TSH (Thyroid Stimulating Hormone) Test
The TSH test measures Thyroid Stimulating Hormone produced by the anterior pituitary gland. It is the recommended first-line screening test for thyroid dysfunction, with elevated TSH indicating primary hypothyroidism and suppressed TSH indicating hyperthyroidism. TSH alone does not assess T4-to-T3 conversion or exclude central hypothyroidism.
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Biomarkers
Blood (serum)
Sample
1–3 business days
Turnaround
No — morning draw recommended for serial monitoring consistency
Fasting
What is the TSH test?
TSH is the single most important first-line thyroid screening marker. It reflects the pituitary gland's assessment of circulating thyroid hormone levels and is the recommended initial test for suspected thyroid dysfunction in primary care and functional medicine settings. An abnormal TSH triggers further investigation with Free T4 and Free T3 to characterise the type and severity of thyroid dysfunction. TSH alone is insufficient for comprehensive thyroid assessment but is the correct starting point for initial screening.
What does the TSH test measure?
TSH is produced by the anterior pituitary gland and acts as the primary regulatory signal for thyroid hormone production. Elevated TSH indicates the pituitary is compensating for insufficient thyroid hormone (hypothyroidism); suppressed TSH indicates excess thyroid hormone (hyperthyroidism).
First-line screening marker for thyroid dysfunction. TSH alone misses central hypothyroidism, T3 thyrotoxicosis, and subclinical presentations where Free T4 and Free T3 are needed for complete assessment.
Why is the TSH test ordered?
- Initial thyroid function screening in patients with fatigue, weight changes, or mood disturbances
- Routine monitoring of patients on levothyroxine or liothyronine therapy
- Pre-conception thyroid screening
- Annual screening in patients with family history of thyroid disease
- Post-treatment monitoring following radioactive iodine or thyroidectomy
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No — morning draw recommended for serial monitoring consistency
Collection method
Venipuncture at patient service centre
Turnaround
1–3 business days
Collection notes
Morning specimen preferred for serial monitoring. Biotin supplementation should be discontinued 48–72 hours prior.
What can affect TSH results?
High-dose biotin (>5mg/day) can interfere with streptavidin-biotin immunoassays, producing falsely low or high TSH depending on assay design. Discontinue biotin 48–72 hours before testing.
TSH peaks between midnight and 4am and is lowest in late afternoon. Morning draws are recommended for consistency in serial monitoring.
Acute illness suppresses TSH transiently. Defer thyroid testing until recovery from acute illness unless thyroid emergency is suspected.
What do TSH results mean?
Elevated
Primary hypothyroidism (most common), recovery phase of non-thyroidal illness, TSH-secreting pituitary adenoma (rare), or assay interference.
Normal
Euthyroid status in most cases. Does not exclude central hypothyroidism or early subclinical thyroid disease.
Reduced
Hyperthyroidism (Graves disease, toxic nodular goitre), central hypothyroidism (paradoxically low TSH with low Free T4), excessive thyroid hormone replacement, or first trimester pregnancy.
Note: TSH alone cannot diagnose central hypothyroidism, T3 thyrotoxicosis, or assess T4-to-T3 conversion. Always pair with Free T4 and Free T3 for comprehensive assessment.
Frequently asked questions
The conventional reference range is 0.4–4.0 mIU/L. Many functional medicine practitioners use a tighter optimal range of 1.0–2.5 mIU/L. Reference ranges vary slightly by laboratory.
How to access the TSH test
The TSH reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: January 15, 2025
Last reviewed: January 15, 2025
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.