Match each laboratory profile of thyroid function to its most likely clinical diagnosis. R… — USMLE Step 3

Match each laboratory profile of thyroid function to its most likely clinical diagnosis. Reference ranges are as follows: TSH $0.4$ - $4.0$ $\mu$ U/mL; Free T4 $0.9$ - $1.7$ ng/dL; 24-hr Radioactive Iodine Uptake (RAIU) $10\%$ - $30\%$ ; Thyroglobulin $3$ - $40$ ng/mL.

Profile 1: TSH $< 0.01$ $\mu$ U/mL, Free T4 $3.2$ ng/dL, 24-hr RAIU $1\%$ , Thyroglobulin $< 0.1$ ng/mLExogenous thyroid hormone ingestion
Profile 2: TSH $< 0.01$ $\mu$ U/mL, Free T4 $2.9$ ng/dL, 24-hr RAIU $2\%$ , Thyroglobulin $145$ ng/mLDestructive thyroiditis
Profile 3: TSH $< 0.01$ $\mu$ U/mL, Free T4 $3.5$ ng/dL, 24-hr RAIU $55\%$ (diffuse), Thyroglobulin $120$ ng/mLGraves disease
Profile 4: TSH $4.5$ $\mu$ U/mL, Free T4 $2.8$ ng/dL, Alpha-subunit to TSH molar ratio: ElevatedTSH-secreting pituitary adenoma

Cevap

Profile 1 matches Exogenous thyroid hormone ingestion; Profile 2 matches Destructive thyroiditis; Profile 3 matches Graves disease; Profile 4 matches TSH-secreting pituitary adenoma.

The correct matches rely on a step-wise interpretation of thyroid laboratories. First, the relationship between TSH and Free T4 distinguishes primary from secondary (central) causes. Second, RAIU separates de novo synthesis (such as in Graves disease) from destructive or exogenous causes. Finally, serum thyroglobulin precisely differentiates destructive thyroiditis (high thyroglobulin) from exogenous ingestion (low thyroglobulin).

Adım Adım Çözüm

Analyze the TSH and Free T4 to determine the primary axis state.

Profiles 1, 2, and 3 show primary thyrotoxicosis (suppressed TSH, high Free T4). Profile 4 shows central hyperthyroidism (inappropriately elevated TSH with high Free T4).

This establishes whether the overproduction source is the thyroid gland itself or the pituitary gland.

Evaluate the Radioactive Iodine Uptake (RAIU) in the profiles showing primary thyrotoxicosis.

Profile 3 has high RAIU, indicating active de novo hormone synthesis. Profiles 1 and 2 have low RAIU, indicating release of preformed hormone or an exogenous source.

RAIU differentiates hyperthyroidism due to active glandular overproduction from thyrotoxicosis without hyperthyroidism.

Use Thyroglobulin levels to differentiate the causes of low-RAIU thyrotoxicosis.

Profile 1 has suppressed thyroglobulin (indicating an exogenous source). Profile 2 has markedly elevated thyroglobulin (indicating destructive thyroiditis).

Thyroglobulin is co-secreted with endogenous thyroid hormone or released during follicular destruction, but its endogenous synthesis is suppressed by exogenous levothyroxine ingestion.

Anahtar Kavram

Interpretation of thyroid function tests, Radioactive Iodine Uptake (RAIU), and serum thyroglobulin to determine the specific etiology of thyrotoxicosis.

Tahmini Süre:1m 30s

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