Differential Diagnosis: Increased or Decreased Total Thyroxine

Shanna Hillsman, LVMT, University of Tennessee

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

ArticleLast Updated September 20181 min readPeer ReviewedWeb-Exclusive
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A gray long-haired cat sits calmly by a window, gazing outside with bright green eyes. Soft natural light illuminates its fur, highlighting its relaxed posture and serene expression.

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