Subclinical Hypothyroidism

🔍 When to Suspect

When a patient presents with non-specific symptoms (fatigue, weight gain) and thyroid function tests show a raised thyroid-stimulating hormone (TSH) with a normal free thyroxine (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 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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