Subacute Thyroiditis (Answer: B)
The most likely diagnosis is subacute thyroiditis, based on the combination of hyperthyroidism (elevated free T4, suppressed TSH), markedly elevated ESR (73 mm/h), neck discomfort, and the 10-day time course of symptoms.
Key Diagnostic Features
The dramatically elevated ESR is the critical distinguishing feature in this case:
Subacute thyroiditis characteristically presents with markedly elevated inflammatory markers (ESR often >50 mm/h), neck pain/discomfort, and thyrotoxicosis due to inflammatory destruction of thyroid follicles releasing preformed thyroid hormone 1
Graves' disease typically presents with suppressed TSH and elevated free T4 in the absence of significantly elevated inflammatory markers and would not explain the ESR of 73 mm/h 2
The 10-day duration with neck discomfort, fever (37.7°C), and systemic symptoms (palpitations, sweating) fits the typical presentation of subacute (de Quervain's) thyroiditis, which is an inflammatory condition often following viral illness 1
Why Not the Other Options
Graves' disease (Option A) would be diagnosed based on elevated FT4 with suppressed TSH in the absence of thyroid mass or nodular goiter and without significant inflammatory markers 2. The markedly elevated ESR (73 mm/h) and neck discomfort are inconsistent with Graves' disease 2.
Hashimoto's thyroiditis (Option C) is the most common cause of hypothyroidism in industrialized nations and would present with elevated TSH, not suppressed TSH 2. While Hashimoto's can have a transient thyrotoxic phase, the clinical presentation here with neck pain and markedly elevated ESR points more toward subacute thyroiditis 1.
Multinodular toxic goiter (Option D) would typically present with palpable thyroid nodules and symptoms from local compression (dysphagia, orthopnea, voice changes), which are not described in this case 1. The elevated ESR and neck discomfort are not characteristic features 1.
Clinical Management Implications
The distinction matters significantly for treatment and prognosis:
Subacute thyroiditis is typically self-limited and may be treated with supportive care including NSAIDs for pain and beta-blockers for symptomatic relief of tachycardia 1
Beta-blockers should be initiated immediately to control the tachycardia (heart rate 116/min) and cardiovascular symptoms while the inflammatory process resolves 2, 3
Antithyroid drugs (methimazole, propylthiouracil) are NOT indicated for subacute thyroiditis because the thyrotoxicosis results from release of preformed hormone, not increased synthesis 1
The thyrotoxic phase typically lasts 2-8 weeks, followed by a hypothyroid phase in some patients, then recovery to euthyroid state in most cases 1
Critical Pitfall to Avoid
Do not start antithyroid medications empirically without confirming the etiology of thyrotoxicosis. If thyroid scintigraphy were performed, subacute thyroiditis would show decreased or absent uptake (distinguishing it from Graves' disease or toxic nodular goiter, which show increased uptake) 1. The elevated ESR combined with neck discomfort makes this distinction clinically without requiring immediate scintigraphy in most cases 1.