GPAtlas🧭 When to suspect
Subclinical hyperthyroidism is a biochemical diagnosis: a low or suppressed thyroid-stimulating hormone (TSH) (below the reference range) with a normal free thyroxine (FT4) and free triiodothyronine (FT3), once non-thyroidal illness, recent thyroiditis and drug effects have been excluded.
• It is most often an incidental finding on thyroid function tests requested for another reason.
• It may be accompanied by non-specific symptoms such as palpitations, tremor, anxiety, heat intolerance or unintended weight loss.
First, repeat the tests in 3–6 months.
• A transient dip in TSH – from thyroiditis, intercurrent illness or drugs – is far more common than true persistent disease.
• More than half of isolated low-TSH results normalise with no treatment.
• Earlier action is reserved for a specific reason, such as a thyroid nodule, pregnancy or new atrial fibrillation.
Two skills then matter: confirm persistence, and grade by how suppressed the TSH is. Risk and management are driven far more by the depth and persistence of TSH suppression than by symptoms.
| TSH (with normal FT4 & FT3) | Classification | What it means |
|---|---|---|
| 0.1–0.4 mU/L (low, still detectable) | Grade 1 (mild) | • Lower risk • Most can be monitored in primary care |
| < 0.1 mU/L (suppressed/undetectable) | Grade 2 (marked) | • Higher AF, fracture and mortality risk • Less likely to recover • Seek specialist advice if persistent |
| FT4 and/or FT3 above range | Overt hyperthyroidism | No longer subclinical – manage as thyrotoxicosis |
Have a lower threshold to check thyroid function in:
• New-onset atrial fibrillation (offer testing).
• Unexplained anxiety or depression.
• Older patients with non-specific symptoms.
• Anyone on amiodarone or lithium.
• Patients on levothyroxine where over-replacement is possible.
Bear in mind that a single symptom alone is a weak pointer to thyroid disease.
Source: NICE NG145
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