What would be the expected thyroid function test results for a patient with suspected subacute thyroiditis, presenting with lethargy, weight gain, and increased sleep need, following a viral upper respiratory illness and a period of hyperthyroidism?

Thyroid Function Tests in Subacute Thyroiditis

The most likely answer is E (Low TSH, Low Thyroglobulin, Low RAIU), representing the recovery phase of subacute thyroiditis following a period of hyperthyroidism.

Clinical Presentation Analysis

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

1. Initial thyrotoxic phase (2 weeks of high energy, weight loss, decreased sleep)
2. Hypothyroid phase (current presentation with lethargy, weight gain, increased sleep)
3. Recovery phase (pending)

This pattern is characteristic of subacute thyroiditis, which typically follows a viral upper respiratory illness, as seen in this patient 1.

Laboratory Findings in Subacute Thyroiditis

During the hypothyroid phase of subacute thyroiditis (the patient's current state), the following laboratory findings would be expected:

• TSH: Low (still suppressed from the previous thyrotoxic phase) 2, 1
• Thyroglobulin: Low (due to depleted thyroid hormone stores following the destructive thyroiditis) 3
• Radioactive Iodine Uptake (RAIU): Low (characteristic of destructive thyroiditis) 3, 4

Explanation of Laboratory Pattern

1. Low TSH: Despite the patient now being clinically hypothyroid, TSH remains suppressed due to the recent thyrotoxic phase. TSH typically takes weeks to normalize after thyroid hormone levels fall 1, 5.

2. Low Thyroglobulin: Following the destructive process of subacute thyroiditis, thyroid follicles are damaged, leading to decreased thyroglobulin production 3.

3. Low RAIU: This is a hallmark finding in subacute thyroiditis. Unlike Graves' disease (which would show diffuse uptake) or toxic nodular disease (which would show focal uptake), subacute thyroiditis shows low uptake due to the inflammatory destruction of thyroid tissue 2, 3.

Differential Diagnosis

• Graves' disease would show low TSH, high thyroglobulin, and diffuse RAIU (option B)
• Toxic nodular goiter would show low TSH, high thyroglobulin, and focal RAIU (option C)
• Factitious thyrotoxicosis would show low TSH, high thyroglobulin, and low RAIU (option D)
• Primary hypothyroidism would show high TSH, low thyroglobulin, and low RAIU (option A)

Clinical Course and Management

The patient is currently in the hypothyroid phase of subacute thyroiditis, which typically follows the initial thyrotoxic phase. This phase is transient in most cases, with spontaneous resolution within 4-6 months 5.

• The median duration of the entire illness is approximately 77 days 5
• Permanent hypothyroidism develops in approximately 10-15% of cases 1, 5
• Treatment is generally supportive, with levothyroxine considered if TSH >10 mIU/L or if the patient is symptomatic with TSH 4-10 mIU/L 6

Key Clinical Pearls

1. The triphasic pattern (thyrotoxicosis → hypothyroidism → recovery) is characteristic of subacute thyroiditis
2. Low RAIU distinguishes subacute thyroiditis from other causes of thyrotoxicosis
3. TSH may remain suppressed even when the patient becomes clinically hypothyroid
4. Most patients recover normal thyroid function within 6 months

Based on the clinical presentation and expected laboratory findings, option E (Low TSH, Low Thyroglobulin, Low RAIU) is the most appropriate answer for this patient with subacute thyroiditis in the hypothyroid phase.