How to read TSH
TSH is a message from the brain telling the thyroid to work harder. Counter-intuitively, a high TSH usually means an underactive thyroid (the brain is 'shouting' at a sluggish gland), while a low TSH usually means an overactive thyroid.
T4 is the actual thyroid hormone and helps confirm the picture: low T4 with high TSH points to an underactive thyroid; high T4 with low TSH points to an overactive thyroid.
What do the results lead to?
An underactive thyroid (hypothyroidism) is often treated with levothyroxine and monitored with repeat TSH. An overactive thyroid (hyperthyroidism) needs further assessment and specific treatment.
Mild or borderline ('subclinical') results are common and are often simply rechecked after a few weeks before any decision, as TSH can fluctuate.
Common questions
Why does a high TSH mean an underactive thyroid?
TSH is the brain's signal to the thyroid. When the thyroid is underactive, the brain raises TSH to try to stimulate it — so a high TSH points to an underactive gland.
Related tools
These calculators are designed for healthcare professionals.