What is the interpretation of a thyroid function test showing a Thyroid-Stimulating Hormone (TSH) level of 2.080 milliunits per liter (mU/L) and a Thyroxine (T4) level of 0.79 nanograms per deciliter (ng/dL) in an adult patient?

Interpretation of TSH 2.080 mU/L and T4 0.79 ng/dL

These results indicate normal thyroid function (euthyroidism), as the TSH falls within the standard reference range of 0.45-4.12 mU/L, though the T4 value requires clarification of units and reference range to fully assess. 1, 2

Primary Assessment

TSH Interpretation:

• A TSH of 2.080 mU/L is solidly within the normal reference range (0.45-4.12 mU/L for adults under 60 years) 1, 2
• This value is close to the geometric mean TSH of 1.4 mU/L observed in disease-free populations, indicating optimal thyroid function 3
• Values below 0.1 mU/L are considered low and values above 6.5 mU/L (or 4.5 mU/L by some definitions) are considered elevated 1

T4 Interpretation - Critical Caveat:

• The T4 value of 0.79 ng/dL appears abnormally low if this represents total T4 (normal range typically 5-12 mcg/dL or 50-120 ng/mL) 4
• This likely represents a unit conversion issue or the value may actually be free T4 in different units 5
• If this is free T4 measured in ng/dL, it would need comparison to the laboratory's specific reference range (typically 0.8-1.8 ng/dL or 10-23 pmol/L) 5, 4

Clinical Significance

Normal TSH with Potentially Low T4:

• When TSH is normal but T4 appears low, consider: 5, 4
  • Laboratory error or unit misinterpretation (most likely in this case)
  • Central (secondary) hypothyroidism - where pituitary dysfunction causes inappropriately normal TSH despite low thyroid hormone 5, 4
  • Non-thyroidal illness affecting T4 levels while TSH remains normal 1

If Results Are Accurate:

• A normal TSH with truly low T4 would be highly unusual and warrants investigation for central hypothyroidism, particularly if the patient has symptoms of hypothyroidism (fatigue, weight gain, cold intolerance) or history of pituitary/hypothalamic disease 5, 4
• In central hypothyroidism, TSH is diagnostically misleading and free T4 should be measured directly 5

Recommended Next Steps

Immediate Actions:

• Verify the T4 units and reference range with the laboratory - this is the most critical step 5
• If T4 is truly low with normal TSH, measure free T4 to clarify thyroid status 1, 5
• Assess for symptoms of thyroid dysfunction (fatigue, weight changes, temperature intolerance, cognitive changes) 1

If Patient is Asymptomatic with Confirmed Normal Values:

• No treatment or further testing is needed 1
• Routine screening intervals are not required unless symptoms develop or risk factors emerge 3

If Central Hypothyroidism is Suspected:

• Evaluate other pituitary hormones, especially the hypothalamic-pituitary-adrenal axis, as hypocortisolism must be corrected before initiating thyroid hormone replacement to avoid precipitating adrenal crisis 4, 3

Important Clinical Caveats

• TSH values can be transiently affected by acute illness, recent iodine exposure, certain medications, or recovery from thyroiditis 1, 3
• Each individual has a unique thyroid function "set point" - a test result within laboratory reference limits may not be normal for a specific individual if their baseline differs significantly 6
• When used for screening in primary care populations, TSH has lower positive predictive value than in specialty settings, and interpretation can be complicated by underlying illness or frailty 1
• The distinction between normal and abnormal thyroid function can be somewhat arbitrary, as individual reference ranges are much narrower than population-based laboratory ranges 6