What is the diagnosis and treatment for a 13-year-old patient with weight loss, exophthalmos, and fever, presenting with hyperthyroidism (elevated free triiodothyronine (fT3), suppressed thyroid-stimulating hormone (TSH), and elevated free thyroxine (fT4)), and weighing 30.5 kilograms?

Diagnosis: Graves' Disease (Thyrotoxicosis)

This 13-year-old patient has severe overt hyperthyroidism (thyrotoxicosis), most likely Graves' disease given the presence of exophthalmos, with markedly elevated free thyroid hormones (fT3 18.2 pmol/L, fT4 58.519 pmol/L) and suppressed TSH (<0.01 mIU/L). The combination of suppressed TSH with dramatically elevated fT4 and fT3, along with exophthalmos (a hallmark of Graves' disease), confirms this diagnosis 1, 2.

Immediate Treatment Recommendation

Start methimazole immediately at a weight-based dose of 0.5-1.0 mg/kg/day, which translates to 15-30 mg daily for this 30.5 kg patient, divided into 2-3 doses. Methimazole inhibits thyroid hormone synthesis and is the first-line antithyroid medication for pediatric Graves' disease 1.

Initial Dosing Strategy

• For severe hyperthyroidism with markedly elevated thyroid hormones (as in this case), start at the higher end: 0.7-1.0 mg/kg/day (approximately 20-30 mg/day for 30.5 kg) 1
• Divide the total daily dose into 2-3 administrations to maintain consistent drug levels 1
• The drug is readily absorbed in the gastrointestinal tract and does not inactivate existing circulating thyroid hormones, so clinical improvement takes several weeks 1

Critical Safety Monitoring

Patients on methimazole require close surveillance with immediate reporting of illness, particularly sore throat, skin eruptions, fever, headache, or general malaise, as agranulocytosis is a serious potential complication 1. In such cases, obtain white blood cell and differential counts immediately 1.

• Monitor for vasculitis symptoms: new rash, hematuria, decreased urine output, dyspnea, or hemoptysis—these require immediate medical attention 1
• Check prothrombin time before any surgical procedures, as methimazole may cause hypoprothrombinemia and bleeding 1
• Monitor thyroid function tests (TSH, fT4, fT3) periodically during therapy 1

Adjunctive Symptomatic Treatment

Consider adding a beta-blocker (propranolol 10-20 mg three times daily or atenolol 25-50 mg daily) to control tachycardia, tremor, and other hyperadrenergic symptoms while waiting for methimazole to take effect 1. Note that hyperthyroidism increases clearance of beta-blockers, so dose reduction will be needed once the patient becomes euthyroid 1.

Monitoring and Follow-up

• Recheck thyroid function tests (TSH, fT4, fT3) in 4-6 weeks after initiating therapy 1
• Once clinical hyperthyroidism resolves and TSH begins rising, reduce the methimazole dose to a lower maintenance level 1
• A rising serum TSH indicates the need for dose reduction to prevent iatrogenic hypothyroidism 1

Diagnostic Confirmation

The FT4/TSH ratio in this patient is extraordinarily high (>5000), which strongly supports Graves' disease as the diagnosis 2. An FT4/TSH ratio >13,948.98 pmol/mIU has 99.4% specificity and 92.31% positive predictive value for Graves' disease 2.

Consider measuring thyroid-stimulating hormone receptor antibodies (TRAb) to definitively confirm Graves' disease, though treatment should not be delayed while awaiting results 2.

Important Caveats

• The fever may represent thyroid storm or an intercurrent infection—assess for signs of thyroid storm (altered mental status, severe tachycardia >140 bpm, hyperthermia >40°C, heart failure) which would require hospitalization and additional urgent interventions 3, 4
• Thyroid function tests can be misleading during acute illness (euthyroid sick syndrome), but the constellation of suppressed TSH with markedly elevated fT4 and fT3 plus exophthalmos confirms true hyperthyroidism rather than nonthyroidal illness 4
• Weight loss in this context is due to the hypermetabolic state of hyperthyroidism and should improve with treatment 3
• The presence of exophthalmos indicates Graves' ophthalmopathy, which may require additional ophthalmologic management 2

Treatment Duration

A reasonable approach is a therapeutic trial of methimazole for 12-24 months to induce remission 5. Many pediatric patients with Graves' disease achieve remission with this duration of therapy, though some require longer treatment or definitive therapy with radioactive iodine or thyroidectomy 5.