Subclinical Hypothyroidism

Definition: an elevated serum TSH with a normal or low-normal free T4. It is a biochemical, not a clinical, diagnosis.

Prevalence: 4–10% of the general population, rising to up to 20% in women over 60. Very common because of widespread sensitive TSH assays.

Confirm before acting: a TSH above 4.5 mIU/L must be repeated at 1–3 months, and thereafter every 6–12 months if the patient is asymptomatic.

Evaluate for a cause: past radioiodine, previous thyroid surgery, goitre, family history of thyroid disease; check anti-thyroid peroxidase antibodies and a lipid profile.

Natural history: progression to overt hypothyroidism is around 5% per year (range 3–20%), higher with goitre and/or positive antibodies. Antibodies help predict progression but rarely change management.

When to treat: if TSH rises above 10 mIU/L, start levothyroxine. Between 4.5 and 10 mIU/L, an individual ex adiuvantibus trial may be considered for subtle symptoms.

TRUST trial (Stott et al., NEJM 2017) — 737 adults aged ≥65 (mean 74.4 years, baseline TSH 6.40 mIU/L). Levothyroxine lowered TSH to 3.63 vs 5.48 mIU/L on placebo but produced no improvement in Hypothyroid Symptoms score, Tiredness score, cognition, blood pressure, weight, Barthel index or IADL.

The cardiovascular link is age-dependent: subclinical hypothyroidism appears detrimental in young and middle-aged people (Whickham re-analysis showed excess coronary events over 20 years), but is harmless or possibly protective in the oldest old. In studies of people over 73 and over 85, high TSH and/or low free T4 were associated with lower all-cause and cardiovascular mortality.

Leiden 85-plus study: 12% of 85-year-olds had hypothyroidism (7% overt, 5% subclinical); none of the subclinical group progressed by age 88, and a raised TSH predicted slower decline in instrumental ADL.

Conclusion for the elderly: do not treat minor TSH elevations in the oldest old — the risks (atrial fibrillation, fracture) outweigh benefit. Decide on comorbidity, frailty and symptom burden, not on a TSH cut-off alone. Latest Updates & Current Practice (2023–Present) — 3 Points Age-specific TSH reference intervals (Thyroid 2024) are reducing the number of older adults labelled with subclinical hypothyroidism at all — many are simply normal for their age. Systematic review and meta-analysis (Holley et al., Systematic Reviews 2024, 6 studies, 3853 participants) found no clear cardiovascular or bone-health benefit from levothyroxine in older people with subclinical hypothyroidism — consistent with TRUST and with the pooled TRUST/IEMO 80-plus data. Current practice therefore diverges from the textbook only in emphasis: where Pathy's leaves the 4.5–10 mIU/L band open to an individual trial, guidelines now favour watchful monitoring without treatment in those over 70–80, and are moving towards active deprescribing of levothyroxine started for this indication.