What is a low Thyroid-Stimulating Hormone (TSH) level called?

Low TSH: Understanding Hyperthyroidism

A low Thyroid-Stimulating Hormone (TSH) level is called hyperthyroidism, which can be further classified as subclinical hyperthyroidism when TSH is low but thyroid hormone levels remain normal. 1, 2

Types of Low TSH Conditions

Low TSH levels can be categorized based on severity:

1. Subclinical Hyperthyroidism (SH):

   • Grade I SH: TSH is detectable but low (0.1-0.4 mIU/L) with normal free T4 and T3 levels 2
   • Grade II SH: TSH is fully suppressed (<0.1 mIU/L) with normal free T4 and T3 levels 2

2. Overt Hyperthyroidism:

   • Low or undetectable TSH with elevated T4 and/or T3 levels 1

Causes of Low TSH

Common causes include:

• Graves' disease: The most common cause of hyperthyroidism
• Toxic multinodular goiter: Multiple autonomous nodules producing excess thyroid hormone
• Toxic adenoma: Single autonomous nodule (hot nodule) 3
• Thyroiditis: Including subacute (De Quervain) thyroiditis 3
• Medication-induced: Amiodarone, excessive thyroid hormone replacement 3, 4
• Iodine overload: From contrast media or supplements 4

Clinical Significance

Low TSH levels are associated with significant health implications:

• Found in approximately 1-3% of the elderly population 2
• Associated with increased morbidity and mortality in longitudinal studies 2, 5
• Mortality risk increases by approximately 11-13% for every 6 months of decreased TSH 5
• After 5 years of decreased TSH, mortality increases by 184% in untreated patients and 239% in treated patients with persistent low TSH 5

Diagnostic Approach

When confronted with a low TSH level:

1. Confirm the finding: Repeat TSH measurement after 3-6 months to rule out transient abnormalities 1, 6
2. Measure free T4 and free T3 levels: To distinguish between subclinical and overt hyperthyroidism 1, 2
3. Consider scintigraphy: Especially useful in multinodular goiter to identify hot nodules 3
4. Rule out non-thyroidal causes: Though rare (only 0.3% of cases), certain non-thyroidal illnesses can cause low TSH 4

Clinical Implications

The clinical significance of low TSH varies by severity:

• Undetectable TSH (<0.04 mIU/L): Associated with thyrotoxicosis in 97% of cases (excluding patients on thyroid hormone therapy) 4
• Mildly suppressed TSH (0.04-0.15 mIU/L): About 41% of patients may be asymptomatic despite having thyroid abnormalities 4

Treatment Considerations

Treatment decisions should be guided by:

• Degree of TSH suppression: More aggressive treatment for fully suppressed TSH (<0.1 mIU/L) 1, 2
• Presence of symptoms: Even asymptomatic patients may benefit from treatment to reduce mortality risk 5
• Underlying cause: Treatment approach differs for Graves' disease versus toxic nodular disease 1

Important Caveats

• Mortality risk persists in treated patients with continued low TSH: Maintaining euthyroidism (normal TSH) is crucial for reducing mortality risk 5
• Meticulous follow-up during treatment: Essential to maintain biochemical euthyroidism 5
• Not all low TSH indicates hyperthyroidism: Other conditions and medications can cause low TSH without thyrotoxicosis 6

Careful monitoring and appropriate treatment of low TSH conditions are essential to prevent the significant increase in mortality associated with persistent hyperthyroidism, whether treated or untreated.