What are the typical serum Thyroid-Stimulating Hormone (TSH) and free Thyroxine (T4) levels in a patient with hyperthyroidism?

Thyroid Function Tests in Hyperthyroidism

In hyperthyroidism, serum TSH is suppressed (below 0.1 mU/L) while free T4 is elevated above the normal reference range. 1

Laboratory Pattern in Hyperthyroidism

The American College of Physicians guidelines clearly define the laboratory patterns seen in thyroid disorders:

• Overt Hyperthyroidism:

  • TSH: Low (<0.1 mU/L)
  • Free T4: Elevated
  • This pattern represents severe hyperthyroidism 1

• Subclinical Hyperthyroidism:

  • TSH: Low (<0.1 mU/L)
  • Free T4: Normal
  • This pattern represents mild hyperthyroidism 1

Subclinical hyperthyroidism can be further categorized:

• Grade I: TSH detectable but low (0.1-0.4 mU/L)
• Grade II: TSH fully suppressed (<0.1 mU/L) 2

Pathophysiology Explanation

The inverse relationship between TSH and thyroid hormones occurs due to the negative feedback mechanism of the hypothalamic-pituitary-thyroid axis:

1. In hyperthyroidism, excess thyroid hormone production (T3 and T4) occurs from the thyroid gland
2. Elevated thyroid hormones exert negative feedback on the pituitary gland
3. This suppresses TSH production, resulting in low or undetectable TSH levels
4. The combination of suppressed TSH with elevated free T4 is diagnostic of overt hyperthyroidism 1

Clinical Considerations

• TSH testing has high sensitivity (98%) and specificity (92%) for thyroid disorders 1
• Values below 0.1 mU/L are considered low 1
• In some cases of hyperthyroidism, T3 levels may be disproportionately elevated compared to T4 (T3 toxicosis)
• Non-thyroidal illness can cause false positive low TSH results 3
• Propranolol treatment in hyperthyroidism may decrease T3 levels while increasing reverse T3, but typically doesn't affect T4 levels significantly 4

Common Pitfalls and Caveats

• Always interpret thyroid function tests together (TSH and free T4), not in isolation
• Ensure proper timing of blood draws as TSH has diurnal variation
• Medication effects can alter thyroid function tests:
  • Levothyroxine therapy can cause iatrogenic subclinical hyperthyroidism 3
  • Glucocorticoids, dopamine, and severe illness can suppress TSH
• Patients with subclinical hyperthyroidism may progress to overt hyperthyroidism at a rate of up to 5% per year 3
• Elderly patients have higher prevalence of subclinical hyperthyroidism (approximately 3%) 3

Remember that while laboratory values are crucial for diagnosis, clinical symptoms must also be considered when evaluating patients with suspected hyperthyroidism, as subclinical hyperthyroidism may be asymptomatic or present with milder versions of classic hyperthyroid symptoms.