In a patient with sinus tachycardia and labs showing normal free thyroxine (FT4) and thyroid‑stimulating hormone (TSH) but low free triiodothyronine (FT3), what is the likely diagnosis and appropriate management?

Likely Diagnosis: Nonthyroidal Illness Syndrome (Euthyroid Sick Syndrome)

In a patient presenting with sinus tachycardia, normal FT4 and TSH, but isolated low FT3, the most likely diagnosis is nonthyroidal illness syndrome (also called euthyroid sick syndrome or low T3 syndrome), which represents a physiologic adaptation to acute systemic illness rather than true thyroid disease. 1, 2, 3

Understanding the Thyroid Pattern

• Low T3 with normal TSH and normal FT4 is the hallmark laboratory finding of nonthyroidal illness syndrome, occurring in 60-70% of critically ill patients and representing the mildest form of this adaptive response 2, 3
• The isolated low FT3 reflects decreased peripheral conversion of T4 to T3 (reduced type 1 deiodinase activity) and increased conversion to reverse T3 (increased type 3 deiodinase activity) 2, 4
• This pattern specifically excludes primary hypothyroidism (which would show elevated TSH) and central hypothyroidism (which would show low-normal FT4, not normal FT4) 5

Relationship to Sinus Tachycardia

• The sinus tachycardia is NOT caused by the low T3—rather, both findings are manifestations of the underlying acute illness 1, 2
• Sinus tachycardia in this context should prompt investigation for the primary cause: sepsis, acute cardiac disease, pulmonary embolism, acute coronary syndrome, heart failure, or other acute medical conditions 6
• The mainstay of managing sinus tachycardia is identifying and treating the underlying cause, not treating the thyroid abnormality 6

Differential Diagnosis to Exclude

Medication-Induced Thyroid Suppression

• Glucocorticoids, dopamine, and dobutamine can suppress the thyroid axis and cause low T3 5
• Review the medication list carefully for these agents 7

Central Hypothyroidism

• Central hypothyroidism presents with low or inappropriately normal TSH alongside LOW FT4 (not normal FT4), distinguishing it from this patient's pattern 5
• This diagnosis is excluded by the normal FT4 in this case 5

Primary Hypothyroidism

• Primary hypothyroidism shows elevated TSH (typically >10 mIU/L in severe cases) with low FT4 and eventually low T3 5
• The normal TSH excludes this diagnosis 7, 8

Diagnostic Workup

Confirm Nonthyroidal Illness

• Repeat thyroid function tests (TSH, FT4, FT3) after 3-6 weeks once the acute illness resolves, as 30-60% of thyroid abnormalities in acute illness normalize spontaneously 7, 8
• Measure reverse T3 if available—elevated reverse T3 with low T3 confirms nonthyroidal illness syndrome 2, 4

Identify the Underlying Acute Illness

• Focus diagnostic efforts on finding the cause of sinus tachycardia: obtain ECG, chest X-ray, cardiac biomarkers, complete blood count, metabolic panel, and consider D-dimer if pulmonary embolism is suspected 6
• The severity of T3 reduction correlates with illness severity and mortality risk—a markedly low T3 suggests severe underlying disease requiring aggressive investigation 1, 9

Exclude Medication Effects

• Review for dopamine, glucocorticoids, dobutamine, amiodarone, or other medications that alter thyroid function 7, 5

Management Approach

Primary Management: Treat the Underlying Illness

• Do NOT treat the low T3 with thyroid hormone replacement—this represents a physiologic adaptation, not true hypothyroidism 2, 3, 4
• Direct all therapeutic efforts toward identifying and treating the acute illness causing the sinus tachycardia 6

Rate Control for Sinus Tachycardia

• Beta blockers are the preferred initial agent for symptomatic sinus tachycardia when the underlying cause is being addressed, particularly in anxiety-related disorders, post-myocardial infarction, or heart failure 6
• Nondihydropyridine calcium-channel blockers (diltiazem or verapamil) are alternatives if beta blockers are contraindicated 6
• For patients with atrial fibrillation or cardiac disease, more aggressive rate control may be warranted within 2 weeks 6, 7

Thyroid Hormone Replacement: When NOT to Treat

• Thyroid hormone treatment in nonthyroidal illness syndrome is controversial and generally NOT recommended, as data on beneficial effects are conflicting 2, 4
• The low T3 typically normalizes as the acute illness resolves without intervention 2, 3, 4
• Treatment with thyroid hormone may be harmful in elderly patients and does not improve outcomes in most critically ill patients 2, 4

Monitoring and Follow-Up

• Recheck TSH, FT4, and FT3 in 4-6 weeks after resolution of the acute illness to confirm normalization 7, 8
• If thyroid abnormalities persist beyond 6 months, consider alternative diagnoses such as evolving primary hypothyroidism or central hypothyroidism 1, 3
• Monitor for progression: low T3 with low T4 and low/normal TSH suggests severe nonthyroidal illness or evolving central hypothyroidism 5

Critical Pitfalls to Avoid

• Never start thyroid hormone replacement based solely on low T3 in the setting of acute illness—this misdiagnoses a physiologic adaptation as thyroid disease 2, 3, 4
• Do not overlook the underlying acute illness—the sinus tachycardia and low T3 are both manifestations of a potentially serious systemic condition requiring urgent evaluation 6, 1, 2
• Avoid rechecking thyroid function too soon (before 3-6 weeks)—transient abnormalities are common and lead to overdiagnosis 7, 8
• If central hypothyroidism is ultimately diagnosed, always rule out adrenal insufficiency and start corticosteroids BEFORE thyroid hormone to avoid precipitating adrenal crisis 5

Prognostic Implications

• The degree of T3 reduction correlates with illness severity (r=0.58) and mortality risk 1
• Patients with low T3 and low T4 combined with low or undetectable TSH have the highest mortality 1, 9
• Recovery of thyroid function generally parallels clinical improvement, making serial measurements useful for monitoring disease trajectory 2, 3, 4