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Difficulty: Very hardEndocrinology and Metabolism

Match the following complex thyroid function laboratory profiles to the most accurate underlying pathophysiologic diagnosis.

  • Suppressed TSH, elevated free T4, suppressed serum thyroglobulin, and near-zero radioactive iodine uptakeExogenous levothyroxine administration (Factitious thyrotoxicosis)
  • Suppressed TSH, decreased free T4, decreased total T3, and elevated reverse T3 in a critically ill patientEuthyroid sick syndrome (Non-thyroidal illness syndrome)
  • Inappropriately normal or elevated TSH, elevated free T4, elevated alpha-subunit, and diffuse radioactive iodine uptakeThyrotropin-secreting pituitary adenoma (TSHoma)
  • Inappropriately normal or elevated TSH, elevated free T4, normal alpha-subunit, and normal anterior pituitary on magnetic resonance imagingSyndrome of thyroid hormone resistance

Answer

Profile 1 matches Exogenous levothyroxine administration; Profile 2 matches Euthyroid sick syndrome; Profile 3 matches Thyrotropin-secreting pituitary adenoma; Profile 4 matches Syndrome of thyroid hormone resistance.
Each laboratory profile contains specific biomarker nuances (thyroglobulin, reverse T3, alpha-subunit) that pinpoint a distinct pathophysiology. Low thyroglobulin proves an exogenous source; high reverse T3 proves altered peripheral metabolism in acute illness; and the alpha-subunit differentiates a neoplastic pituitary process (TSHoma) from a genetic receptor defect (Refetoff syndrome).

Step-by-Step Solution

1
Analyze the profile with suppressed TSH, high free T4, and low radioactive iodine uptake.
Identify that this represents thyrotoxicosis without hyperthyroidism. The suppressed thyroglobulin confirms an exogenous source (factitious thyrotoxicosis) rather than thyroiditis.
Destructive thyroiditis would release preformed thyroglobulin into the serum, whereas exogenous hormone suppresses its synthesis.
2
Analyze the profile with low TSH, low free T4, low T3, and high reverse T3.
Identify euthyroid sick syndrome.
The elevated reverse T3 is the distinguishing marker here, reflecting altered 5'-deiodinase activity typical of severe physiologic stress.
3
Analyze the two profiles featuring central hyperthyroidism (inappropriately normal/high TSH with high free T4).
Differentiate them based on the alpha-subunit level.
Both TSH-secreting adenomas and thyroid hormone resistance present with unsuppressed TSH and high T4. However, TSHomas overproduce the alpha-subunit, whereas resistance syndromes do not.
4
Assign the remaining diagnoses based on the differentiation in Step 3.
The profile with elevated alpha-subunit matches TSHoma; the profile with normal alpha-subunit matches Thyroid hormone resistance.
This confirms the final two pathophysiologic mechanisms accurately.

Key Concept

Interpretation of advanced thyroid function testing (thyroglobulin, reverse T3, pituitary alpha-subunit) to differentiate atypical and complex neuroendocrine thyroid profiles.
Estimated Time:2m 0s
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Topics

Endocrinology and Metabolism

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