Thyroid Tests
Answers to common questions about thyroid tests.
Questions on this page
What is TSH?
TSH (thyroid-stimulating hormone) is produced by the pituitary gland and regulates thyroid hormone production.
It is usually the first blood test used to evaluate thyroid function.
What does a high TSH mean?
A high TSH commonly suggests that the thyroid is not producing enough thyroid hormone.
However, interpretation depends on the free T4 level and the clinical situation.
What does a low TSH mean?
A low TSH may occur when the thyroid is producing too much hormone.
It can also occur because of certain medications, pregnancy-related changes, pituitary disorders or other conditions.
What is T4?
T4 is the main hormone produced by the thyroid gland.
What is free T4?
Free T4 represents the unbound fraction of thyroxine available to tissues.
What is T3?
T3 is the more biologically active thyroid hormone. It can be particularly useful when hyperthyroidism is suspected.
Why can TSH be normal when I have symptoms?
Symptoms such as tiredness, weight change or hair loss are not specific to thyroid disease.
A normal thyroid profile makes a thyroid explanation less likely, but the symptoms may have another cause.