Why we measure it

Thyroid hormones regulate the metabolic rate of virtually every cell in the body. They govern heart rate and contractility, lipid metabolism, glucose utilisation, gut motility, cognitive function, mood regulation, bone turnover, and thermogenesis — a reach so broad that thyroid dysfunction is among the most systemic of all hormonal disorders. Subclinical hypothyroidism — defined as an elevated TSH with normal free T4 — is prevalent in approximately 5–8% of the general adult population and can persist for years without classic symptoms of fatigue, weight gain, or cold intolerance. A major individual participant meta-analysis by Rodondi et al., published in JAMA, pooled data from 55,287 adults across eleven prospective cohort studies and found that subclinical hypothyroidism was associated with a significantly increased risk of coronary heart disease events and cardiovascular mortality — with the greatest risk at TSH > 10 mIU/L. Subclinical hyperthyroidism (suppressed TSH with normal free hormones) is associated with increased risk of atrial fibrillation, stroke, and accelerated bone loss, even at TSH values just below the lower reference limit. These are clinically actionable conditions that can be detected and treated before causing overt organ-level consequences. Six-monthly testing provides the resolution needed to catch the gradual onset of thyroid dysfunction in its subclinical phase.

Why every 6 months

TSH changes slowly — the regulatory feedback loop it participates in operates on a timescale of weeks to months. This means that TSH measured six-monthly captures meaningful trends: the gradual onset of Hashimoto's thyroiditis (the most common cause of hypothyroidism in iodine-replete populations), the progression of subclinical to overt disease, and the stability or drift of thyroid function in those already receiving thyroid hormone replacement. For individuals without known thyroid disease, the six-month interval also captures the gradual TSH changes that accompany ageing — older adults show a modest upward shift in the TSH reference range, and the upper limit appropriate for an 80-year-old differs from that appropriate for a 35-year-old.

What movement means

The American Thyroid Association (ATA) defines the normal TSH reference range as approximately 0.4–4.0 mIU/L for most adults, though population-based studies show this range shifts upward with age. Subclinical hypothyroidism is conventionally defined as TSH 4.0–10.0 mIU/L with normal free T4; overt hypothyroidism as TSH > 10.0 mIU/L with low free T4. Subclinical hyperthyroidism is defined as TSH < 0.4 mIU/L with normal free T4 and T3; overt hyperthyroidism as TSH suppressed below detectable limits with elevated free hormones. These thresholds describe clinical categories derived from reference populations — not optimal health targets — and the appropriate TSH level for any individual depends on age, symptoms, thyroid antibody status, and clinical context.

In the protocol