What is the clinical significance and recommended management of a 67‑year‑old woman with mild leukocytosis (white‑blood‑cell count 13 × 10⁹/L), relative lymphocytosis (17.6 % lymphocytes) and neutropenia (9.7 % neutrophils), normal hemoglobin and hematocrit, who is euthyroid on thyroid hormone replacement therapy (TSH 0.574 mIU/L, free thyroxine 1.26 ng/dL, free triiodothyronine 2.09 pg/mL)?

Clinical Significance of Laboratory Findings

This 67-year-old woman's laboratory values are essentially normal and require no intervention. Her mild leukocytosis (WBC 13 × 10⁹/L) with relative lymphocytosis (17.6%) and neutropenia (9.7%) likely represents normal physiologic variation or a transient viral response, while her thyroid function is adequately controlled on current replacement therapy.

Hematologic Assessment

White Blood Cell Evaluation

The WBC count of 13 × 10⁹/L represents only mild elevation above the normal range (typically 4-11 × 10⁹/L), and the differential pattern does not suggest serious pathology. 1

• The absolute lymphocyte count (approximately 2.3 × 10⁹/L if calculated from 17.6% of 13) falls within normal limits, as normal lymphocyte counts range from 1.0-4.8 × 10⁹/L 1
• The reported "neutropenia" at 9.7% is actually a relative neutropenia (low percentage) but the absolute neutrophil count (approximately 1.26 × 10⁹/L) is only mildly reduced and does not meet criteria for clinically significant neutropenia (typically <1.5 × 10⁹/L) 1
• This pattern—mild leukocytosis with relative lymphocytosis—is commonly seen in viral infections, stress responses, or normal variation in older adults 1

When to Investigate Further

Leukocytosis warrants additional investigation only when accompanied by concerning features that are absent in this patient: 1

• Fever, weight loss, bruising, or fatigue suggesting hematologic malignancy 1
• Marked elevation (WBC >20-25 × 10⁹/L) 1
• Immature white blood cells (blasts, bands) on peripheral smear 1
• Other cytopenias (anemia, thrombocytopenia) suggesting bone marrow dysfunction 1

Since this patient has normal hemoglobin (16 g/dL) and hematocrit (49%), no constitutional symptoms are mentioned, and the WBC elevation is mild, no immediate hematologic workup is indicated. 1

Recommended Approach

• Repeat CBC with differential in 4-6 weeks to confirm this is transient rather than persistent 1
• If WBC normalizes, no further action needed 1
• If WBC remains elevated or increases, obtain peripheral blood smear review to evaluate white cell morphology and maturity 1
• Consider referral to hematology only if: WBC >20 × 10⁹/L, constitutional symptoms develop, or other cytopenias appear 1

Thyroid Function Assessment

Current Thyroid Status

The patient's thyroid function tests indicate adequate replacement therapy with no adjustment needed:

• TSH 0.574 mIU/L falls within the normal reference range (0.45-4.5 mIU/L) and represents optimal replacement 2
• Free T4 1.26 ng/dL is within normal limits 2
• Free T3 2.09 pg/mL is at the lower end of normal, which is expected and acceptable on levothyroxine monotherapy 3

Understanding the T3/T4 Ratio

The relatively low-normal free T3 with normal free T4 is a physiologic consequence of levothyroxine monotherapy and does not indicate inadequate treatment: 3

• Patients treated with levothyroxine alone consistently show a lower free T3 to free T4 ratio compared to euthyroid individuals with intact thyroid glands 3
• The normal thyroid gland produces approximately 20% of circulating T3 directly, whereas levothyroxine monotherapy relies entirely on peripheral conversion of T4 to T3 3
• This pattern is identical in primary and central hypothyroidism treated with T4 alone and does not require combination T4/T3 therapy 3

Monitoring Recommendations

For stable patients on levothyroxine with TSH in the target range:

• Recheck TSH every 6-12 months to ensure continued adequacy of replacement 2
• No dose adjustment is needed when TSH is 0.5-4.5 mIU/L with normal free T4 2
• Free T3 measurement is not routinely indicated for monitoring levothyroxine therapy 2

Erythrocyte Parameters

Hemoglobin and Hematocrit Assessment

The hemoglobin of 16 g/dL and hematocrit of 49% are normal for a woman and indicate no anemia. 4

• These values exclude thyroid-related anemia, which occurs in 40.9% of hyperthyroid patients and 57.1% of hypothyroid patients 4
• Normal erythrocyte parameters in a patient on adequate thyroid replacement confirm appropriate treatment 4

Clinical Integration

No Intervention Required

This patient requires no immediate diagnostic workup or treatment changes:

1. Hematologic: Mild leukocytosis without concerning features—repeat CBC in 4-6 weeks 1
2. Thyroid: Adequately replaced with TSH in target range—continue current levothyroxine dose and recheck TSH in 6-12 months 2
3. Erythrocytes: Normal hemoglobin and hematocrit—no action needed 4

Common Pitfalls to Avoid

• Do not overinterpret relative lymphocytosis when absolute lymphocyte count is normal 1
• Do not adjust levothyroxine dose based on low-normal free T3 in asymptomatic patients with normal TSH and free T4 3
• Do not pursue extensive hematologic workup for mild, isolated leukocytosis without constitutional symptoms or other cytopenias 1
• Do not assume neutropenia based on percentage alone—calculate absolute neutrophil count 1

When to Reassess

Recheck laboratory values if any of the following develop:

• Constitutional symptoms (fever, weight loss, night sweats, fatigue) 1
• Hypothyroid symptoms (cold intolerance, constipation, weight gain) 2
• Hyperthyroid symptoms (palpitations, tremor, heat intolerance) 2
• Bruising or bleeding 1
• WBC continues to rise on repeat testing 1