Initial Endocrine Laboratory Evaluation for Suspected Endocrine Disorders
For suspected endocrine disorders, the initial laboratory evaluation should include TSH and free T4 for thyroid function, morning cortisol and ACTH for adrenal function, and fasting glucose and HbA1c for diabetes, followed by appropriate treatment based on the specific diagnosis.
Thyroid Function Assessment
Initial Laboratory Tests
- TSH and Free T4: Essential first-line tests for suspected hypothyroidism 1
- Central hypothyroidism presents with low/normal TSH and low Free T4
- Primary hypothyroidism presents with elevated TSH and low Free T4
- Free T3: Less essential but provides additional insights if TSH and Free T4 are normal 1
Treatment for Hypothyroidism
- Levothyroxine: First-line treatment for hypothyroidism 2
- Initial dosing based on age, weight, and cardiac status
- Taken as a single dose on an empty stomach, 30-60 minutes before breakfast
- Avoid taking with iron, calcium supplements, and antacids (separate by at least 4 hours)
- Monitor TSH and Free T4 6-8 weeks after starting therapy
- Goal: Free T4 in upper half of reference range 1
Adrenal Function Assessment
Initial Laboratory Tests
- Morning cortisol and ACTH (8 AM preferred): First-line tests for adrenal insufficiency 3, 1
- If morning cortisol results are indeterminate (3-15 μg/dL):
Etiologic Diagnosis
- 21-hydroxylase antibodies (21OH-Ab): To identify autoimmune adrenal insufficiency 3, 4
- If 21OH-Ab negative, consider:
Treatment for Adrenal Insufficiency
- Hydrocortisone: First-line treatment 5, 4
- Adults: 15-25 mg/day in 2-3 divided doses (typically 2/3 in morning, 1/3 in afternoon)
- Children: ~8 mg/m²/day 4
- Adjust dosage based on clinical response
- Fludrocortisone: For mineralocorticoid replacement (median dose 0.1 mg daily) 4
- Patient education on stress dosing and emergency administration is essential 3, 4
Diabetes Assessment
Initial Laboratory Tests
- Fasting plasma glucose and HbA1c: First-line tests for suspected diabetes 1
- Renal function and electrolytes: To assess for complications 1
Treatment for Type 2 Diabetes
- Metformin: First-line pharmacologic therapy
- Starting dose: 500 mg once or twice daily with meals
- Gradually increase to effective dose (typically 2000 mg/day in divided doses)
- Monitor renal function before and during treatment
Pituitary Function Assessment
Initial Laboratory Tests
- Complete anterior pituitary axis evaluation 1:
- Thyroid: TSH and Free T4
- Adrenal: Morning cortisol and ACTH
- Gonadal: FSH, LH, testosterone (males), estradiol (females)
- Growth hormone: IGF-1
- Prolactin: Serum prolactin
Treatment for Hypopituitarism
- Replace deficient hormones in order of priority:
Common Pitfalls and Considerations
- Adrenal insufficiency must be treated before thyroid replacement: Starting thyroid hormone without addressing adrenal insufficiency can precipitate adrenal crisis 7
- TSH alone is insufficient for thyroid assessment: In central hypothyroidism, TSH may be normal or low despite low Free T4 1, 8
- Laboratory interference: Biotin supplements, heterophile antibodies, and certain medications can interfere with hormone assays 9
- Timing of testing: Hormone testing should be performed in the morning when possible, particularly for cortisol assessment 1
- Reversible hypothyroidism: Patients with adrenal insufficiency may have reversible hypothyroidism that normalizes with adrenal hormone replacement alone 7
Special Considerations
- Cushing's syndrome evaluation: If suspected, use 24-hour urinary free cortisol (UFC), late-night salivary cortisol (LNSC), or dexamethasone suppression test (DST) 3
- Immune checkpoint inhibitor therapy: Can cause immune-related endocrinopathies requiring specific management approaches 3
- Pregnancy: Women on levothyroxine who become pregnant should increase dose by approximately 30% and monitor TSH monthly 1
By following this systematic approach to endocrine laboratory evaluation and treatment, clinicians can effectively diagnose and manage patients with suspected endocrine disorders.