By Dr Syed Kashif Hussain Kazmi, Consultant Endocrinologist. Published.View professional profile on Top Doctors.
TSH is the pituitary signal that tells the thyroid how hard to work. A high TSH with a low free T4 usually points to an underactive thyroid; a low TSH with a high free T4 usually points to an overactive one. Borderline results are often repeated, because levels fluctuate and one result rarely settles the question.
What does TSH measure?
Thyroid-stimulating hormone (TSH) is made by the pituitary gland at the base of the brain. When thyroid hormone levels fall, the pituitary releases more TSH to push the thyroid harder. When they are high, it releases less. That feedback is why TSH is usually the first thyroid test.
What do free T4 and free T3 add?
Free T4 and free T3 are the thyroid hormones themselves, measured in the blood. Read together with TSH, they show whether the thyroid is making too little, too much or a normal amount. Free T3 is usually added when an overactive thyroid is suspected.
What does a borderline or subclinical result mean?
Subclinical means the TSH is outside the usual range while free T4 is still normal. Levels can shift with illness, recovery from illness, some medicines and the time of day, so national guidance from NICE generally advises repeating the test before deciding on treatment. Whether treatment helps depends on how far the TSH is from normal, your symptoms, thyroid antibodies, age and any pregnancy plans.
What can affect thyroid test results?
Several things can make a single result misleading:
- A recent illness, or recovering from one
- Some medicines, such as amiodarone, lithium or steroids
- Pregnancy
- Biotin (vitamin B7) supplements, which can interfere with some laboratory methods. Tell the person taking your blood if you use them
When is a specialist review useful?
Your GP can manage most straightforward thyroid conditions. A specialist opinion can help when results stay borderline or keep changing, when symptoms do not fit the numbers, when thyroid antibodies are positive, when there is a lump or an enlarged thyroid, or when you are planning a pregnancy.
Common questions
Do I need to fast for a thyroid blood test?
Usually not. Follow the instructions you are given, and tell the person taking your blood about any supplements or medicines, including biotin.
Can stress or tiredness alone cause an abnormal thyroid result?
Everyday stress does not usually change thyroid tests, but illness, some medicines and pregnancy can. Persistent tiredness has many causes, which is why results are read alongside your symptoms.
Further reading
- NHS: Underactive thyroid (hypothyroidism)
- NHS: Overactive thyroid (hyperthyroidism)
- NICE guideline NG145: Thyroid disease, assessment and management
Clinical content awaiting consultant review.