🧭 When to suspect

Hypothyroidism is the clinical syndrome of inadequate thyroid hormone, and in the UK the great majority of cases are primary and autoimmune (Hashimoto's thyroiditis).

The presentation is typically insidious and non-specific:

• Fatigue

• Weight gain

• Cold intolerance

• Constipation

• Dry skin and thinning hair

A single symptom is rarely diagnostic, so the diagnosis is ultimately biochemical, made on thyroid function tests (TFTs): chiefly thyroid-stimulating hormone (TSH) and free thyroxine (FT4).

Have a low threshold to test in people with:

• Type 1 diabetes mellitus or other autoimmune disease

• New-onset atrial fibrillation

• Depression or unexplained anxiety

• Heavy or irregular periods or subfertility (women)

Be aware that thyroid symptoms in perimenopausal women are easily mistaken for the menopause.

Do not test:

• During an acute illness (the results are distorted)

• Solely because someone has type 2 diabetes

When you do test:

• Confirm the biochemical pattern

• Decide whether to treat (this differs for overt and subclinical disease)

• Never miss the two dangerous variants – the rare emergency of myxoedema coma and the pituitary picture of secondary hypothyroidism

Pattern	TSH	Free T4 (FT4)	Interpretation & action
Overt primary hypothyroidism	Raised	Low	Treat with levothyroxine
Subclinical hypothyroidism	Raised	Normal	• Repeat at ~3 months • Treat by threshold/symptoms
Secondary (central) hypothyroidism	Low or inappropriately normal	Low	Refer – suspected pituitary disease
Non-thyroidal illness (‘sick euthyroid’)	Often low	Low or normal	• Do not test in acute illness • Recheck on recovery

Source: NICE NG145

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