Lab

Thyroid Function Test

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Thyroid Function Test

  • Approach
  1. TSH alone is sufficiently sensitive and specific
    1. TSH is a good single screening outpatient test
    2. Bauer (1996) Arch Intern Med 156:2333-7 [PubMed]
  1. Euthyroid
  2. No additional testing indicated
  3. Normal Serum TSH range is decreased in pregnancy
    1. See Thyroid Dysfunction in Pregnancy
  1. Normal Free Thyroxine Index or Free T4
    1. Diagnosis: Subclinical Hypothyroidism
    2. Non-specific Symptoms
      1. See Subclinical Hypothyroidism
      2. Consider Levothyroxine
    3. Goiter OR Microsomal Antibodies exceed 1:1600
      1. Positive: Levothyroxine
      2. Negative: Follow clinically
  2. Low Free Thyroxine Index or Free T4
    1. Diagnosis: Hypothyroidism
    2. Management: Levothyroxine
  1. Normal Free Thyroxine Index or Free T4
    1. Normal Triiodothyronine (T3)
      1. Diagnosis: Subclinical Hyperthyroidism
      2. Management
        1. Consider following clinically
        2. Age over 60 years
          1. Risk of Atrial Fibrillation or Osteoporosis
          2. Consider definitive treatment
          3. Observe closely
    2. High Triiodothyronine (T3)
      1. Diagnosis: T3 Toxicosis
      2. Management: Treat Hyperthyroidism
  2. High Free Thyroxine Index or Free T4
    1. Diagnosis: Hyperthyroidism
    2. Radioiodine Uptake scan Elevated:
      1. Grave's Disease
      2. Toxic nodular Goiter
      3. Toxic Thyroid Adenoma
    3. Radioiodine Uptake scan Low or Absent:
      1. Acute viral Thyroiditis
      2. Silent Thyroiditis
      3. Struma ovarii
      4. Excessive Levothyroxine ingestion