What labs should be checked for a patient presenting with heat intolerance?

Laboratory Evaluation for Heat Intolerance

For a patient presenting with heat intolerance, check thyroid-stimulating hormone (TSH) and free thyroxine (fT4) as the primary screening tests, followed by serum creatine kinase (CK) if there is concern for underlying myopathy or rhabdomyolysis. 1

Primary Thyroid Screening

The most critical initial laboratory evaluation targets thyroid dysfunction, as both hyperthyroidism and hypothyroidism can manifest with altered heat tolerance:

• Hyperthyroidism screening: Order TSH (which will be low) and fT4/T3 (which will be elevated) in patients reporting heat intolerance, warm/moist skin, nervousness, tremulousness, or weight loss 1
• Hypothyroidism consideration: While less commonly presenting as heat intolerance, check TSH (elevated) and fT4 (low or normal) if the patient has cold intolerance, dry skin, or weight gain 1

Muscle-Related Laboratory Tests

When heat intolerance occurs in specific contexts suggesting muscle pathology:

• Serum creatine kinase (CK): Essential for patients with exertional heat intolerance, recurrent rhabdomyolysis, or persistently elevated CK after neurological work-up has excluded other myopathies 1
• CK timing: Check this particularly if there is a history of exertional heat stroke requiring hospitalization or postoperative rhabdomyolysis 1

Specialized Adrenal Testing (Context-Dependent)

For patients with heat intolerance accompanied by hyponatremia or suspected adrenal insufficiency:

• Baseline cortisol and 17-hydroxyprogesterone: Consider in patients with prior heat illness episodes complicated by hyponatremia, as non-classic congenital adrenal hyperplasia (21-hydroxylase deficiency) may contribute to heat illness susceptibility 2
• ACTH stimulation test: Reserve for cases where adrenal insufficiency is suspected based on clinical presentation and initial screening 2

Additional Baseline Metabolic Assessment

While not specific to heat intolerance, obtain these if the patient will undergo further heat tolerance testing or has systemic symptoms:

• Complete blood count: To assess for anemia or infection that could impair thermoregulation 1
• Serum electrolytes (including calcium and magnesium): Particularly important if dehydration or electrolyte disturbances are suspected 1
• Blood urea nitrogen and creatinine: To evaluate renal function and hydration status 1
• Fasting glucose: To exclude diabetes mellitus, which can affect thermoregulation 1

Clinical Context Matters

The specific laboratory panel should be guided by the clinical presentation:

• Primary heat intolerance with hypermetabolic symptoms: TSH, fT4, T3 first 1
• Exertional heat intolerance with muscle symptoms: CK, TSH, consider genetic testing for RYR1 variants if family history or recurrent episodes 1
• Heat intolerance with hyponatremia: Cortisol, 17-hydroxyprogesterone, consider CYP21A2 genotyping 2
• Unexplained persistent heat intolerance: Full metabolic panel including TSH, fT4, CK, electrolytes, CBC 1

Common Pitfalls to Avoid

• Don't overlook thyroid disease: Hyperthyroidism is the most common endocrine cause of heat intolerance and requires only simple screening tests 1
• Don't miss malignant hyperthermia susceptibility: Patients with idiopathic hyperCKemia (persistently elevated CK without identified cause) warrant consideration of MH susceptibility testing 1
• Don't assume all heat intolerance is benign: Recurrent episodes, especially with rhabdomyolysis, require thorough investigation including CK and potentially genetic testing 1, 2