In hyperthyroidism, is the Thyroid-Stimulating Hormone (TSH) level low and the Thyroxine (T4) level high?

Thyroid Function Tests in Hyperthyroidism

In hyperthyroidism, TSH is low and T4 is high. This classic pattern reflects the negative feedback loop of the hypothalamic-pituitary-thyroid axis, where excess thyroid hormone suppresses TSH production 1, 2.

Laboratory Patterns in Hyperthyroidism

• Overt hyperthyroidism is defined biochemically by a suppressed TSH level and elevated free T4 and/or T3 levels 1
• Subclinical hyperthyroidism is characterized by low TSH with normal T4 and T3 levels, affecting approximately 0.7% to 1.4% of people worldwide 1
• TSH suppression occurs due to negative feedback from elevated thyroid hormones on the pituitary gland 3
• TSH levels in hyperthyroidism are typically below the lower threshold of laboratory reference intervals (usually <0.4 mIU/L) 3

Diagnostic Considerations

• Multiple tests should be performed over a 3-6 month interval to confirm abnormal findings, especially in asymptomatic individuals 3
• Subclinical hyperthyroidism can be further classified based on TSH levels:
  • "Low but detectable" (0.1-0.4 mIU/L) 1
  • "Clearly low" or "undetectable" (<0.1 mIU/L) 1
• Even when total T4 and T3 appear normal, patients with suppressed TSH may have elevated free T4 levels, indicating biochemical hyperthyroidism 4

Common Causes of Hyperthyroidism

• Graves' disease (accounts for approximately 70% of cases) 2
• Toxic nodular goiter (accounts for approximately 16% of cases) 2
• Thyroiditis (in the thyrotoxic phase) 1
• Medication-induced thyroid dysfunction 2

Clinical Implications

• Untreated hyperthyroidism can lead to serious complications including:
  • Cardiac arrhythmias and heart failure 1
  • Osteoporosis and increased fracture risk 1
  • Adverse pregnancy outcomes 1
  • Increased mortality 1, 2
• Common symptoms include anxiety, insomnia, palpitations, unintentional weight loss, diarrhea, and heat intolerance 1

Treatment Considerations

• Treatment options for overt hyperthyroidism include antithyroid drugs, radioactive iodine ablation, and surgery 1
• For subclinical hyperthyroidism, treatment is recommended for patients at highest risk of complications:
  • Those older than 65 years 1
  • Patients with persistent serum TSH levels <0.1 mIU/L 1
• Treatment decisions should be based on the underlying cause, severity of biochemical abnormalities, and patient risk factors 2

Important Caveats

• Laboratory reference intervals for TSH are based on statistical distribution rather than clinical outcomes, leading to some professional disagreement about appropriate cut points 3
• TSH secretion can be affected by conditions other than thyroid dysfunction, requiring careful interpretation 3
• Non-thyroidal illness can cause false positive low TSH results 5
• Approximately 50% of subjects with subclinical hyperthyroidism may be taking levothyroxine 5
• Patients with low but detectable TSH values (0.1-0.4 mIU/L) often recover spontaneously when retested 5