TSH
Thyroid-stimulating hormone — the primary screening marker for thyroid function and metabolic rate.
Optimal Range
0.5-2.5 mIU/L (optimal) · 0.4-4.0 mIU/L (lab range)
Risk-Stratified Targets
| Population / Context | Target |
|---|---|
| Optimal (functional) | 0.5–2.0 mIU/L |
| Acceptable | 0.4–2.5 mIU/L |
| Subclinical hypothyroidismWarrants investigation if symptomatic | 2.5–4.5 mIU/L |
| Overt hypothyroidism | > 4.5 mIU/L |
| Subclinical hyperthyroidism | 0.1–0.4 mIU/L |
| Overt hyperthyroidism | < 0.1 mIU/L |
Why It Matters
Thyroid dysfunction affects energy, weight, cognition, and cardiovascular health. Even subclinical hypothyroidism (TSH 2.5-4.0) can impair metabolism and accelerate aging in some individuals.
Understanding TSH
Thyroid-stimulating hormone (TSH) is produced by the pituitary gland and acts as the master controller of thyroid function. It operates through a negative feedback loop: when thyroid hormone levels drop, TSH rises to stimulate the thyroid; when thyroid hormones are sufficient, TSH falls. This makes TSH the most sensitive screening marker for thyroid dysfunction — it changes before the thyroid hormones themselves become abnormal.
Conventional lab reference ranges for TSH span roughly 0.4–4.5 mIU/L, but these ranges are derived from population averages that include people with undiagnosed thyroid disease. Studies of rigorously screened, disease-free populations suggest that a truly normal TSH falls between 0.5 and 2.5 mIU/L. A TSH above 2.5, while 'normal' by lab standards, may indicate early (subclinical) hypothyroidism — associated with fatigue, weight gain, elevated cholesterol, depression, and accelerated cardiovascular aging.
Thyroid assessment should not rely on TSH alone. A complete thyroid panel includes Free T4, Free T3, and thyroid antibodies (TPO-Ab, TG-Ab). TSH can be normal while Free T3 is low, indicating poor T4-to-T3 conversion — a common issue in chronic stress, inflammation, and caloric restriction. Conversely, a suppressed TSH with normal free hormones warrants investigation for subclinical hyperthyroidism, which increases the risk of atrial fibrillation and osteoporosis.
Key Research
The Clinical Significance of Subclinical Thyroid Dysfunction
Biondi B, Cooper DS · Endocr Rev (2008)
Key finding: Subclinical hypothyroidism (TSH 4.5–10 mIU/L) is associated with increased cardiovascular risk, dyslipidemia, and progression to overt hypothyroidism.
The Evidence for a Narrower Thyrotropin Reference Range Is Compelling
Wartofsky L, Dickey RA · J Clin Endocrinol Metab (2005)
Key finding: Proposed narrowing the TSH upper limit to 2.5 mIU/L based on evidence from rigorously screened, disease-free populations.