What does a Thyroid-Stimulating Hormone (TSH) level of 0.42 indicate?

TSH 0.42: Normal Low-End Value Requiring No Treatment

A TSH of 0.42 mIU/L falls within the normal reference range (0.4-4.5 mIU/L) and does not indicate thyroid dysfunction requiring intervention. 1, 2

Clinical Significance of This Value

• This TSH level represents the lower end of normal and does NOT indicate hyperthyroidism. The geometric mean TSH in disease-free populations is 1.4 mIU/L, and values between 0.4-4.5 mIU/L are considered normal. 1

• TSH values below 0.1 mIU/L define subclinical hyperthyroidism, while values between 0.1-0.4 mIU/L represent a "low but detectable" category that may warrant monitoring. 2 Your value of 0.42 mIU/L exceeds both these thresholds.

• In older adults without hyperthyroidism, low TSH values are common and often clinically insignificant. A study of 2,575 ambulatory persons over 60 years found that 3.9% had TSH <0.1 mIU/L, but only 12% of these actually had hyperthyroidism. 3

What This Value Rules Out

• Normal TSH combined with normal free T4 definitively excludes both overt and subclinical thyroid dysfunction. 1 If free T4 is also normal, no thyroid pathology is present.

• The positive predictive value of a low TSH alone for hyperthyroidism is only 12% in older adults, rising to 67% only when combined with elevated T4. 3 At TSH 0.42 mIU/L, the likelihood of thyroid disease is negligible.

When to Recheck Thyroid Function

• For asymptomatic individuals with normal thyroid function tests, routine screening intervals are not necessary. 1 Recheck only if symptoms develop, such as:

  • Unexplained fatigue, weight changes, or temperature intolerance 1
  • Tachycardia, tremor, or palpitations 1
  • New cardiac arrhythmias or bone health concerns 1

• If rechecking is warranted, measure both TSH and free T4 simultaneously after 3-6 weeks, as TSH secretion is highly variable and sensitive to acute illness, medications, and physiological factors. 1, 2

Important Caveats

• TSH values can be transiently affected by non-thyroidal factors: 1

  • Acute illness or hospitalization
  • Recent iodine exposure (CT contrast)
  • Certain medications
  • Recovery phase from thyroiditis

• A single borderline TSH value should never trigger treatment decisions. 30-60% of mildly abnormal TSH levels normalize spontaneously on repeat testing. 1, 2

• Persons with TSH between 0.1-0.45 mIU/L are unlikely to progress to overt hyperthyroidism. 2 Approximately 25% of individuals with subclinical hyperthyroidism revert to euthyroid state without intervention. 2

Bottom Line

No action is required for TSH 0.42 mIU/L in an asymptomatic patient. This value sits comfortably within the normal reference range and does not warrant repeat testing, further workup, or treatment unless clinical symptoms develop. 1, 2