What laboratory tests should be ordered to evaluate hypothyroidism?

Laboratory Tests for Hypothyroidism

The essential laboratory tests for evaluating hypothyroidism are TSH and free T4, with selective use of anti-thyroid peroxidase antibodies (TPOAb) in specific clinical scenarios. 1

Core Testing Algorithm

Initial Evaluation

TSH is the primary screening test for suspected hypothyroidism in non-pregnant adults. 1, 2, 3 This single test should be ordered first, as it has the highest sensitivity for detecting primary hypothyroidism, which accounts for the vast majority of cases.

Free T4 should be measured alongside TSH to:

• Confirm the diagnosis when TSH is elevated
• Distinguish between overt and subclinical hypothyroidism
• Exclude central hypothyroidism (where TSH may be normal or low despite low free T4)
• Assess patients recently treated for hyperthyroidism (where TSH may remain suppressed for weeks to months)

1, 3

Use free T4, not total T4, in non-pregnant patients because total T4 is affected by binding protein alterations (pregnancy, estrogen therapy, liver disease, nephrotic syndrome) that don't reflect actual thyroid status. 1

Antibody Testing - When to Order

TPOAb should be measured in these specific situations:

• Subclinical hypothyroidism (elevated TSH with normal free T4): Positive antibodies predict progression to overt hypothyroidism at 4.3% per year versus 2.6% per year when negative, informing treatment decisions 1

• Nodular thyroid disease suspected to be autoimmune in origin 1

• Recurrent miscarriage with or without infertility (Grade A recommendation) 1

TSH receptor antibodies (TSHRAb) should be measured only in hypothyroid pregnant patients with prior Graves' disease treated with radioactive iodine or thyroidectomy, tested at 20-26 weeks gestation to assess fetal risk. 1

Pregnancy-Specific Testing

In pregnant women, the testing approach differs:

• Measure total T4 or free T4 index plus TSH, not free T4 by direct immunoassay (unless method-specific and trimester-specific reference ranges are available) 1

• Use trimester-specific TSH reference ranges: First trimester upper limit 2.5 mIU/L, second trimester 3.0 mIU/L, third trimester 3.5 mIU/L 1

Tests That Should NOT Be Ordered

Do not use these tests to diagnose hypothyroidism:

• Clinical scoring systems 1
• Reflex relaxation time 1
• Cholesterol levels 1
• Muscle enzymes (CK, LDH) 1
• Total or free T3 - this is explicitly discouraged by Choosing Wisely guidelines for monitoring levothyroxine therapy 4

These tests lack sufficient sensitivity and specificity for diagnosis, though they may be abnormal in hypothyroidism. 1

Monitoring Established Hypothyroidism

TSH alone is sufficient for monitoring levothyroxine replacement therapy in primary hypothyroidism:

• Check at 4-8 weeks after initiating treatment or dose changes
• Once stable, check every 6 months, then annually
• Blood should be drawn before the daily levothyroxine dose

1, 5

Important Caveats

Hospitalized patients: Only order TSH if there is specific clinical suspicion for thyroid dysfunction. Non-thyroidal illness can cause misleading TSH suppression or elevation during recovery phase, leading to inappropriate interventions. 1

Central hypothyroidism: If suspected (pituitary/hypothalamic disease), TSH cannot be used for monitoring—use free T4 and T3 concentrations instead. 3

Interfering factors: Be aware that heterophilic antibodies, biotin supplementation, and certain medications (dopamine, glucocorticoids, octreotide, bexarotene) can cause spurious results. 1, 6