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Thyroid Function Test

General
1. TSH alone is sufficiently sensitive and specific
2. TSH is a good single screening outpatient test
3. References
  1. Bauer (1996) Arch Intern Med 156:2333
Normal Thyroid Stimulating Hormone (TSH): 0.3 to 5.0 mU/L
1. Normal Free Thyroxine Index or Free T4
  1. Diagnosis: EuThyroid
2. Low Free Thyroxine Index or Free T4
  1. Diagnosis: Secondary Hypothyroidism (Central)
High Thyroid Stimulating Hormone (TSH): >5-20 mU/L
1. Normal Free Thyroxine Index or Free T4
  1. Diagnosis: Subclinical Hypothyroidism
  2. Management
    1. Non-specific Symptoms: Consider Levothyroxine
    2. 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
Low Thyroid Stimulating Hormone (TSH): <0.3 mU/L
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
        
        Risk of Atrial Fibrillation or Osteoporosis
        
        Consider definitive treatment
        
        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
References
1. Costa (1995) Am Fam Physician 52(8):2325
2. Pittman (1996) Am Fam Physician 54(3):961

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