Reference — Endocrine
Endocrine Laboratory Values Reference
Key endocrine lab values for nurses — glucose, A1C, TSH, free T4/T3, cortisol, ACTH, aldosterone, C-peptide, and insulin: normal ranges, elevated and decreased causes, and interpretation notes.
Educational use only. This material supports nursing education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional policy, or medical direction. Always follow facility protocols and current provider orders.
Reference ranges vary by lab.Always confirm with your institution's reference ranges and clinical context. These values reflect typical adult ranges from common clinical references.
Glucose Monitoring
| Lab Test | Normal Range | Elevated Causes | Decreased Causes |
|---|---|---|---|
| Fasting plasma glucose | 70–99 mg/dL | Diabetes (≥126), prediabetes (100–125), steroid use, Cushing syndrome, stress hyperglycemia, pancreatitis, acromegaly | Hypoglycemia (<70), insulin overdose, prolonged fasting, Addison disease, insulinoma |
| Random glucose | 70–139 mg/dL (post-meal) | Same as fasting glucose causes | Same as fasting hypoglycemia causes |
| HbA1c (A1C) | <5.7% | Diabetes (≥6.5%); reflects average glucose over ~3 months; prediabetes 5.7–6.4% | Hemolytic anemia, blood transfusion (falsely low — shorter RBC lifespan) |
Fasting plasma glucose — Interpretation note:
Fast ≥8 hours. Values ≥126 on two occasions = diabetes diagnosis.
Random glucose — Interpretation note:
≥200 mg/dL + classic symptoms = diagnostic for diabetes without repeat testing.
HbA1c (A1C) — Interpretation note:
Each 1% A1C ≈ 28–30 mg/dL average glucose. Not valid in sickle cell, hemolytic anemia.
Thyroid Function
| Lab Test | Normal Range | Elevated Causes | Decreased Causes |
|---|---|---|---|
| TSH (Thyroid-Stimulating Hormone) | 0.4–4.0 mIU/L | Primary hypothyroidism (TSH high because thyroid fails → pituitary releases more TSH to stimulate); TSH >10 = overt hypothyroidism | Hyperthyroidism (thyroid produces excess T3/T4 → pituitary suppresses TSH); secondary hypothyroidism (pituitary failure) |
| Free T4 (Thyroxine) | 0.8–1.8 ng/dL | Hyperthyroidism (Graves disease, toxic multinodular goiter, thyroid storm); exogenous thyroid hormone excess | Hypothyroidism; severe illness (euthyroid sick syndrome) |
| Free T3 (Triiodothyronine) | 2.3–4.2 pg/mL | Hyperthyroidism; T3 thyrotoxicosis; T4→T3 hypercovnersion | Hypothyroidism; euthyroid sick syndrome (severe nonthyroidal illness) |
TSH (Thyroid-Stimulating Hormone) — Interpretation note:
TSH is the best FIRST screening test for thyroid disorders. Low T4 + HIGH TSH = primary hypothyroid. Low T4 + LOW TSH = secondary/central hypothyroid.
Free T4 (Thyroxine) — Interpretation note:
Free T4 is active unbound fraction — more reliable than total T4. Use with TSH for full thyroid picture.
Free T3 (Triiodothyronine) — Interpretation note:
T3 is the active hormone at tissue level (T4 is converted to T3 peripherally). Less commonly ordered routinely; important in suspected thyroid storm or T3 toxicosis.
Adrenal Function
| Lab Test | Normal Range | Elevated Causes | Decreased Causes |
|---|---|---|---|
| Cortisol (AM serum) | 6–23 mcg/dL (AM); PM values are lower | Cushing syndrome/disease (tumor, exogenous steroids, ectopic ACTH); stress response; severe illness; obesity | Adrenal insufficiency (primary = Addison disease; secondary = pituitary failure) |
| ACTH (Adrenocorticotropic Hormone) | 7–50 pg/mL (AM) | Primary adrenal insufficiency (Addison — adrenal fails → ACTH rises trying to stimulate); Cushing DISEASE (pituitary adenoma secreting ACTH) | Secondary/tertiary adrenal insufficiency (pituitary or hypothalamic failure → ACTH falls → adrenals atrophy); Cushing SYNDROME from exogenous steroids or adrenal tumor (ACTH suppressed) |
| Aldosterone | 3–16 ng/dL (supine/normal Na intake) | Primary hyperaldosteronism (Conn syndrome — aldosterone-producing adrenal adenoma); secondary hyperaldosteronism (low renal perfusion, HF, cirrhosis) | Adrenal insufficiency; Addison disease; ACE inhibitor/ARB use; hypoaldosteronism |
Cortisol (AM serum) — Interpretation note:
Cortisol follows diurnal rhythm — highest AM, lowest midnight. AM cortisol <3 mcg/dL is highly suspicious for adrenal insufficiency.
ACTH (Adrenocorticotropic Hormone) — Interpretation note:
KEY distinction: High ACTH + high cortisol = Cushing disease (pituitary adenoma). Low ACTH + high cortisol = Cushing syndrome (adrenal tumor or exogenous steroids). High ACTH + low cortisol = Addison disease.
Aldosterone — Interpretation note:
Aldosterone retains sodium and excretes potassium. Hyperaldosteronism → hypertension + hypokalemia. Hypoaldosteronism → hypotension + hyperkalemia.
Pancreatic / Other
| Lab Test | Normal Range | Elevated Causes | Decreased Causes |
|---|---|---|---|
| C-peptide | 0.5–2.0 ng/mL (fasting) | Endogenous insulin excess — insulinoma, sulfonylurea use; Type 2 DM with residual beta cell function | Type 1 DM (destroyed beta cells cannot produce C-peptide or insulin); insulin factitious hypoglycemia (injected insulin raises glucose-lowering effect, suppresses C-peptide) |
| Insulin (serum) | 2–25 mcIU/mL (fasting) | Insulinoma; insulin resistance (Type 2 DM, obesity); exogenous insulin injection (if measured) | Type 1 DM (absolute deficiency); late-stage Type 2 DM |
C-peptide — Interpretation note:
C-peptide is co-secreted with insulin from beta cells but NOT present in injected insulin. Used to assess endogenous insulin production. Type 1 = low/absent C-peptide.
Insulin (serum) — Interpretation note:
Clinical use primarily to evaluate hypoglycemia etiology. Fasting insulin elevated inappropriately during hypoglycemia = insulinoma or sulfonylurea effect.
Related Resources
Standards & sources
Fact-checked Jun 20, 2026This page is written to align with American Diabetes Association (ADA) Standards of Care · American Association of Clinical Endocrinology (AACE). It is an educational summary, not a citation of any single document — always verify specific doses, values, and protocols against current guidelines and your facility policy. How we source content →