A 15-year-old with fatigue, weight loss, and a 'normal' blood count

The most likely diagnosis is anorexia nervosa (restrictive type), with secondary hypothalamic amenorrhoea, relative energy deficiency in sport (RED-S), and mild electrolyte disturbance.

Supporting features:

• Low BMI (16.2) with significant weight loss (4 kg) and food restriction, particularly avoidance of carbohydrates.
• Amenorrhoea for 4 months in a previously regular 15-year-old - hypothalamic suppression from energy deficit, not primary endocrine disease.
• Bradycardia (52), hypotension (90/60), cool peripheries, lanugo hair, dry skin - classic signs of starvation and increased vagal tone.
• Hypophosphataemia/hypokalaemia (K 3.2) - from poor intake and possibly vomiting or laxative use; must be asked about.
• Normal TSH and free T4 - makes primary hypothyroidism unlikely; in starvation, T4 may be low-normal with a normal TSH (sick euthyroid pattern).
• Negative coeliac serology - makes coeliac disease unlikely.
• Normal Hb, MCV, and platelets - does not exclude anorexia; anaemia is common but not universal.

Why not an endocrine cause?

• Thyroid function is normal; hypothyroidism would cause weight gain, not loss, and would not explain the lanugo or the food restriction.
• Addison's disease could cause fatigue, weight loss, and hypotension, but would usually show hyperpigmentation, hyponatraemia with hyperkalaemia, and a raised ACTH; the potassium is low here, not high.
• Diabetes mellitus would cause weight loss but with hyperglycaemia, polyuria, and ketosis; glucose is 3.6.

Why not a gastrointestinal cause?

• Coeliac serology is negative; there is no diarrhoea; and the bradycardia, lanugo, and amenorrhoea are not explained by malabsorption alone.

Important: the patient's denial ('I'm fine, everyone is overreacting') is itself a feature of the illness and should be documented, not argued with.

Immediate medical risks:

1. Refeeding syndrome - the most dangerous early risk. Hypokalaemia (3.2), low BMI, and poor intake put her at high risk. Refeeding can cause a shift of phosphate, potassium, and magnesium intracellularly, leading to arrhythmia, heart failure, and seizures.
2. Cardiac complications - bradycardia, hypotension, and electrolyte disturbance predispose to QT prolongation and arrhythmia. She needs an ECG.
3. Hypoglycaemia - glucose 3.6; monitor and treat if symptomatic.
4. Dehydration and renal impairment - urea 7.8 with normal creatinine suggests pre-renal impairment.
5. Osteoporosis - prolonged amenorrhoea and low BMI; bone density should be assessed.
6. Suicidal risk and psychiatric comorbidity - must be assessed; anorexia has a high mortality.

Assessment:

• Full history including vomiting, laxative use, exercise, and body image; use a validated tool (e.g. SCOFF) and a confidential HEADSS assessment.
• Examination including weight, height, BMI, temperature, HR, BP, and lying/standing BP.
• ECG for QT interval and bradycardia.
• Bloods: phosphate, magnesium, calcium, full electrolytes, LFTs, glucose, and amylase; repeat electrolytes daily during early refeeding.
• Bone density (DEXA) if amenorrhoea persists.
• Psychiatric assessment for depression, anxiety, and suicidality.

Management:

• Admission if there is significant medical compromise (bradycardia, hypotension, hypokalaemia, hypoglycaemia, or rapid weight loss).
• Start refeeding cautiously - begin at a low calorie target and increase gradually; monitor phosphate, potassium, and magnesium daily; give thiamine before feeding and multivitamins; replace electrolytes.
• Do not aim for rapid weight gain; the goal is steady, medically supervised re-feeding.
• Multidisciplinary team - paediatrician, dietitian, psychologist/psychiatrist, and specialist eating disorder service.
• Family-based therapy is first-line for adolescents.
• Safety-netting: return if palpitations, syncope, or worsening weakness.

Answering 'is this just a phase?':

• No. Anorexia nervosa is a serious mental illness with the highest mortality of any psychiatric disorder. It is not attention-seeking or a phase, and it will not resolve by simply insisting she eats.
• Be clear and compassionate: "This is a real illness. Her body is in a state of starvation, and that affects her thinking as well as her body. She is not doing this to upset you."

Answering 'should we make her eat?':

• Forcing food creates conflict and can worsen the illness. The goal is supported, supervised meals with a clear plan, not battles at the table.
• Parents should be involved as partners in the treatment, not as enforcers. Family-based therapy (the Maudsley approach) empowers parents to take charge of meals in a supportive way, initially with the therapist's guidance, and gradually hands control back to the adolescent.
• Practical advice: eat together, avoid commenting on her body or food, do not label foods as 'good' or 'bad', and keep the focus on health rather than weight.

Role of the family:

• Monitoring - parents should observe meals and snacks, and watch for vomiting or excessive exercise.
• Emotional support - acknowledge the illness without blaming; siblings also need support.
• Practical support - reduce exercise, remove scales, and avoid diet talk at home.
• Engagement with the team - attend appointments, and be part of the treatment plan.

Confidentiality:

• Explain that you will keep some things confidential (as is appropriate for a 15-year-old) but that safety concerns will be shared with her parents.
• Assess for suicidality and self-harm, and involve the mental health team.

Prognosis:

• With early and consistent treatment, many adolescents recover. Relapse is common, so follow-up should be long-term.
• Be honest that recovery takes time and that the family's involvement is one of the strongest predictors of a good outcome.