Successful Detection and Removal of a Functional Parathyroid Adenoma in a Pony Using Technetium Tc 99m Sestamibi Scintigraphy

A 20-year-old, 275-kg, Welsh pony gelding was referred to New Bolton Center for treatment of primary hyperparathyroidism. The horse was diagnosed with primary hyperparathyroidism based on persistently high total (19.5 mg/dL; reference range 10.8–13.5 mg/dL) and ionized serum calcium (9.36– 13.16 mg/dL; reference range 6–7.6 mg/dL) concentrations with concurrently increased serum parathyroid hormone (PTH) concentration (Immunoradiometric assay) (128.63 pmol/L; reference range 0.60–11.00 pmol/L), and lack of evidence of renal failure (serum creatinine concentration, 1.7 mg/dL; reference range 0.8–2.2 mg/dL). Hypercalcemia was first identified 19 months earlier (total serum calcium concentration, 14.6 mg/dL; reference range 10.4–12.9 mg/dL), but was not accompanied by clinical signs until 3 weeks before presentation when lethargy and inappetence were noted. The gelding had been previously diagnosed with pituitary pars intermedia dysfunction (PPID) and was being treated with pergolide (1 mg PO q24h). The pony had no reported lameness problems, other than a mild bout of laminitis presumably associated with PPID. No treatment for hypercalcemia was initiated before referral. On presentation, the pony was bright and alert, with a normal rectal temperature (38.4°C), heart rate (44 beats/min [bpm]), and respiratory rate (16 breaths/ min). Body condition score was 4/9 with visible ribs and decreased musculature over the neck and back. Hypertrichosis was present. Dentition was normal for the pony’s age and no loosening of the teeth was present. Visual assessment and palpation of the limbs and skull did not detect any skeletal abnormalities. The pony had intermittent weight shifting of the hind limbs, strong to bounding digital pulses, and a moderately shortened stride but did not show overt lameness at a walk. Intermittent full body fine muscle fasciculations were noted. Polyuria and polydipsia were not evident. A urine sample was not obtained. Complete blood count disclosed mild normocytic, hypochromic anemia (Hct, 28%; reference range 34–46%; MCHC, 33.2; reference range 33.3–38.5 g/ dL), moderate hyperfibrinogenemia (plasma fibrinogen concentration, 651 mg/dL; reference range 100–400 mg/dL), and a normal leukogram. A serum biochemistry profile disclosed marked hypercalcemia (total calcium concentration, 21.47 mg/dL; reference range 10.70–13.40 mg/dL; ionized calcium concentration, 10.08 mg/dL; reference range 5.48–7.04 mg/dL), mildly decreased creatine kinase activity (CK, 85 U/L; reference range 90–270 U/L), and mildly increased gamma glutamyl transferase activity (GGT, 57 U/L; reference range 12–45 U/L). Serum creatinine (1.6 mg/dL; reference range 0.6–1.8 mg/dL) and phosphorus (3.07 mg/ dL; reference range 1.90–5.40 mg/dL) concentrations were within reference ranges. Although laboratory analysis identified increased PTH, parathyroid hormone–related protein (PTHrP) was not measured, and a paraneoplastic syndrome was not ruled out. Abdominal ultrasonography was performed to identify lymphosarcoma or tumors that could produce PTHrp. No evidence of neoplasia was found. A hyperechoic medullary rim sign was detected in both kidneys, which is a nonspecific finding indicative of tubulonephrosis or nephrocalcinosis. Both kidneys were normal in size for a pony. Technetium Tc 99m nuclear scintigraphy of the neck using 200 millicuries of technetium Tc 99m sestamibi IV was performed from the ramus of the mandible to the heart base. Lateral and ventrodorsal images of the neck were acquired using a large field-of-view scintillation camera with a low-energy, all-purpose collimator. Images were obtained at 10 minutes, 2.5 hours, 5.5 hours, and 24 hours after technetium injection. Uptake of radionuclide was identified in areas attributed to the salivary and thyroid glands. An abnormal circular area of uptake was identified at the thoracic inlet at the level of the point of the shoulder on From the New Bolton Center, University of Pennsylvania, Kennett Square, PA (Tomlinson, Johnson, Ross, Engiles, Levine, Sweeney); and the Green Glen Equine Hospital, Glen Rock, PA (Wisner). The work was done at the New Bolton Center Widener Hospital for Large Animals in Kennett Square, PA. Corresponding author: A. Johnson, DVM, DACVIM, New Bolton Center, University of Pennsylvania, 382 West Street Road, Kennett Square, PA 19348; [email protected] Submitted June 10, 2013; Revised October 20, 2013; Accepted November 20, 2013. Copyright © 2014 by the American College of Veterinary Internal Medicine 10.1111/jvim.12285 Abbreviations: