Corticotropin (ACTH)
Corticotropin (ACTH) is a 39-amino acid polypeptide hormone secreted by the anterior pituitary. In veterinary medicine, it is used primarily as a **diagnostic agent** to evaluate adrenal gland function. - **Primary Use**: The ACTH stimulation test is used to diagnose hypoadrenocorticism (Addison's disease), identify iatrogenic hyperadrenocorticism (Cushing's syndrome), and monitor the efficacy of adrenal-suppressive therapies (e.g., mitotane or trilostane). - **Limitations**: It is considered relatively insensitive as a screening test for naturally occurring hyperadrenocorticism in dogs compared to the low-dose dexamethasone suppression test (LDDST). - **Clinical Pearl**: Due to the high cost, limited availability, and variable potency of natural porcine ACTH gel products, synthetic **cosyntropin** is now the preferred agent for ACTH stimulation testing in modern clinical practice.
Mechanism: Endogenous ACTH release is controlled by corticotropin-releasing factor (CRF) from the hypothalamus and is subject to negative feedback by glucocorticoids. Exogenous corticotropin binds to **melanocortin-2 receptors (MC2R)** in the **zona fasciculata** of the adrenal cortex $\rightarrow$ stimulates the conversion of cholesterol to pregnenolone via cholesterol desmolase $\rightarrow$ rapidly increases the synthesis and release of **glucocorticoids** (primarily **cortisol** in mammals and **corticosterone** in birds and reptiles).
Dosing by species
- ACTH Stimulation Test ยท 2.2 Units/kg IM ยท IM ยท Once ยท Draw baseline blood sample, administer gel, then draw samples at 60 minutes and 120 minutes after injection.
- ACTH Stimulation Test ยท 0.5 Units/kg IM ยท IM ยท Once ยท Draw baseline blood sample, administer gel, then draw sample 120 minutes after injection.
- ACTH deficiency or primary bovine ketosis ยท 200-600 Units initially followed by daily or semi-daily dose of 200-300 Units ยท IM ยท daily or semi-daily
- ACTH Stimulation Test ยท 1 Unit/kg IM ยท IM ยท Once ยท Draw baseline blood sample, administer gel, then draw second sample 8 hours later.
- ACTH Stimulation Test ยท 1 Unit/kg IM ยท IM ยท Once ยท Administer between 8 and 10 AM; take post ACTH cortisol levels at 2 and 4 hours post dose.
- ACTH Stimulation Test ยท 16-25 Units IM ยท IM ยท Once ยท Draw baseline blood sample for corticosterone (not cortisol), administer IM, draw second sample 1-2 hours later.
- ACTH Stimulation Test ยท 2.2 Units/kg IM ยท IM ยท Once ยท Draw baseline blood sample, administer gel, then draw second sample 120 minutes after injection.
Routes of administration
Contraindications
- Hypersensitivity to porcine proteins
- Intravenous (IV) administration of the repository gel formulation
Adverse effects
- Fluid and electrolyte disturbances (with prolonged/chronic use)
- Hypersensitivity reactions (rare)
Drug interactions
- Anticholinesterases (e.g., pyridostigmine) ยท May antagonize effects in patients with myasthenia gravis
- Diuretics ยท May increase electrolyte loss (e.g., hypokalemia) if used chronically
- Glucocorticoids (hydrocortisone, cortisone) ยท Exogenous administration on the day of the test will cross-react with cortisol assays and invalidate test results
Monitoring
- Serum cortisol levels (pre- and post-injection for mammals)
- Serum corticosterone levels (pre- and post-injection for birds/reptiles)
- Electrolytes and fluid balance (only if used chronically)
Overdose
When used for diagnostic purposes, acute inadvertent overdoses are highly unlikely to cause any significant adverse effects. Monitor the patient as required and treat symptomatically if necessary.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.