How the test is done
A blood sample only. No fasting is required for a thyroid panel.
TSH shifts through the day and is usually highest late at night and in the early morning. Taking the sample at a similar time each round gives a more consistent comparison between tests.
High-dose biotin supplements can interfere with thyroid immunoassays and produce misleading readings. Tell the doctor what supplements you are taking before the test is run.
How thyroid results are read
The TSH reference range most laboratories use is roughly 0.4 to 4.0 mIU/L, and the exact range differs by laboratory and assay method.
- High TSH with normal FT4 is called subclinical hypothyroidism. It does not automatically call for treatment, and that decision rests on symptoms, antibodies and a repeat test.
- Low TSH with high FT4 or FT3 points to an overactive thyroid and needs prompt assessment.
- Positive anti-TPO indicates an autoimmune process against the thyroid. Antibodies can be present before TSH moves, which makes them useful for judging risk earlier.
A single reading is rarely enough. Thyroid levels shift with acute illness, certain medicines, pregnancy and physiological stress, so borderline results are usually repeated before any conclusion is drawn.
