Relationship Between Thyroid Function Tests and White Blood Cell Count
Direct Relationship Between TFT and WBC
Full blood count including white cell count is recommended in all patients undergoing thyroid evaluation, as it provides prognostic information and helps identify potential causes of ischemia or systemic illness that may affect thyroid function. 1
The relationship between abnormal thyroid function and WBC count is primarily indirect rather than causal:
Thyroid dysfunction itself does not directly alter WBC count in most cases, but both abnormalities may coexist in the context of systemic illness, autoimmune conditions, or as part of comprehensive metabolic assessment 1
WBC count serves as a marker of concurrent illness that may affect thyroid function test interpretation, particularly in the context of non-thyroidal illness syndrome where acute infection or inflammation can transiently suppress TSH 2, 3
Clinical Context for Combined Assessment
When Both Tests Are Indicated
In acute medical admissions, screening both thyroid function and complete blood count is appropriate when clinical suspicion exists for thyroid disease, fever, infection, or unexplained metabolic derangement 1, 4
For patients with suspected autoimmune thyroid disease (Hashimoto's thyroiditis or Graves' disease), WBC count helps exclude concurrent autoimmune conditions or infection that may complicate the clinical picture 1, 5
In critically ill patients, both tests are part of comprehensive metabolic assessment, as non-thyroidal illness can cause abnormal TFTs while elevated WBC suggests infection or inflammatory response 3, 4
Specific Clinical Scenarios
Thyroid storm requires assessment of WBC count as part of evaluation for precipitating infection or inflammatory triggers, with fever and tachycardia being key diagnostic features 1
Postpartum thyroiditis may present with symptoms overlapping with postpartum infection; WBC count helps distinguish between thyroiditis and infectious etiologies 1
Drug-induced thyroid dysfunction (particularly from immunotherapy) requires monitoring of both thyroid function and WBC count, as immune checkpoint inhibitors can cause both thyroid dysfunction (6-9% with anti-PD-1/PD-L1 therapy) and hematologic abnormalities 6
Management Approach
Initial Assessment Algorithm
Measure TSH as the primary screening test with sensitivity >98% and specificity >92% for thyroid dysfunction 6, 5
If TSH is abnormal, measure free T4 to distinguish subclinical (normal free T4) from overt (abnormal free T4) thyroid dysfunction 6, 7
Obtain complete blood count including WBC when evaluating suspected thyroid disease to assess for concurrent illness, infection, or systemic disease 1
Confirm abnormal TSH with repeat testing after 3-6 weeks, as 30-60% of elevated TSH levels normalize spontaneously, particularly in the context of acute illness 6
Critical Pitfalls to Avoid
Do not attribute abnormal TFTs solely to thyroid disease without considering non-thyroidal illness, as acute infection (often marked by elevated WBC) can transiently suppress TSH or alter thyroid hormone levels 2, 3, 4
Avoid treating based on single abnormal TFT values obtained during acute illness when WBC count is elevated, as thyroid function often normalizes after recovery from the acute illness 6, 3
Do not overlook medication effects that may affect both thyroid function and WBC count, particularly immunotherapy agents, amiodarone, or lithium 6, 2
When to Recheck After Acute Illness
For patients with abnormal TFTs during acute illness (suggested by elevated WBC, fever, or systemic symptoms), recheck TSH and free T4 in 4-6 weeks after resolution of the acute illness 6
If TSH remains elevated but <10 mIU/L and the patient is asymptomatic with normal free T4, continue monitoring without treatment, as this may represent recovery phase from non-thyroidal illness 6
Special Populations
Elderly Patients
- For patients >70 years with elevated WBC suggesting infection, use conservative approach to thyroid hormone replacement, starting with lower doses (25-50 mcg/day) if treatment becomes necessary after acute illness resolves 6
Patients on Immunotherapy
Monitor TSH every 4-6 weeks for the first 3 months in patients receiving immune checkpoint inhibitors, as thyroid dysfunction occurs in 6-9% with anti-PD-1/PD-L1 therapy 6
Continue immunotherapy in most cases even with thyroid dysfunction, as high-dose corticosteroids are rarely required for thyroid-related adverse events 6