Subclinical Hypothyroidism

🔍 When to Suspect

When a patient presents with non-specific symptoms (fatigue, weight gain) and thyroid function tests show a raised TSH with a normal FT4 level; requires confirmation with a repeat test after 3 months

From the full topic in The Ocean Library: Subclinical Hypothyroidism

🧭 When to suspect

Subclinical hypothyroidism (SCH) is a biochemical diagnosis: a TSH above the reference range with a normal free thyroxine (FT4) (and FT3). It is common – affecting up to around 1 in 10 adults – and is more frequent in women, older people, and 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.

The single most important step is to confirm persistence. A raised TSH is frequently transient – recovery from a non-thyroidal illness, recent thyroiditis, or assay interference can all raise it – so repeat TSH and FT4 after about 3 months before labelling or treating. A substantial proportion normalise with no intervention at all.

The clinical task in primary care is therefore threefold: decide who genuinely benefits from levothyroxine (driven by TSH level, age, symptoms and antibody status), avoid over-treatment, and 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 and 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, and always repeat TSH and FT4 after about 3 months. A large proportion normalise without any treatment.

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