Thyroid Function Tests
TFTs / TSH / T3 / T4
Thyroid function tests (TFTs) measure the levels of thyroid hormones and thyroid-stimulating hormone (TSH) in the blood. They are used to diagnose and monitor thyroid disorders including hypothyroidism, hyperthyroidism, ...
About this Test
Thyroid function tests (TFTs) measure the levels of thyroid hormones and thyroid-stimulating hormone (TSH) in the blood. They are used to diagnose and monitor thyroid disorders including hypothyroidism, hyperthyroidism, thyroiditis, and thyroid nodules. The TSH test alone is the most sensitive initial screening test; it is supplemented with free T4 and free T3 for a complete picture.
Why is this test ordered?
- Suspected hypothyroidism (fatigue, weight gain, cold intolerance, constipation)
- Suspected hyperthyroidism (weight loss, heat intolerance, palpitations, anxiety)
- Monitoring thyroid replacement therapy (levothyroxine)
- Screening in women over 35 or during pregnancy
- Infertility or irregular menstrual cycles
- Unexplained depression, anxiety, or cognitive changes
- High blood cholesterol (hypothyroidism can cause this)
- Monitoring after thyroid surgery or radioactive iodine treatment
How to Prepare
- β’No fasting required for TSH alone
- β’Inform doctor of all medications β many affect thyroid hormone levels (biotin supplements should be stopped 72h before)
- β’Ideally take thyroid medication after, not before, the blood draw on the day of testing
- β’Time of day matters for TSH β levels are slightly higher in the morning
The Procedure
Blood is drawn from a vein in the arm. TSH is always measured first; if abnormal, the laboratory may reflexively add free T4 and free T3 measurement. Results are typically available within 24β48 hours. In pregnancy or thyroid disease monitoring, free T4 is typically included with TSH.
Normal Reference Ranges
TSH: 0.4β4.0 mIU/L (some labs 0.5β5.0 mIU/L). Free T4: 0.8β1.8 ng/dL (10β23 pmol/L). Free T3: 2.3β4.2 pg/mL (3.5β6.5 pmol/L). Note: reference ranges vary by laboratory and trimester in pregnancy.
Reference ranges may vary between laboratories. Always interpret results with your doctor.
Interpreting Your Results
When results are HIGH
High TSH with low free T4 = primary hypothyroidism (underactive thyroid). High TSH with normal free T4 = subclinical hypothyroidism. High free T3/T4 with low TSH = hyperthyroidism. High TSH with high T4 = rare (TSH-secreting pituitary tumor).
When results are LOW
Low TSH with high T3/T4 = primary hyperthyroidism (Graves' disease, toxic nodule). Low TSH with normal T3/T4 = subclinical hyperthyroidism. Low TSH with low T4 = secondary (pituitary) hypothyroidism β rare. TSH < 0.1 suppressed on levothyroxine therapy may require dose adjustment.
Frequently Asked Questions
What TSH level indicates hypothyroidism?
TSH above 4.0β5.0 mIU/L (depending on lab reference range) with a low free T4 is diagnostic for hypothyroidism. Subclinical hypothyroidism is defined as elevated TSH with normal free T4. Treatment decisions depend on the degree of elevation, symptoms, and individual factors.
Should I stop my thyroid medication before a blood test?
It is generally recommended to have your blood drawn before taking your morning levothyroxine dose. Taking the medication just before testing can cause a temporary spike in free T4 levels, giving a misleadingly elevated result. However, this has minimal effect on TSH. Ask your doctor for specific instructions.
Can supplements affect thyroid test results?
Yes. Biotin (vitamin B7) supplements at high doses can interfere with thyroid immunoassays, causing falsely abnormal TSH and thyroid hormone results. Stop biotin supplements at least 72 hours before thyroid testing. Other supplements like iron and calcium can affect levothyroxine absorption but not the blood test itself.
Quick Reference
- Test name
- Thyroid Function Tests
- Abbreviation
- TFTs / TSH / T3 / T4
- Category
- Endocrinology
- Turnaround
- 24β48 hours. Urgent TSH results available within hours in hospital settings.
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