A medical student is reviewing thyroid laboratory panels in the endocrine clinic. Match th… — USMLE Step 2 CK

A medical student is reviewing thyroid laboratory panels in the endocrine clinic. Match the following thyroid function test and radioactive iodine uptake (RAIU) laboratory profiles with the most accurate underlying clinical diagnosis.

TSH $0.01$ $\mu$ U/mL (Normal: $0.5-5.0$ ), Free T4 $2.8$ ng/dL (Normal: $0.8-1.8$ ), $24$ -hour RAIU $65\%$ (diffusely increased)Graves disease
TSH $0.02$ $\mu$ U/mL, Free T4 $2.5$ ng/dL, $24$ -hour RAIU $<1\%$ , non-tender thyroid glandPainless (silent) thyroiditis
TSH $14.5$ $\mu$ U/mL, Free T4 $0.6$ ng/dL, Anti-TPO antibodies positiveOvert primary hypothyroidism (Hashimoto)
TSH $6.5$ $\mu$ U/mL, Free T4 $1.2$ ng/dL, Anti-TPO antibodies positiveSubclinical hypothyroidism

Answer

The laboratory profiles correctly match to Graves disease, painless thyroiditis, overt Hashimoto thyroiditis, and subclinical hypothyroidism based on the respective TSH, free T4, and RAIU patterns.

Each diagnosis possesses a distinct biochemical and diagnostic signature. Graves disease demonstrates increased hormone synthesis (high RAIU). Painless thyroiditis involves follicular destruction and passive hormone release (low RAIU). Overt hypothyroidism presents with low free T4 and high TSH, whereas subclinical hypothyroidism features normal free T4 with an elevated TSH.

Step-by-Step Solution

Evaluate the thyrotoxic profiles (suppressed TSH, elevated free T4) and differentiate them using the RAIU value.

High RAIU points to Graves disease, while low RAIU with a non-tender gland points to painless (silent) thyroiditis.

RAIU distinguishes between conditions of increased thyroid hormone synthesis and conditions involving destructive release of preformed hormone.

Evaluate the hypothyroid profiles (elevated TSH) and differentiate them using the free T4 level.

A low free T4 confirms overt primary hypothyroidism, whereas a normal free T4 indicates subclinical hypothyroidism.

Subclinical disease is defined biochemically by an abnormal TSH but normal peripheral thyroid hormone levels.

Key Concept

Interpretation of thyroid function tests (TSH, free T4) and radioactive iodine uptake (RAIU) to differentiate causes of thyrotoxicosis and hypothyroidism.

Estimated Time:1m 30s

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