Lab test, explained · Reviewed by the Medical Records Vault clinical team · Last reviewed June 2026
What is TSH? Your thyroid test, explained
TSH (thyroid-stimulating hormone) is a signal from the brain telling the thyroid how much hormone to make. It is the usual first test of thyroid function. A high TSH can suggest an underactive thyroid and a low TSH an overactive one, but TSH is interpreted with other thyroid values and your symptoms.
- Measures
- The brain's signal to the thyroid gland
- High TSH may suggest
- An underactive thyroid (hypothyroidism)
- Low TSH may suggest
- An overactive thyroid (hyperthyroidism)
How the signal works
Your brain's pituitary gland releases TSH to tell the thyroid how much hormone to make. Because it is a feedback signal, TSH often moves in the opposite direction to thyroid hormone: when the thyroid makes too little, the brain pushes harder and TSH rises; when the thyroid makes too much, TSH falls.
Reading your result
A high TSH can suggest an underactive thyroid (hypothyroidism) and a low TSH an overactive one (hyperthyroidism). TSH is usually the first thyroid test, and an out-of-range value is often confirmed with T3 and T4 and read alongside your symptoms.
Questions to ask your doctor
Bring these to your next visit. They are written to help you understand what was already decided — not to second-guess your doctor.
- 1.What is my TSH, and is it outside the range you expect for me?
- 2.Do I need further thyroid tests (T3, T4) to interpret this?
- 3.Should this be repeated, and when?
Common questions
- What does a high TSH mean?
- A high TSH often suggests the thyroid is underactive (hypothyroidism), because the brain is sending a stronger signal to a gland that is making too little hormone. It is confirmed alongside other thyroid values and your symptoms, so ask your doctor to interpret it.
This is an educational explanation, not a diagnosis or medical advice. Medical Records Vault explains what documents say and what is commonly done; it never tells you what you have or what to change. Always consult your doctor for any medical decision.
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