Best Initial Screening Test for Thyroid Dysfunction
TSH (thyroid-stimulating hormone) is the single best initial screening test for both hyperthyroidism and hypothyroidism. 1, 2, 3
Why TSH is the Optimal Screening Test
TSH has superior diagnostic accuracy with 98% sensitivity and 92% specificity when used to confirm suspected thyroid disease in clinical settings. 1 This makes it the most reliable single test for detecting both thyroid hormone excess and deficiency before other laboratory abnormalities become apparent. 1
Diagnostic Performance
- TSH detects thyroid dysfunction earlier than other tests, identifying abnormalities before free T4 or T3 levels change 1
- In outpatient settings, TSH is the most sensitive test for detecting mild thyroid hormone excess or deficiency 2, 3
- TSH values below 0.1 mU/L indicate hyperthyroidism, while values above 6.5 mU/L indicate hypothyroidism 1
When to Add Free T4 Testing
After obtaining an abnormal TSH, measure free T4 to distinguish between subclinical and overt thyroid dysfunction. 4, 5
- Subclinical hypothyroidism: Elevated TSH with normal free T4 1, 6
- Overt hypothyroidism: Elevated TSH with low free T4 4, 6
- Subclinical hyperthyroidism: Low TSH with normal free T4 1
- Overt hyperthyroidism: Low TSH with elevated free T4 5
Important Caveats and Limitations
When TSH May Be Unreliable
TSH accuracy is compromised in hospitalized patients with acute illness, where the positive predictive value drops dramatically. 1 In acute care settings, a low TSH has only 24% positive predictive value for hyperthyroidism and 6% for hypothyroidism. 1
Additional situations affecting TSH interpretation include:
- Non-thyroidal illness (sick euthyroid syndrome) 1
- Recent iodine exposure (>20 mGy radiation) 1
- Recovery phase from thyroiditis 4
- Certain medications 4
Central Hypothyroidism Exception
In suspected central (secondary) hypothyroidism due to pituitary or hypothalamic disease, TSH cannot be used as the primary screening test. 5 Instead, measure free T4 directly, as TSH may be inappropriately normal or only mildly elevated despite low thyroid hormone levels. 5
Practical Screening Algorithm
- Order TSH as the initial test for suspected thyroid dysfunction 1, 2, 3
- If TSH is abnormal, add free T4 to classify the disorder 4, 5
- If TSH is undetectable and free T4 is normal, consider adding T3 to detect T3 toxicosis 5
- Confirm elevated TSH with repeat testing after 3-6 weeks, as 30-60% normalize spontaneously 4
Cost-Effectiveness Considerations
Case-finding in clinical settings (testing patients already seeing a physician) is more cost-effective than community screening. 7 The yield is highest in women over 40 years (approximately 1%) and lowest in young men (approximately 0%). 7
TSH alone is more cost-effective as an initial screening test compared to ordering multiple thyroid function tests simultaneously. 7