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Difficulty: MediumEndocrinology and Metabolism

Match each diagnostic laboratory profile with the most likely etiology of thyrotoxicosis.

  • Decreased TSH, increased free T4, decreased radioactive iodine uptake (RAIU), undetectable serum thyroglobulinExogenous levothyroxine intake
  • Decreased TSH, increased free T4, decreased radioactive iodine uptake (RAIU), elevated serum thyroglobulinSubacute granulomatous thyroiditis
  • Decreased TSH, increased free T4, increased radioactive iodine uptake (RAIU) with a diffuse, homogeneous patternGraves disease
  • Normal or elevated TSH, increased free T4, increased alpha-subunit to TSH molar ratioTSH-secreting pituitary adenoma

Answer

Exogenous levothyroxine intake matches the profile of low RAIU and undetectable thyroglobulin. Subacute granulomatous thyroiditis matches low RAIU and elevated thyroglobulin. Graves disease matches diffusely increased RAIU. A TSH-secreting pituitary adenoma matches central hyperthyroidism with an increased alpha-subunit.
The diagnosis of thyrotoxicosis relies on a structured laboratory algorithm. Primary hyperthyroidism presents with a suppressed TSH and elevated Free T4. Within this category, high radioactive iodine uptake (RAIU) indicates new hormone synthesis (e.g., Graves disease with a diffuse pattern). Low RAIU indicates release of preformed hormone or an exogenous source. These are distinguished by serum thyroglobulin, which is released from damaged follicles in destructive thyroiditis (elevated) but suppressed in exogenous hormone ingestion (undetectable). Conversely, central hyperthyroidism presents with a normal or elevated TSH despite high Free T4; TSH-secreting adenomas typically also hypersecrete the alpha-subunit.

Step-by-Step Solution

1
Evaluate the TSH and Free T4 relationship to determine the origin.
Low TSH with high Free T4 indicates primary hyperthyroidism. Normal or high TSH with high Free T4 indicates central hyperthyroidism (TSH-secreting adenoma).
This establishes whether the source of thyrotoxicosis is driven by the pituitary or is independent of it.
2
Assess the radioactive iodine uptake (RAIU) in patients with primary hyperthyroidism.
High RAIU suggests active de novo hormone synthesis (e.g., Graves disease). Low RAIU suggests release of preformed hormone or exogenous intake.
RAIU distinguishes hyperactive glandular synthesis from thyroid destruction or factitious thyrotoxicosis.
3
Use serum thyroglobulin to differentiate the specific causes of low RAIU thyrotoxicosis.
Elevated thyroglobulin indicates thyroid follicular destruction (subacute thyroiditis). Undetectable thyroglobulin indicates an exogenous hormone source (exogenous levothyroxine intake).
Thyroglobulin is released during follicular damage but is suppressed when taking exogenous pure levothyroxine.

Key Concept

Diagnostic evaluation of thyrotoxicosis using thyroid function tests, radioactive iodine uptake, and serum markers.
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Topics

Endocrinology and Metabolism

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