Is low T3 with normal TSH, T4, and free T4 in a patient with chronic hidradenitis suppurativa (HS) indicative of hypothyroidism or another condition related to chronic inflammation?

Low T3 with Normal TSH, T4, and Free T4 in Hidradenitis Suppurativa: Euthyroid Sick Syndrome vs. Hypothyroidism

Low T3 with normal TSH, T4, and free T4 in a patient with chronic hidradenitis suppurativa most likely represents euthyroid sick syndrome (non-thyroidal illness syndrome) due to chronic inflammation rather than true hypothyroidism.

Understanding the Laboratory Pattern

This laboratory pattern (low T3 with normal TSH, T4, and free T4) is characteristic of euthyroid sick syndrome, which occurs in the setting of chronic inflammatory conditions. The American Thyroid Association guidelines recognize that comprehensive thyroid function assessment should include measuring TSH, free T4, and free T3 levels 1.

Key considerations in this case:

• Normal TSH: Primary hypothyroidism is defined by elevated TSH levels, which is not present in this case 2
• Normal T4 and free T4: These values being normal further argues against primary hypothyroidism 1
• Low T3 only: Isolated low T3 in the setting of chronic inflammation suggests adaptation to chronic disease rather than thyroid gland dysfunction 1

Hidradenitis Suppurativa and Thyroid Function

Research on thyroid function in HS patients shows mixed findings:

• A Danish population-based study found HS was associated with hyperthyroidism (lower TSH and higher T3) 3, which is the opposite pattern of what this patient exhibits
• A more recent 2023 study found increased rates of subclinical hypothyroidism in HS patients 4, but subclinical hypothyroidism is characterized by elevated TSH with normal thyroid hormones

The chronic inflammatory nature of HS may affect thyroid hormone metabolism peripherally without affecting the thyroid gland itself. HS is associated with numerous comorbid conditions that contribute to poor health outcomes 2.

Euthyroid Sick Syndrome vs. Hypothyroidism

Euthyroid Sick Syndrome (Non-thyroidal Illness Syndrome):

• Occurs in patients with chronic inflammatory conditions
• Characterized by low T3 with normal TSH and T4
• Represents an adaptive response to illness
• Does not typically require thyroid hormone replacement

Primary Hypothyroidism:

• Characterized by elevated TSH with low or normal T4/T3
• Represents thyroid gland dysfunction
• Requires thyroid hormone replacement therapy

Management Approach

1. Monitor thyroid function:

   • Recheck thyroid function tests in 3-6 months
   • Focus on TSH and free T4 as primary indicators of thyroid function 1, 5
   • Consider testing for thyroid antibodies (TPO-Ab) to rule out autoimmune thyroiditis 1

2. Address the underlying HS:

   • Controlling inflammation from HS may improve thyroid hormone metabolism
   • HS management should follow established guidelines including medical and surgical approaches when appropriate 2
   • Consider factors that may worsen HS severity such as smoking and obesity 6

3. When to consider thyroid hormone replacement:

   • Not indicated for isolated low T3 with normal TSH and T4
   • Would be indicated if TSH becomes elevated or free T4 decreases 1
   • A therapeutic trial of levothyroxine is reasonable only if TSH becomes elevated and the patient is symptomatic 1

Cautions and Considerations

• Avoid overtreatment: Treating euthyroid sick syndrome with thyroid hormone can be harmful and is not recommended
• Monitor for progression: Some patients with euthyroid sick syndrome may eventually develop true hypothyroidism
• Consider other comorbidities: HS is associated with multiple comorbid conditions that may affect thyroid function 2
• Recognize limitations: Free T3 to free T4 ratio is typically lower in treated hypothyroidism compared to euthyroidism 7, but this is not directly applicable to untreated patients with isolated low T3

Conclusion

The laboratory pattern of low T3 with normal TSH, T4, and free T4 in a patient with chronic hidradenitis suppurativa most likely represents euthyroid sick syndrome due to chronic inflammation rather than true hypothyroidism. Management should focus on treating the underlying HS and monitoring thyroid function for any changes that would indicate development of true thyroid dysfunction.