What lab values indicate hypothyroidism?

Laboratory Values that Indicate Hypothyroidism

The primary laboratory values that indicate hypothyroidism are an elevated thyroid-stimulating hormone (TSH) level and a low free thyroxine (T4) level, which together confirm overt primary hypothyroidism. 1, 2

Primary Hypothyroidism

Primary hypothyroidism is characterized by:

• Elevated TSH: Values exceeding the upper threshold of the laboratory reference interval (commonly defined as >4.5 mIU/L) 1
• Low free T4: Values below the normal reference range 2, 3

Subclinical Hypothyroidism

Subclinical hypothyroidism is identified by:

• Elevated TSH: Values above the reference range (>4.5 mIU/L) 1
• Normal free T4: Values within the normal reference range 4, 5
• Further classification often divides subclinical hypothyroidism into:
  • Mild: TSH between 4.5 and 10.0 mIU/L 1
  • More severe: TSH greater than 10.0 mIU/L 1, 6

Central (Secondary/Tertiary) Hypothyroidism

Central hypothyroidism due to pituitary or hypothalamic dysfunction is characterized by:

• Low or inappropriately normal TSH: Values within or below the reference range 7, 8
• Low free T4: Values below the reference range 7, 2

Clinical Considerations and Pitfalls

Laboratory Testing Approach

• Initial screening: TSH is the recommended initial test for evaluating suspected hypothyroidism 1
• Confirmatory testing: Both TSH and free T4 (or FTI) should be measured in patients with suspected hypothyroidism 1
• Serial measurements: A single abnormal TSH value is insufficient for diagnosis; serial TSH measurements are essential to establish persistent thyroid dysfunction 1
• Daily variation: TSH levels can vary by as much as 50% of mean values on a day-to-day basis, with up to 40% variation in values obtained from serial measurements performed at the same time of day 1

Factors Affecting Laboratory Values

• Acute illness: Serum TSH is frequently suppressed during phases of acute illness 1
• Medications: TSH levels may be affected by drugs such as iodine, dopamine, glucocorticoids, octreotide, or bexarotene 1
• Other conditions: Adrenal insufficiency, pregnancy (particularly first trimester), anorexia nervosa, certain autoimmune diseases, and pituitary adenomas can interfere with normal TSH levels 1
• Beta-blockers: Thyroid dysfunction may be masked if the patient is taking beta-blockers 9

Special Populations

• Pregnancy: TSH reference ranges are trimester-specific during pregnancy, with generally lower values compared to non-pregnant adults 8
• Elderly patients: Interpretation of TSH values should consider age-related changes in thyroid function 4, 5

Treatment Considerations Based on Lab Values

• Overt hypothyroidism: All patients with elevated TSH and low free T4 should receive treatment 4, 3
• Subclinical hypothyroidism:
  • TSH >10 mIU/L: Treatment is generally recommended 4, 6
  • TSH between 4.5-10 mIU/L: Treatment decisions should consider symptoms, presence of thyroid antibodies, and cardiovascular risk factors 4, 6
• Central hypothyroidism: Treatment is monitored with free T4 levels, which should be maintained in the upper half of the normal range 8

Important Caution

• When both adrenal insufficiency and hypothyroidism are present, always start corticosteroids first before thyroid hormone replacement to prevent precipitating adrenal crisis 9, 7