The Somogyi effect

An important reason for conducting a blood glucose curve, ie, evaluating blood glucose levels every 2 hours following the morning Vetsulin® (porcine insulin zinc suspension) treatment is the possibility of Somogyi effect or rebound hyperglycemia. In cats, the “Somogyi overswing” occurs when the insulin dose is too high and the patient’s blood glucose plummets below 60 mg/dL.

The Somogyi effect occurs when the body attempts to counteract the life-threatening decline in the blood glucose concentration through a chain of reactions:

  • The blood glucose concentration falls rapidly or approaches hypoglycemia (blood glucose concentrations of less than 60 mg/dL [3.3 mmol/L]) following the injection of insulin. The cat becomes hungry and restless or lethargic.
  • In response to a declining blood glucose concentration in the central nervous system, adrenaline and subsequently cortisol, glucagons, and growth hormone are released.
  • These hormones increase blood glucose concentration (through gluconeogenesis, release of glucose from hepatic glycogen, and increased peripheral resistance to insulin).
  • The resulting hyperglycemia produces polyuria and polydipsia. This can be mistakenly attributed to an inadequate insulin dose.

If the morning polyuria is thought to result from an insufficient insulin dose and a higher dose is given, the problem will be aggravated. An even more pronounced Somogyi effect will follow. Eventually the counter-regulatory mechanisms may become exhausted, resulting in severe hypoglycemia.

The Somogyi effect can occur in both cats and dogs, but cats are particularly prone to develop this rebound hyperglycemia. The appropriate corrective action is to decrease the patient’s insulin dose to prevent insulin-induced hypoglycemia.

When to suspect a Somogyi overswing

  • Minimal glycemia: <65 mg/dL or 3.6 mmol/L
  • Maximum glycemia: 400–800 mg/dL or 22–44 mmol/L
  • Persistent morning glucosuria: >1% or 1–2 g/dL (strips)
  • Morning glycemia: >400–450 mg/dL or 22 mmol/L
  • Clinical signs:
    • Polyuria, polydipsia, weight loss
    • Hypoglycemia (weakness, convulsions, ataxia, behavior changes)
  • High insulin dose: close to 2.2 IU/kg and greater

Diagnosing Somogyi effect

A blood glucose curve can help detect a Somogyi effect and confirm that a cat’s insulin dose needs to be reduced.

Any of the following blood glucose curves can be suggestive of the Somogyi effect:

  • Hypoglycemia (low nadir) followed by rebound hyperglycemia.
  • A rapid decrease in glycemia with an adequate nadir followed by rebound hyperglycemia.
  • Persistently high blood glucose values with no discernible nadir (rebound hyperglycemia can persist for a few days following the hypoglycemic event).

See the following graph for an example of a blood glucose curve in a case of rebound hyperglycemia. Insulin was injected at time=0 hours.

Blood glucose curve indicating Somogyi effect

Blood glucose measured in mg/dL
Click on thumbnail for full image:


Decrease dose by 50% or return to starting dose of 1 IU twice daily, whichever is lower.

See About glucose curves for details on making a glucose curve.

Hyperglycemia due to a Somogyi effect can persist for as long as 3 days after a single hypoglycemic episode. As a result, blood glucose concentrations do not always stabilize within a few days after lowering the insulin dose.