PPID, Hyperinsulinaemia & the Silent Threat of Laminitis in Geriatric Equids

The ageing equine population is steadily growing, thanks to advances in veterinary care, nutrition, and management that now allow many horses and ponies to remain active well into their 20s and beyond. Along with this demographic shift comes a sharp rise in endocrine disorders—particularly Pituitary Pars Intermedia Dysfunction (PPID), one of the most common endocrinopathies affecting aged equids¹. 

Studies suggest that up to 30% of older equids may develop PPID, most commonly after 15 years of age¹. Yet, the real clinical challenge is often not the diagnosis of PPID itself—it is recognising the metabolic consequences that silently accompany it, especially insulin dysregulation (ID) and endocrinopathic laminitis. 

Understanding the Pathophysiology Behind PPID 

Under normal physiological conditions, the pars intermedia of the pituitary gland remains under tonic dopaminergic inhibition from hypothalamic neurons. Age-related neurodegeneration disrupts this control, leading to hyperplasia, microadenoma, or macroadenoma formation within the pars intermedia¹. 

The diseased tissue secretes excessive quantities of proopiomelanocortin (POMC)-derived peptides including¹: 

• Adrenocorticotropic hormone (ACTH) 

• α-melanocyte stimulating hormone (α-MSH) 

• Corticotropin-like intermediate lobe peptide 

• β-endorphin 

These hormonal disturbances significantly alter glucose and insulin dynamics, predisposing affected equids to metabolic dysfunction and laminitis. 

The PPID–Laminitis Connection: More Dangerous Than It Appears 

Laminitis remains one of the most devastating complications associated with PPID. It can permanently end athletic careers and, in severe cases, necessitate euthanasia. 

What makes endocrinopathic laminitis particularly concerning is its insidious nature. Unlike acute inflammatory laminitis, these cases often develop gradually and quietly. Many owners fail to detect clinical signs until substantial laminar damage has already occurred. 

Horses with hyperinsulinaemia were more likely to have laminitis recognised by owners, and the likelihood of recognition increased with the severity of hyperinsulinaemia². Furthermore, animals with moderate to severe radiographic hoof changes had significantly higher serum insulin concentrations than those with mild or absent radiographic abnormalities^1,3. 

These findings reinforce an important clinical message: 

Absence of owner-reported lameness does not rule out clinically significant laminar pathology. 

Why Hyperinsulinaemia Matters 

Insulin dysregulation is reported in approximately 30–60% of PPID-affected equids. Among the various components of ID, persistent hyperinsulinaemia appears to be the most clinically significant risk factor for laminitis^1,3. 

Insulin dysregulation may include: 

• Fasting hyperinsulinaemia 

• Postprandial hyperinsulinaemia 

• Exaggerated insulin responses 

• Insulin resistance 

• Dyslipidaemia  

However, hyperinsulinaemia appears to be the primary driver of laminar injury. 

Experimental studies have demonstrated that administering supraphysiological insulin doses to healthy ponies and horses can induce laminitis within 48–72 hours^1,4. This has fundamentally shifted the understanding of endocrinopathic laminitis from being merely inflammatory to metabolically mediated. 

How Hyperinsulinaemia Damages the Lamellae 

Excess insulin is believed to aberrantly activate insulin-like growth factor-1 (IGF-1) receptors within laminar epithelial cells. These receptors share structural homology with insulin receptors and become responsive when insulin concentrations are pathologically elevated^1,5,6. 

IGF-1 signalling regulates: 

• Cell growth 

• Adhesion 

• Differentiation 

• Apoptosis 

• Tissue repair 

When overstimulated by excessive insulin concentrations, the result is^1,5,6: 

• Abnormal keratinocyte proliferation 

• Cytoskeletal dysfunction 

• Extracellular matrix disruption 

• Laminar weakening and stretching 

• Final separation of the epidermal and dermal structures 

Clinically, horses with moderate to severe radiographic changes consistently demonstrate higher serum insulin concentrations compared with animals showing mild or absent radiographic lesions. 

The Clinical Trap: “Normal” Horses May Still Be Laminitic 

An especially important takeaway for practitioners is that normoinsulinaemic PPID horses are not necessarily free from laminitis. 

Several horses classified as normoinsulinaemic still showed evidence of laminar pathology, although lesions were often milder and less obvious clinically. This raises the possibility that subtle or early insulin dysregulation may be missed when relying solely on resting insulin measurements¹. 

Histological studies have shown that horses with PPID and concurrent hyperinsulinaemia demonstrate characteristic laminar lesions including elongation and widening of primary epidermal laminae, fusion of epidermal laminae, keratinocyte proliferation, apoptosis, and epidermal separation from the dermis. In contrast, laminar histology in PPID horses without hyperinsulinaemia is often relatively normal¹. 

For this reason, dynamic insulin testing should be strongly considered in PPID cases where suspicion remains high despite apparently normal basal insulin concentrations. 

Relationship Between PPID and Insulin Dysregulation 

The relationship between PPID and insulin dysregulation remains an area of ongoing debate, as not all PPID horses develop ID. 

One theory suggests that PPID and ID are independent disorders that may coexist in ageing horses. Given that obesity and metabolic dysfunction are increasingly common in equine populations—with obesity prevalence reaching up to 50% in some groups—many metabolically unhealthy horses may simply develop PPID concurrently as they age¹. 

