Laboratory Testing for Hypothyroidism Diagnosis
TSH is the single most important initial test for diagnosing hypothyroidism, followed by free T4 measurement when TSH is elevated to distinguish between subclinical and overt disease. 1
First-Line Testing
Measure serum TSH as the primary screening test - it has 98% sensitivity and 92% specificity for detecting thyroid dysfunction and is the most sensitive initial test for evaluating thyroid function 1, 2
Follow abnormal TSH with free T4 measurement to differentiate between:
Confirm abnormal results with repeat testing after 3-6 weeks before initiating treatment, as 30-60% of elevated TSH levels normalize spontaneously on repeat measurement 1, 3
Additional Diagnostic Tests
Measure anti-thyroid peroxidase (anti-TPO) antibodies to confirm Hashimoto's thyroiditis (autoimmune thyroiditis), the most common cause of primary hypothyroidism in iodine-sufficient areas 1, 2
Check anti-thyroglobulin antibodies along with anti-TPO when autoimmune etiology is suspected, particularly in patients with diabetes type 1 or other autoimmune conditions 1
Anti-TPO positivity predicts higher progression risk to overt hypothyroidism (4.3% per year versus 2.6% in antibody-negative individuals) 3
Critical Interpretation Points
Normal TSH range is typically 0.4-4.5 mIU/L, though these thresholds are somewhat arbitrary and not based on adverse health outcomes 1
TSH levels vary by age, sex, and ethnicity - for example, 12% of people over 80 years without thyroid disease have TSH >4.5 mIU/L 1
Serial TSH measurements over 3-6 months are essential because TSH can vary up to 50% day-to-day due to pulsatile secretion and physiological factors 1
Tests to Avoid or Use Selectively
Do NOT routinely measure T3 levels for hypothyroidism diagnosis - T3 is more appropriate for hyperthyroidism diagnosis 4, 5
Avoid relying on total T4 or total T3 as these are affected by thyroid-binding globulin levels and do not reliably reflect thyroid status 5
Free T4 is superior to total T4 because it is independent of transport protein concentrations 5
Special Populations
Pregnant women or those planning pregnancy: Measure both TSH and free T4 when hypothyroidism is suspected, as even subclinical hypothyroidism is associated with adverse pregnancy outcomes including preeclampsia and low birth weight 6, 1
Patients on immunotherapy: Check TSH every 4-6 weeks initially, as thyroid dysfunction occurs in 6-9% with anti-PD-1/PD-L1 therapy 3
Patients with diabetes type 1: Measure anti-TPO and anti-thyroglobulin antibodies shortly after diabetes diagnosis due to increased autoimmune thyroid disease risk 1
Common Pitfalls to Avoid
Never diagnose based on a single abnormal TSH value - always confirm with repeat testing as transient elevations are common 1, 3
Be aware of interfering factors: Acute illness, medications (iodine, dopamine, glucocorticoids, amiodarone), adrenal insufficiency, pregnancy, and recent iodine exposure (CT contrast) can all affect TSH levels 1, 3
Do not screen asymptomatic adults routinely - targeted testing is only recommended for high-risk patients (those with type 1 diabetes, autoimmune disease, family history, or symptoms) 2, 7
For central hypothyroidism (pituitary or hypothalamic dysfunction): TSH cannot be used reliably; instead monitor free T4 and T3 levels directly 4