Lab

Thyroid Function Test

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Thyroid Function Test

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Approach

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]

Interpretation
Normal Thyroid Stimulating Hormone (TSH): 0.3 to 5.0 mU/L

Euthyroid
No additional testing indicated
Normal Serum TSH range is decreased in pregnancy
1. See Thyroid Dysfunction in Pregnancy

Interpretation
High Thyroid Stimulating Hormone (TSH): >5-20 mU/L

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
Low Free Thyroxine Index or Free T4
1. Diagnosis: Hypothyroidism
2. Management: Levothyroxine

Interpretation
Low Thyroid Stimulating Hormone (TSH): <0.3 mU/L

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
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