the results together gives more information than TSH alone.
Symptoms, medicines, serious illness and pregnancy can affect the assessment. Tell the clinician about medicines and supplements you take. Do not stop prescribed treatment just to change a test result.

BaluMed makes medicine understandable! Our extensive medical report explanation 'translates' complex medical terms from MRI findings and discharge summaries into easily understandable language. With a combination of artificial intelligence and medical expertise, we offer quickly available and reliable information.