What labs should be drawn if a patient is suspected to have hypothyroidism?

Laboratory Tests for Suspected Hypothyroidism

For patients with suspected hypothyroidism, both thyroid-stimulating hormone (TSH) and free thyroxine (FT4) should be drawn as the initial laboratory tests. 1, 2

Primary Hypothyroidism Evaluation

• Initial testing:

  • TSH and FT4 should be used for case detection in symptomatic patients 1
  • This combination allows proper differentiation between primary and central hypothyroidism 2

• Rationale for combined testing:

  • TSH alone may be misleading in central hypothyroidism, where TSH can remain within normal range despite low FT4 1
  • Low TSH with low FT4 indicates central hypothyroidism, requiring different evaluation 1
  • High TSH with low FT4 indicates primary hypothyroidism 3

Central Hypothyroidism Evaluation

If initial testing suggests central hypothyroidism (low FT4 with normal/low TSH), additional testing should include:

• Morning cortisol and ACTH (to rule out central adrenal insufficiency)
• Gonadal hormones
• Complete blood count
• Comprehensive metabolic panel 2

Special Considerations

• Monitoring during treatment:

  • For primary hypothyroidism: TSH is the most important parameter 4
  • For central hypothyroidism: Free T4 and T3 concentrations should be used 4

• Pregnancy considerations:

  • TSH and FT4 testing should be performed in pregnant women with suspected thyroid dysfunction 1
  • Monitoring should occur at minimum during each trimester 5

Common Pitfalls to Avoid

1. Relying on TSH alone for initial diagnosis:

   • This can miss central hypothyroidism cases where TSH may be normal despite low FT4 1
   • Drawing both TSH and FT4 is especially important when patients are symptomatic 1

2. Misinterpreting subclinical hypothyroidism:

   • Defined as elevated TSH with normal FT4 6
   • Different management approach than overt hypothyroidism

3. Overlooking T3 testing in specific scenarios:

   • Consider T3 testing if TSH is undetectable and free T4 is normal 4

4. Failing to consider medication timing when interpreting results:

   • Levothyroxine should be taken on an empty stomach, preferably 30 minutes before breakfast 2
   • Timing of blood draw relative to medication can affect results

By obtaining both TSH and FT4 as the initial laboratory tests for suspected hypothyroidism, clinicians can accurately diagnose the condition, determine its etiology (primary vs. central), and establish appropriate treatment plans based on laboratory findings.