Hormone Blood Tests for Endocrine Evaluation
Order morning (8-9 AM) serum cortisol, free T4 with TSH, testosterone with FSH/LH/prolactin (males), or estradiol with FSH/LH (females with menstrual irregularities), based on the specific endocrine axis you're evaluating.
Algorithmic Approach by Endocrine Axis
Adrenal Axis (Central Adrenal Insufficiency)
- First-line test: 8-9 AM serum cortisol 1
- Interpretation algorithm:
- Cortisol <3 µg/dL → confirms adrenal insufficiency
- Cortisol >15 µg/dL → excludes adrenal insufficiency
- Cortisol 3-15 µg/dL → proceed to corticotropin stimulation test (peak <18.1 µg/dL at 30-60 minutes indicates insufficiency) 1
Critical pitfall: Never use random cortisol levels—they are unreliable and should be avoided entirely 1. If the patient is on hydrocortisone, wait at least 18-24 hours after the last dose before testing; wait even longer for synthetic glucocorticoids 1.
Gonadal Axis - Males (Central Hypogonadism)
Order all four tests simultaneously before 10 AM after overnight fast 1:
- Serum testosterone
- FSH
- LH
- Prolactin
This combination is essential because low testosterone with inappropriately normal/low FSH and LH indicates central hypogonadism, while elevated prolactin may reveal a prolactinoma as the underlying cause. The morning timing is critical as testosterone levels vary diurnally 1.
Important caveat: Do not test during acute or subacute illness, as this will yield falsely abnormal results 1.
Gonadal Axis - Females (Central Hypogonadism)
For premenopausal women with oligomenorrhea/amenorrhea 1:
- Serum estradiol
- FSH
- LH
- Also exclude: Pregnancy, hyperprolactinemia, hyperandrogenism, thyroid disease
Low estradiol with inappropriately normal/low FSH and LH indicates central hypogonadism. The guideline emphasizes excluding other causes of menstrual irregularities, particularly if no other pituitary deficits exist 1.
For postmenopausal women: Simply check FSH and LH—the absence of elevated levels (which should normally be high post-menopause) confirms gonadotrope dysfunction, provided the patient isn't on hormone replacement 1.
Do not order GnRH stimulation testing—it provides no useful diagnostic information 1.
Thyroid Axis (Central Hypothyroidism)
Order both tests together 1:
- Free T4 (fT4)
- TSH
Interpretation: Low fT4 with low, normal, or even mildly elevated TSH in the setting of pituitary disease confirms central hypothyroidism 1. This differs from primary hypothyroidism where TSH would be markedly elevated.
For borderline cases: If fT4 is low-normal with suggestive symptoms, either start levothyroxine empirically or monitor fT4 over time—initiate treatment if it decreases by ≥20% 1.
Avoid dynamic TSH-secretion testing—it's not recommended 1.
Posterior Pituitary (Central Diabetes Insipidus)
For patients with polyuria (>50 mL/kg/24h or >3.5 L/day in a 70-kg person), simultaneously measure 1:
- Serum osmolarity
- Urine osmolarity
- Urine dipstick (to exclude glucose)
Interpretation: High serum osmolarity (>295 mOsmol/L) should trigger urine concentration to ~600 mOsmol/L (urine/plasma ratio ≥2) if the posterior pituitary is intact. Failure to concentrate urine appropriately with negative glucose on dipstick suggests diabetes insipidus 1.
Key Principles
The Endocrine Society guidelines 1 provide the strongest framework for hormone testing in suspected hypopituitarism. These recommendations prioritize:
- Timing matters: Morning samples for cortisol and testosterone are non-negotiable
- Paired measurements: Always measure the pituitary hormone with its target hormone (e.g., TSH with fT4, FSH/LH with sex steroids)
- Context is critical: Avoid testing during acute illness, ensure adequate washout from exogenous hormones
The older research studies [2-3] support these guideline recommendations but focus on specific populations (erectile dysfunction, amenorrhea, PCOS) rather than comprehensive endocrine evaluation. The most recent studies [4-5] address hormone replacement outcomes rather than diagnostic testing and should not influence your initial diagnostic approach.