What thyroid studies are expected in a patient with suspected subacute thyroiditis, presenting with lethargy, weight gain, and an enlarged but nontender thyroid gland, following a recent viral upper respiratory illness and a period of hyperthyroidism?

Thyroid Studies in Subacute Thyroiditis

Based on the clinical presentation and suspected diagnosis of subacute thyroiditis, the most appropriate set of thyroid studies would be option E: Low TSH, Low Thyroglobulin, and Low RAIU.

Understanding Subacute Thyroiditis

Subacute thyroiditis (SAT) is an inflammatory thyroid disorder typically following a viral upper respiratory tract infection 1. The patient's clinical presentation strongly suggests this diagnosis:

• Recent viral upper respiratory illness
• Initial hyperthyroid phase (energy, weight loss, decreased sleep)
• Current hypothyroid phase (lethargy, weight gain, excessive sleep)
• Enlarged but nontender thyroid gland
• Typical triphasic pattern of thyroid function

Laboratory and Imaging Findings in Subacute Thyroiditis

Thyroid Function Tests

• TSH: Low during both the hyperthyroid and early recovery phases 2, 3
  • The patient is currently in the recovery phase but likely still has suppressed TSH

Thyroglobulin

• Thyroglobulin: Initially elevated during acute inflammatory phase but decreases during recovery 4, 5
  • Since the patient is now 4 weeks into recovery, thyroglobulin levels would be low

Radioactive Iodine Uptake (RAIU)

• RAIU: Characteristically low in subacute thyroiditis 4, 2
  • This is a key diagnostic finding that distinguishes subacute thyroiditis from other causes of thyrotoxicosis like Graves' disease or toxic nodular goiter
  • In subacute thyroiditis, inflammation causes thyroid follicle destruction and release of preformed thyroid hormone, but impairs the gland's ability to trap iodine

Differential Diagnosis

1. Graves' Disease: Would show low TSH, high thyroglobulin, and diffuse uptake on RAIU (Option B) 1

2. Toxic Adenoma: Would show low TSH, high thyroglobulin, and focal uptake on RAIU (Option C) 1

3. Factitious Thyrotoxicosis: Would show low TSH, low thyroglobulin, and low RAIU (Option E), but patient's history doesn't suggest this

4. Hashimoto's Thyroiditis: Would typically show high TSH (Option A) during hypothyroid phase 6

Clinical Course of Subacute Thyroiditis

The patient's presentation follows the classic triphasic pattern of subacute thyroiditis:

1. Thyrotoxic Phase (first 2-8 weeks):

   • Symptoms: Increased energy, weight loss, decreased sleep
   • Labs: Low TSH, elevated free T4 and T3

2. Hypothyroid Phase (weeks to months):

   • Symptoms: Fatigue, weight gain, excessive sleep
   • Labs: Initially low TSH (still recovering from thyrotoxic phase)

3. Recovery Phase:

   • Return to euthyroid state in majority of patients
   • 25% may develop permanent hypothyroidism 2

Key Points to Remember

• Subacute thyroiditis often presents with a history of preceding viral illness 5
• The condition follows a triphasic pattern with initial thyrotoxicosis followed by hypothyroidism and then recovery
• Low radioactive iodine uptake is a hallmark finding that distinguishes it from other causes of thyrotoxicosis 1
• Despite the enlarged thyroid, the gland is often nontender in the recovery phase 7

Therefore, option E (Low TSH, Low Thyroglobulin, Low RAIU) best represents the thyroid studies expected in this patient with subacute thyroiditis who is currently in the recovery phase.