GPAtlas🧭 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 – so a single symptom is rarely diagnostic and 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, or depression or unexplained anxiety, and in women with heavy or irregular periods or subfertility. 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), and do not test solely because someone has type 2 diabetes.
The clinical task is threefold: confirm the biochemical pattern, decide whether to treat (this differs for overt and subclinical disease), and 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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