Thyroid
Thyroid-stimulating hormoneTSH
Pituitary feedback signal; the most sensitive screen for primary thyroid dysfunction.
Sample
Serum
Clinical reference range
0.4 to 4 mIU/L
Adult
Conv: 0.4–4 mIU/LFunc: 1–2.5 mIU/L
Biology
What it measures
Pulsatile pituitary glycoprotein under TRH control. Logarithmic relationship to free T4 — small changes in T4 produce large changes in TSH.
Clinical use
Why we order it
First-line thyroid screen and treatment monitor.
Lens · Clinical
Interpretation by lens
Clinical interpretation
Subclinical hypothyroidism (TSH 4.5–10 with normal free T4): treat if symptomatic, pregnant, infertility, or TPO antibodies.
Functional interpretation
Functional 'optimal' is 1.0–2.5 mIU/L; pregnancy-specific upper limits are 2.5 (1st), 3.0 (2nd/3rd) per ATA.
Research note
Population-based age-adjusted TSH ranges shift upward with age; the 97.5 percentile in adults >70 may be 6–7 mIU/L without disease.
Differential
Causes of abnormal values
Causes of HIGH
- ↑Primary hypothyroidism (Hashimoto's most common)
- ↑Recovery from non-thyroidal illness
- ↑Inadequate L-T4 dose
- ↑Drugs: amiodarone, lithium
- ↑Pituitary TSH-oma (rare)
Causes of LOW
- ↓Primary hyperthyroidism (Graves', toxic nodule)
- ↓Excess L-T4
- ↓Central hypothyroidism (with low free T4)
- ↓Pregnancy 1st trimester (β-hCG cross-reactivity)
Pitfalls
Pre-analytic & interpretation traps
- !TSH lags free T4 by 6–8 weeks — recheck at this interval after dose change.
- !Diurnal variation; morning values run higher.
- !Biotin supplements can interfere with immunoassays — hold ≥3 days.
Follow-up orders
Logical next-step labs
Evidence-graded claims
What the data says
A
TSH is the best initial test for thyroid dysfunction
All major endocrine guidelines.
E
Subclinical hypothyroidism (TSH 4.5–10) should always be treated
TRUST and other RCTs found no symptomatic benefit in mild cases.
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