Laboratory Testing for Hypothyroidism

TSH is the single most important initial test for diagnosing hypothyroidism, followed by free T4 measurement when TSH is abnormal. 1, 2, 3

Primary Screening Test

Measure serum TSH as the first-line test for evaluating suspected hypothyroidism, with sensitivity above 98% and specificity greater than 92% 1, 2, 3
TSH testing using monoclonal antibodies is the recommended initial approach for both screening and evaluating symptomatic patients 4
A single abnormal TSH value should never trigger treatment decisions—confirm with repeat testing after 3-6 weeks, as 30-60% of elevated TSH levels normalize spontaneously 1, 2

Measure free T4 (or free thyroxine index) when TSH is abnormal to distinguish between subclinical hypothyroidism (normal free T4) and overt hypothyroidism (low free T4) 4, 1, 2, 3
Both TSH and free T4 should be performed together in pregnant women with suspected thyroid dysfunction 4
For suspected central (secondary) hypothyroidism, measure free T4 alongside TSH, as TSH may be inappropriately normal or only mildly elevated despite low thyroid hormone levels 5, 3, 6

Measure anti-thyroid peroxidase (anti-TPO) antibodies when elevated TSH is confirmed, to identify autoimmune thyroiditis (Hashimoto's disease) and predict progression risk to overt hypothyroidism 1, 2, 7
Anti-TPO positive patients have 4.3% annual progression risk versus 2.6% in antibody-negative individuals 1
In children with type 1 diabetes, measure anti-TPO and anti-thyroglobulin antibodies shortly after diabetes diagnosis 2
Measure T3 levels only if TSH is undetectable and free T4 is normal, to evaluate for possible hyperthyroidism 3

Perform serial TSH measurements over 3-6 months to confirm abnormal results, as TSH levels can vary up to 50% day-to-day 2
In pregnant women with pre-existing hypothyroidism, measure TSH and free T4 as soon as pregnancy is confirmed and at minimum during each trimester 8
For patients on levothyroxine therapy, recheck TSH 6-8 weeks after any dose adjustment, then every 6-12 months once stable 1, 8, 9

TSH reference ranges vary by age, sex, and ethnicity—12% of people over 80 years without thyroid disease have TSH above 4.5 mIU/L 2
Acute illness, medications (iodine, dopamine, glucocorticoids, amiodarone, immune checkpoint inhibitors), adrenal insufficiency, and pregnancy can all interfere with TSH levels 2, 9
Never rely on T4 alone to assess thyroid status—subclinical hypothyroidism (elevated TSH with normal T4) represents a clinically significant condition requiring evaluation 1
In central hypothyroidism, TSH cannot be used as the primary monitoring parameter—use free T4 and T3 concentrations instead 8, 3

For patients on immune checkpoint inhibitors, monitor TSH (with optional free T4) every 4-6 weeks during the first 3 months, then every second cycle thereafter 4, 1
Before initiating thyroid hormone replacement in suspected central hypothyroidism or hypophysitis, always rule out adrenal insufficiency first and start corticosteroids before levothyroxine to prevent adrenal crisis 4, 1
In pediatric patients, measure both TSH and total or free T4 at 2 and 4 weeks after treatment initiation, 2 weeks after dose changes, then every 3-12 months until growth is complete 8