The diagnosis of obesity is defined by a body mass index over 30 kilograms per metre squared.

Baseline investigations - all cases

Full blood count

Useful indicator of general health and underlying disease. Anaemia may exacerbate tiredness and breathlessness and so reduce exercise capacity.

Urea and electrolytes

Hypokalaemia may be present in Cushing's syndrome.

Liver function test

Useful indicator of general health and underlying disease. Fatty liver is common in obesity, diabetes and Cushing’s, though abnormal liver function should be investigated if indicated in the usual way. This is also needed as baseline before starting medical treatments.

Bone profile

Undiagnosed hyperparathyroidism may contribute to depression and tiredness and so exacerbate weight gain.

Thyroid stimulating hormone

Untreated hypothyroidism is associated with weight gain and depressed mood.

Thyroid antibodies

Positive thyroid antibodies indicate an increased risk of future thyroid dysfunction and should trigger annual monitoring of TSH.

Fasting lipids, glucose, HbA1c

Essential to stratify and address other cardiovascular risk factors in all patients with possible metabolic syndrome.

Investigation for possible Cushing's syndrome

Consider screening investigations for Cushing's only if this is clinically indicated. 2x24hr urinary free cortisol and low dose dexamethasone suppression test comprise the usual first line screen.

Further investigations - selected cases only

Investigation for possible polycystic ovarian syndrome

Consider performing screening investigations for PCOS if clinically indicated. Where this is suspected clinically, diagnoses such as prolactinoma, hypothyroidism, virilising tumours, congenital adrenal hyperplasia and Cushing's syndrome must all be ruled out.