Background
N-terminal type III procollagen propeptide (PIIINP) is a biomarker of soft tissue proliferation. Hypersomatotropism (HS) is associated with soft tissue proliferation.

Hypothesis
Serum PIIINP is increased in cats with HS and decreases with effective treatment, and may be an additional tool in the diagnosis and treatment of feline HS.

Animals
Cats with uncomplicated diabetes mellitus (DM; n = 30) and with HS-induced DM (HSDM; n = 30). Pre- and posttreatment samples were available from 5 cats undergoing radiotherapy (RT) and 16 cats undergoing hypophysectomy (HPX).

Methods
Retrospective and prospective cross-sectional study. Analytical performance of a serum PIIINP ELISA was assessed and validated for use in cats. PIIINP and insulin-like growth factor 1 (IGF-1) radioimmunoassays (RIA) were performed pre- and post-treatment in cats with DM and HSDM. PIIINP and IGF-1 were compared between cats treated by RT and HPX.

Results
Serum PIIINP concentrations were significantly higher (P < .001) in HSDM cats (median, 19.6 ng/mL; range, 1.7–27.9) compared to DM cats (median, 5.0 ng/mL; range, 2.1–10.4). A cut-off of 10.5 ng/mL allowed differentiation between DM and HSDM cats with 87% sensitivity and 100% specificity (area under the curve [AUC], 0.91; 95% confidence interval [CI], 0.82-1). After RT, PIIINP increased significantly (P = .043) with no significant change in IGF-1 concentrations. After HPX, serum PIIINP (P = .034) and IGF-1 concentrations (P < .001) decreased significantly.

Conclusion and clinical importance
PIIINP concentrations are increased in cats with untreated HSDM compared to those with DM, demonstrating the effect of excess GH on soft tissue. PIIINP concentrations decreased after HPX in most HSDM cats.