GPAtlas🧭 When to suspect
Subclinical hyperthyroidism is a biochemical diagnosis: a low or suppressed TSH (below the reference range) with a normal FT4 and 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, though it may be accompanied by non-specific symptoms such as palpitations, tremor, anxiety, heat intolerance or unintended weight loss.
The single most important first step is to 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, and 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, and 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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