GPAtlas🧭 When to suspect
Subclinical hypothyroidism (SCH) is a biochemical diagnosis: a thyroid-stimulating hormone (TSH) above the reference range with a normal free thyroxine (FT4) (and free triiodothyronine [FT3]).
It is common – affecting up to around 1 in 10 adults – and is more frequent in:
• Women
• Older people
• Those with autoimmune disease or positive thyroid antibodies
Most patients are asymptomatic, or have non-specific symptoms (fatigue, weight gain, low mood, cold intolerance) that overlap with many other conditions.
First confirm persistence: repeat TSH and FT4 after about 3 months before labelling or treating. A substantial proportion normalise with no intervention at all.
A raised TSH is frequently transient; each of these can raise it:
• Recovery from a non-thyroidal illness
• Recent thyroiditis
• Assay interference
Then:
• Decide who genuinely benefits from levothyroxine, driven by TSH level, age, symptoms and antibody status
• Avoid over-treatment
• Recognise the situations that change the rules – above all pregnancy
| TSH (with normal FT4) | Severity | Primary-care action |
|---|---|---|
| Above range but < 10 mU/L | Mild subclinical | • Confirm on repeat • Watchful waiting – 6-month levothyroxine trial only if under 65 and symptomatic |
| ≥ 10 mU/L | Marked subclinical | Confirm on 2 occasions 3 months apart – consider levothyroxine |
Raise suspicion in patients with persistent fatigue, low mood or weight gain, and test for thyroid dysfunction in those with new-onset atrial fibrillation, type 1 diabetes or other autoimmune disease.
The commonest cause is autoimmune (Hashimoto's) thyroiditis; also consider:
• Previous radioiodine or thyroid surgery
• Drugs (amiodarone, lithium)
Do not test during an acute illness unless thyroid disease is the suspected cause.
Source: NICE NG145
|
⚠️ Common pitfall • Committing a patient to lifelong levothyroxine on a single raised TSH. • TSH rises transiently during recovery from any acute or non-thyroidal illness, so do not test during acute illness. • Always repeat TSH and FT4 after about 3 months. • A large proportion normalise without any treatment. |
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