Lab test glossary · Thyroid
TSH: what it means
Also known as: thyroid stimulating hormone, thyrotropin
TSH is the pituitary's signal to the thyroid and the primary screen for an under- or over-active thyroid — read counterintuitively, since high TSH usually means low thyroid activity.
What it measures
TSH (thyroid-stimulating hormone) is made by the pituitary gland to tell the thyroid how much hormone to produce. It works like a thermostat: when thyroid hormone is low, the pituitary raises TSH to push the thyroid harder — so a high TSH usually points to an underactive thyroid, and a low TSH to an overactive one.
Typical range
Often roughly 0.4–4.0 mIU/L, though the 'right' range is debated and shifts in pregnancy and with age. Your clinician interprets it with your symptoms and, often, free T4.
Reference ranges differ between labs and are printed on your own report — always read your result against that range, not a general one.
What a high result can mean
- Underactive thyroid (hypothyroidism), including Hashimoto's
- Recovering from illness
- Certain medications
A high value is a starting point for a conversation, not a diagnosis — many causes are benign or temporary.
What a low result can mean
- Overactive thyroid (hyperthyroidism), including Graves' disease
- Too much thyroid-hormone replacement
- Pituitary problems (less common)
Why the trend matters
TSH can drift gradually, and it's followed over time to fine-tune thyroid-hormone dosing — the trend is central to managing a known thyroid condition.
When to talk to your doctor
A TSH outside the reference range, or one that's changing over successive tests, is worth reviewing with your doctor, especially with fatigue, weight changes, or temperature intolerance.
Frequently asked questions
What does a high TSH mean?
A high TSH usually points to an underactive thyroid (hypothyroidism) — the pituitary raising its signal because thyroid hormone is low. It's interpreted together with free T4 and your symptoms.
What does a low TSH mean?
A low TSH usually points to an overactive thyroid (hyperthyroidism), or sometimes to taking too much thyroid-hormone replacement.
Why does TSH seem to work backwards?
TSH is the pituitary's signal telling the thyroid how hard to work, so it moves opposite to thyroid activity: a high TSH generally means low thyroid function, and a low TSH means high.
This page is general education about a common lab test — it isn’t medical advice and can’t interpret your specific result. What a value means depends on your history, symptoms, and the rest of your labs, which is a conversation for you and your clinician.