Basic hormone tests that assess whether the thyroid gland is functioning adequately, underactive, or overactive; the first step in evaluating fatigue, weight changes, and goiter findings.
Indication
- Suspicion of hypothyroidism: fatigue, weight gain, cold intolerance, constipation
- Suspicion of hyperthyroidism: palpitations, weight loss, heat intolerance, tremor
- Detection of goiter or thyroid nodule
- Pre-pregnancy and pregnancy follow-up
- Monitoring efficacy of levothyroxine or antithyroid therapy
- Investigation of infertility, menstrual irregularity, and depression
- Follow-up of autoimmune thyroid disease (Hashimoto, Graves)
Preparation
- Fasting is not required; morning collection is preferred
- If on therapy, the morning dose of levothyroxine is taken after the blood draw
- Biotin-containing supplements should be discontinued at least 48 hours in advance
- The test is postponed in case of acute illness or infection
- Medications in use (amiodarone, lithium, estrogen) should be reported to the physician
How it's performed
- Approximately 3-5 mL of blood is drawn from a vein with a single-use needle
- The sample is transferred to a serum tube and centrifuged
- Hormone analyses are measured by chemiluminescence or immunoassay method
- TSH, fT3, and fT4 levels are reported separately
- If needed, additional tests such as anti-TPO, anti-Tg, TRAb may be ordered
- Results are usually evaluated on the same day
Post-procedure
- TSH reference range is generally 0.4-4.0 mIU/L; trimester-specific targets exist in pregnancy
- fT4 and fT3 reference values vary by laboratory technique
- Hypothyroidism: TSH high, fT4 low; in subclinical form fT4 is normal
- Hyperthyroidism: TSH suppressed, fT4/fT3 high
- In levothyroxine therapy, follow-up every 6-8 weeks is recommended to reach target TSH
Risks
- Temporary bruising at the blood draw site
- Rarely vasovagal lightheadedness
- Biotin use may cause falsely high/low results
- In acute illness, transient 'sick euthyroid' findings may be misleading
FAQ
Is TSH alone sufficient?
For screening purposes, TSH is the starting point in most cases; if abnormal, fT4 and if needed fT3 are added. In pituitary disease or pregnancy, the combination may be recommended from the outset.
Can the test be done on the day of levothyroxine?
Yes, but the medication dose should be taken after the blood draw; otherwise, the fT4 value may appear artificially high.
How often are results repeated?
Without treatment, every 1-2 years; at the start of treatment every 6-8 weeks, and every 6-12 months in the stable period.
Why does biotin affect the test?
High-dose B7 (biotin) supplements can interfere with the immunoassay technique and cause hormone results to appear artificially high/low; they should be discontinued at least 48 hours in advance.
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