What does the Thyroid-Stimulating Hormone test measure?

Why is Thyroid-Stimulating Hormone measured?

TSH is the first-line and most sensitive thyroid screening marker across all clinical contexts — primary hypothyroidism, subclinical hypothyroidism, hyperthyroidism, and subclinical hyperthyroidism are all identified first by TSH abnormality before free hormone levels become overtly abnormal. In a comprehensive thyroid panel, TSH serves as the gateway marker that contextualises the free thyroid hormone values — an elevated TSH indicates reduced thyroid hormone output or pituitary resistance; a suppressed TSH indicates excess thyroid hormone or pituitary suppression from exogenous thyroid hormone, thyroid autonomy, or thyrotoxicosis. TSH alone is insufficient for complete thyroid assessment — it does not identify the cause of thyroid dysfunction, does not reflect peripheral thyroid hormone activity, and cannot detect central hypothyroidism.

Reference ranges

Standard reference

0.45–4.50 mIU/L # REVIEWER FLAG: Confirm Access Medical Laboratories reference range for TSH — values vary between immunoassay platform generations. Third-generation TSH assays with functional sensitivity below 0.02 mIU/L are the clinical standard; confirm Access Medical Laboratories' assay sensitivity and reference interval.

Functional medicine optimal

1.0–2.5 mIU/L (functional medicine and integrative endocrinology optimal — this range reflects the TSH distribution in populations without thyroid pathology and is used in functional medicine as the target zone for thyroid hormone optimisation, particularly in symptomatic patients and those planning pregnancy). # REVIEWER FLAG: The 1.0–2.5 mIU/L functional optimal is widely cited in integrative practice but is not a treatment threshold in mainstream ATA or AACE guidelines — reviewer to confirm appropriate framing.

Units

mIU/L, µIU/mL

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.