What laboratory tests should I order to evaluate thyroid function, including T4 and T3, in a patient with suspected hypo‑ or hyperthyroidism?

Laboratory Testing for Thyroid Function Evaluation

Order TSH as your initial screening test, and if abnormal, follow with free T4 to distinguish between subclinical and overt thyroid dysfunction. 1, 2

Initial Testing Strategy

First-Line Test: TSH

• TSH is the most sensitive initial screening test with sensitivity above 98% and specificity greater than 92% for detecting thyroid dysfunction 2, 3
• TSH testing using monoclonal antibodies is the recommended first test for screening and evaluating patients with symptomatic thyroid disease 1
• A single TSH measurement effectively identifies both hypo- and hyperthyroidism in most uncomplicated cases 4, 3

Second-Line Test: Free T4

• Measure free T4 (FT4) when TSH is abnormal to distinguish between subclinical hypothyroidism (normal FT4) and overt hypothyroidism (low FT4) 2, 3
• Free T4 provides a more accurate assessment than total T4 because it is not influenced by variations in thyroid hormone-binding proteins 5, 6
• In pregnant women with suspected thyroid dysfunction, measure both TSH and FT4 or free thyroxine index (FTI) together 1

When to Add T3 Testing

Limited Indications for T3

• Measure free T3 (FT3) only in specific scenarios:
  • When TSH is suppressed but free T4 is normal, to detect T3 toxicosis 4, 3
  • When hyperthyroidism is suspected but FT4 is normal 5
  • For early diagnosis of hyperthyroidism 7

T3 Is NOT Useful for Hypothyroidism

• Do not routinely measure T3 when evaluating suspected hypothyroidism because FT3 may remain normal even in patients with subclinical or mild thyroid failure 6
• Serum T3 is not useful for verification of clinical hypothyroidism 7
• Circulating T3 is a less reliable reflection of thyroid hormone production than T4 because 80% is produced extrathyroidally from T4 deiodination 5

Algorithmic Approach

For Suspected Hypothyroidism:

1. Order TSH first 1, 2, 3
2. If TSH is elevated: Add free T4 to distinguish subclinical (normal FT4) from overt (low FT4) hypothyroidism 2, 3
3. If TSH is normal but clinical suspicion remains high: Consider central hypothyroidism and measure free T4 directly 3
4. Do NOT order T3 for hypothyroidism evaluation 7, 6

For Suspected Hyperthyroidism:

1. Order TSH first 1, 4
2. If TSH is suppressed: Add free T4 4, 3
3. If TSH is suppressed but FT4 is normal: Add free T3 to detect T3 toxicosis 4, 3

Common Pitfalls to Avoid

• Avoid ordering total T4 or total T3 instead of free hormones, as total hormone measurements are influenced by binding protein abnormalities (pregnancy, estrogen therapy, familial dysalbuminemic hyperthyroxinemia, TBG deficiency) 5, 6
• Do not order T3 when evaluating hypothyroidism—it adds no diagnostic value and may remain normal despite significant thyroid failure 7, 6
• Recognize that thyroid tests are often abnormal in non-thyroidal illnesses and drug administration can affect results; interpret cautiously in hospitalized or acutely ill patients 7, 5
• Confirm elevated TSH with repeat testing after 3–6 weeks before initiating treatment, as 30–60% of elevated TSH values normalize spontaneously 2

Special Populations

Pregnancy

• Measure both TSH and FT4 (or FTI) together in pregnant women with suspected thyroid dysfunction 1
• TSH and FT4 should be performed in pregnant women with suspected hyperthyroidism or hypothyroidism 1

Central Hypothyroidism

• When pituitary or hypothalamic disease is suspected, TSH cannot be used as a screening test—measure free T4 and free T3 directly 3
• TSH may be inappropriately normal or even low despite low thyroid hormone levels in central hypothyroidism 3

Monitoring Thyroid Replacement

• For primary hypothyroidism on treatment: TSH is the most important parameter to monitor 3
• For central hypothyroidism on treatment: Use free T4 and T3 concentrations because TSH levels cannot be used 3