TSH Interpreter.
Your lab calls a wide TSH range "normal," but optimal is narrower. Enter your TSH — and free T4 if you have it — to see your thyroid tier and what it means.
Why "normal" TSH is wider than optimal.
TSH (thyroid-stimulating hormone) rises when the pituitary senses the thyroid is underperforming, so it moves inversely to thyroid output — a high TSH usually means an underactive thyroid. Lab reference ranges (roughly 0.4–4.5 mIU/L) were built from broad populations that include people with undiagnosed early thyroid disease, which is why many thyroid-aware clinicians treat 0.4–2.5 as the functionally optimal band.
The number is only half the story. A raised TSH with a normal free T4 is "subclinical"; with a low free T4 it is overt hypothyroidism. TPO antibodies reveal whether autoimmune Hashimoto's is driving it. And TSH is genuinely noisy — it shifts with time of day and illness, and biotin supplements can distort the assay — so a single borderline value should be rechecked, not acted on.
- Garber JR, Cobin RH, Gharib H, et al. (2012). Clinical practice guidelines for hypothyroidism in adults (ATA/AACE). Thyroid, 22(12), 1200–1235.
- Surks MI, Ortiz E, Daniels GH, et al. (2004). Subclinical thyroid disease: scientific review and guidelines for diagnosis and management. JAMA, 291(2), 228–238.
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