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Differential Diagnosis: Increased or Decreased Total Thyroxine

Shanna Hillsman, LVMT, University of Tennessee

M. Katherine Tolbert, DVM, PhD, DACVIM (SAIM), Texas A&M University

Endocrinology & Metabolic Diseases

|September 2018|Peer Reviewed|Web-Exclusive

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Following are differential diagnoses, listed in order of likeliness, for patients presented with increased or decreased total thyroxine (T4).

Increased Total Thyroxine

  • Hyperthyroidism
    • Functional benign adenomatous hyperplasia
    • Functional thyroid carcinoma
    • Thyroxine oversupplementation
    • Dietary causes
  • Analytical error (eg, false positive)

Decreased Total Thyroxine

  • Nonthyroidal illness (eg, euthyroid sick syndrome)
  • Hypothyroidism
    • Lymphocytic thyroiditis
    • Thyroid atrophy
    • Iatrogenic secondary to radioactive iodine therapy
    • Methimazole therapy
    • Thyroid neoplasia
    • Sulfonamides
    • Congenital
  • Hyperadrenocorticism
  • Drug effects
    • Phenobarbital
    • Potassium bromide
    • Carprofen
    • Clomipramine
    • Glucocorticoids
    • Propranolol
  • Analytical error

References

For global readers, a calculator to convert laboratory values, dosages, and other measurements to SI units can be found here.

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