Alternatively, PPID itself may induce or worsen insulin dysregulation. Elevated pituitary hormone concentrations may stimulate pancreatic β-cells and promote insulin secretion. Additionally, hepatic insulin clearance may decline due to obesity, ageing, mild hepatic dysfunction, or PPID-associated metabolic changes^1,7. 

Variability in pituitary lesion severity and hormonal secretion profiles may explain why only some PPID horses develop clinically significant ID. 

Ageing Itself Increases Metabolic Vulnerability 

Age-related decline in insulin sensitivity further complicates the picture. 

As horses age, they naturally develop¹: 

• Reduced insulin sensitivity 

• Exaggerated insulin responses 

• Increased visceral adiposity 

• Reduced lean muscle mass 

• Oxidative stress and mitochondrial dysfunction 

• Chronic low-grade inflammation (“inflammaging”) 

This means geriatric equids are already metabolically predisposed before PPID or obesity enter the equation. 

When PPID and insulin dysregulation overlap in these animals, the risk of hyperinsulinaemia and endocrinopathic laminitis rises dramatically. 

Prognostic Importance of Serum Insulin 

Hyperinsulinaemia may also carry prognostic significance. 

In one study evaluating horses with PPID, serum insulin concentrations below 62 μU/ml predicted survival, whereas concentrations above 188 μU/ml predicted non-survival within 1–2 years, with sensitivity and specificity exceeding 90%⁸. Most non-survivors ultimately succumbed to complications associated with laminitis. 

Another study demonstrated that reductions in plasma insulin concentrations over time correlated with improvements in laminitis severity⁹. 

These findings reinforce a critical principle in PPID management: 

Insulin control is not just metabolic management—it is laminitis prevention. 

Final Takeaway 

PPID is no longer simply a “haircoat disease” of older horses. It is a complex endocrine disorder with profound metabolic consequences. 

The combination of PPID, insulin dysregulation, ageing-related insulin resistance, and hyperinsulinaemia creates the perfect environment for endocrinopathic laminitis to develop—often silently. 

For veterinarians, early identification of insulin abnormalities and subtle laminar changes may represent the single most important opportunity to preserve both soundness and quality of life in geriatric equids. 

By recognising the metabolic dimension of PPID earlier, clinicians can intervene before laminar damage becomes catastrophic. 

References  

1. Tadros EM, Fowlie JG, Refsal KR, Marteniuk J, Schott HC. Association between hyperinsulinaemia and laminitis severity at the time of pituitary pars intermedia dysfunction diagnosis. Equine veterinary journal. 2019 Jan;51(1):52-6. https://doi.org/10.1111/evj.12963  

2. Sillence M, Meier A, de Laat M, Klee R, Reiche D. Demographic, morphologic, hormonal and metabolic factors associated with the rate of improvement from equine hyperinsulinaemia-associated laminitis. BMC veterinary research. 2022 Jan 18;18(1):49. https://link.springer.com/content/pdf/10.1186/s12917-022-03149-z.pdf  

3. Frank N, Tadros EM. Insulin dysregulation. Equine veterinary journal. 2014 Jan;46(1):103-12. https://guabiequinos.com.br/pdf/2025/panfleto/Effect_of_type_of_grain_and_feed_processing_on_gas.pdf  

4. Karikoski NP, McGowan CM, Singer ER, Asplin KE, Tulamo RM, Patterson-Kane JC. Pathology of natural cases of equine endocrinopathic laminitis associated with hyperinsulinemia. Veterinary pathology. 2015 Sep;52(5):945-56. https://journals.sagepub.com/doi/pdf/10.1177/0300985814549212  

5. Mezi S, Todi L, Orsi E, Angeloni A, Mancini P. Involvement of the Src-cortactin pathway in migration induced by IGF-1 and EGF in human breast cancer cells. International journal of oncology. 2012 Sep 26;41(6):2128-38. https://www.spandidos-publications.com/10.3892/ijo.2012.1642/download 

6. Doyle SL, Donohoe CL, Finn SP, Howard JM, Lithander FE, Reynolds JV, Pidgeon GP, Lysaght J. IGF-1 and its receptor in esophageal cancer: association with adenocarcinoma and visceral obesity. Official journal of the American College of Gastroenterology| ACG. 2012 Feb 1;107(2):196-204. https://www.researchgate.net/profile/Suzanne-Doyle-2/publication/51857291 

7. Thatcher CD, Pleasant RS, Geor RJ, Elvinger F. Prevalence of overconditioning in mature horses in southwest Virginia during the summer. Journal of Veterinary Internal Medicine. 2012 Nov;26(6):1413-8. https://academic.oup.com/jvim/article-pdf/26/6/1413/66683283/jvim995.pdf  

8. McGowan CM, Frost R, Pfeiffer DU, Neiger R. Serum insulin concentrations in horses with equine Cushing's syndrome: response to a cortisol inhibitor and prognostic value. Equine veterinary journal. 2004 Apr;36(3):295-8. https://beva.onlinelibrary.wiley.com/doi/abs/10.2746/0425164044877288  

9. Walsh DM, McGowan CM, McGowan T, Lamb SV, Schanbacher BJ, Place NJ. Correlation of plasma insulin concentration with laminitis score in a field study of equine Cushing's disease and equine metabolic syndrome. Journal of equine veterinary science. 2009 Feb 1;29(2):87-94. https://www.academia.edu/download/93950289/2009Walshetal.pdf