What is the management approach for a patient with normal thyroid labs except for decreased Triiodothyronine (T3)?

Likely Diagnosis: Nonthyroidal Illness Syndrome (Euthyroid Sick Syndrome)

When thyroid labs show only decreased T3 with normal TSH and T4, the most likely diagnosis is nonthyroidal illness syndrome (euthyroid sick syndrome), which represents an adaptive physiological response to acute or chronic systemic illness rather than true thyroid disease. 1

Understanding the Pathophysiology

• Decreased serum T3 occurs primarily due to reduced peripheral conversion of T4 to T3, not from thyroid gland dysfunction 1
• The free T3 concentration may be normal or reduced, while TSH typically remains within normal range, confirming the euthyroid state 1
• This represents an adaptive change that enables sick patients to conserve protein during illness, not a pathological condition requiring treatment 1

Key Diagnostic Features

• Normal TSH is the critical finding that distinguishes nonthyroidal illness from true hypothyroidism, as TSH would be elevated in genuine thyroid failure 1, 2
• Normal free T4 further confirms the absence of primary thyroid disease 1
• The isolated decrease in T3 without TSH elevation indicates preserved thyroid axis regulation 1

Clinical Context That Supports This Diagnosis

Look for underlying systemic illness:

• Acute critical illness or hospitalization 1
• Chronic medical conditions (heart failure, renal failure, liver disease) 1
• Recent surgery or trauma 1
• Severe infection or sepsis 1
• Malnutrition or starvation states 1

Management Approach

Do not treat with thyroid hormone replacement. 1

• Treatment with levothyroxine to restore serum T3 concentrations to normal range is not indicated, as the decreased T3 represents an adaptive physiological response 1
• The normal TSH confirms that the thyroid axis is functioning appropriately despite the low T3 1
• Patients with nonthyroidal illness and decreased T3 do not appear clinically hypothyroid, supporting the adaptive rather than pathological nature of this finding 1

Monitoring Strategy

• Recheck thyroid function tests (TSH, free T4, and T3) after resolution of the acute illness, typically 4-6 weeks after recovery 3
• If TSH remains normal and T3 normalizes after illness resolution, this confirms nonthyroidal illness syndrome as the diagnosis 1
• If TSH becomes elevated on repeat testing after illness resolution, this would indicate true hypothyroidism requiring treatment 3

Alternative Diagnoses to Consider

If the patient is on levothyroxine therapy:

• Patients treated with levothyroxine commonly have relatively lower serum T3 concentrations compared to the general population, even when TSH is normalized 4, 5
• The T3 to T4 ratio is significantly lower in levothyroxine-treated patients compared to untreated euthyroid individuals 5
• These patients remain clinically euthyroid despite lower T3 levels, and serum T3 determination is the procedure of choice for evaluation 5
• No dose adjustment is needed if TSH remains in target range (0.5-4.5 mIU/L) 3

If TSH is suppressed (<0.1 mIU/L) with low T3:

• This pattern suggests central (pituitary/hypothalamic) hypothyroidism rather than hyperthyroidism 6
• Measure free T4 to distinguish from subclinical hyperthyroidism 6
• Low or low-normal free T4 with suppressed TSH confirms central hypothyroidism requiring endocrine evaluation 6

Critical Pitfalls to Avoid

• Never initiate levothyroxine based solely on low T3 with normal TSH, as this represents overtreatment of a physiological adaptation 1
• Do not assume thyroid disease without confirming TSH elevation on repeat testing after illness resolution 3, 1
• Avoid checking T3 levels in isolation—TSH is the best screening test, and T3 should only be measured when TSH is abnormal or in specific clinical contexts 2, 7
• Remember that 30-60% of mildly abnormal thyroid tests normalize spontaneously on repeat testing 3