What is the most likely diagnosis for a patient presenting with palpitations, sweating, and neck discomfort, tachycardia (heart rate 116/min), normal blood pressure (110/70 mmHg), and laboratory results showing hyperthyroidism (elevated free Thyroxine (T4) level and suppressed Thyroid-Stimulating Hormone (TSH) level)?

Subacute Thyroiditis

Based on the clinical presentation of 10 days of palpitations, sweating, neck discomfort, tachycardia (HR 116/min), elevated free T4 (76.5 pmol/L), suppressed TSH (0.15 U/L), and markedly elevated ESR (73 mm/h), the most likely diagnosis is subacute thyroiditis (Option B).

Key Diagnostic Features Supporting Subacute Thyroiditis

Clinical Presentation Timeline

• The 10-day duration of symptoms is characteristic of subacute thyroiditis, which typically presents with an acute to subacute onset over days to weeks 1
• Neck discomfort/pain is a hallmark feature that distinguishes subacute thyroiditis from other causes of hyperthyroidism 1
• Palpitations, sweating, and tachycardia represent the thyrotoxic phase where stored thyroid hormone is released from inflamed thyroid tissue 1

Laboratory Pattern

• Elevated ESR (73 mm/h) is the critical distinguishing feature—subacute thyroiditis causes significant thyroid inflammation with markedly elevated inflammatory markers 1
• The combination of suppressed TSH (0.15 U/L) with elevated free T4 (76.5 pmol/L) confirms thyrotoxicosis 1
• Normal WBC (5.2 x10⁹/L) helps exclude infectious thyroiditis 1

Pathophysiology

• Subacute thyroiditis is a destructive inflammatory process where thyroid follicles rupture and release preformed thyroid hormone into circulation, causing transient thyrotoxicosis 1
• This is followed by a hypothyroid phase as stored hormone is depleted, then eventual recovery in most cases 1

Why Other Diagnoses Are Less Likely

Graves' Disease (Option A) - Excluded

• Graves' disease accounts for 95% of hyperthyroidism cases but typically has a more gradual onset over weeks to months, not 10 days 1
• Neck discomfort is NOT a feature of Graves' disease—patients typically have a painless, diffuse goiter 1
• ESR is typically normal or only mildly elevated in Graves' disease, not markedly elevated at 73 mm/h 1
• Distinctive features of Graves' (ophthalmopathy, pretibial myxedema) are not mentioned in this case 1

Hashimoto's Thyroiditis (Option C) - Excluded

• Hashimoto's disease is the most common cause of hypothyroidism, not hyperthyroidism 1
• While Hashimoto's can have a transient thyrotoxic phase (Hashitoxicosis), this typically occurs over months, not 10 days 1
• Neck pain is not characteristic of Hashimoto's thyroiditis 1
• The markedly elevated ESR is more consistent with subacute thyroiditis than Hashimoto's 1

Multinodular Toxic Goiter (Option D) - Excluded

• Toxic multinodular goiter develops gradually over years, not acutely over 10 days 1
• Neck discomfort is not a typical feature—patients usually have a long-standing, painless nodular goiter 1
• ESR is typically normal in toxic multinodular goiter, as there is no inflammatory component 1

Clinical Management Implications

Immediate Assessment

• Confirm diagnosis with radioactive iodine uptake scan, which will show low uptake in subacute thyroiditis (distinguishing it from Graves' disease which shows high uptake) 2, 3
• Monitor for progression through thyrotoxic, hypothyroid, and recovery phases 1

Treatment Approach

• Beta-blockers (e.g., propranolol) for symptomatic relief of tachycardia and palpitations 3
• NSAIDs or corticosteroids for neck pain and inflammation if severe 1
• Avoid antithyroid drugs (methimazole, propylthiouracil) as they are ineffective—the thyrotoxicosis is due to hormone release, not overproduction 2, 3

Critical Pitfall to Avoid

• Do not treat with antithyroid medications as this represents destructive thyroiditis with hormone release, not increased hormone synthesis 2, 3
• The thyrotoxic phase is self-limited and will transition to hypothyroidism in 4-8 weeks, requiring monitoring for potential need for temporary levothyroxine replacement 1