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Summary Background Primary hyperparathyroidism is uncommon in equids. Objectives To describe the diagnostic findings and efficacy of treatment in equids with primary hyperparathyroidism. Study design Retrospective case series describing 16 horses and one mule. Methods Cases were identified by retrospective review of records at Cornell University and via an ACVIM listserv query. Inclusion criteria were an equid with hypercalcemia, normal renal function and high parathyroid hormone (PTH) or histopathological diagnosis of a parathyroid adenoma. Equids with normal PTH and PTH‐related protein (PTHrP) in the face of hypercalcemia were included as suspect cases. Results The most common presenting complaints were weight loss (12/17) and hypercalcemia (10/17). PTH was above reference range in 12/17 cases. Suspected parathyroid tumours were localised in 12/14 equids imaged using ultrasonography alone (2/3), technetium 99m Tc sestamibi scintigraphy alone (1/1) or both modalities (9/10). Three horses did not have imaging performed. Surgical exploration successfully excised tumours in six of 10 cases. Five were located at the thoracic inlet, and surgery resulted in complete cure. One tumour was excised from the thyroid lobe, and the horse remained hypercalcemic. Four other cases explored surgically, four treated medically and three that were not treated also remained hypercalcemic. Main limitations The small study size prohibited statistical analysis. Conclusions Parathyroid adenomas in equids can be successfully localised with ultrasonography and scintigraphy. Surgical excision appears more likely to be successful for single gland disease at the thoracic inlet.

Background Trigeminal‐mediated headshaking results from low‐threshold firing of the trigeminal nerve resulting in apparent facial pain. Magnesium may have neuroprotective effects on nerve firing that potentially dampen signs of neuropathic pain. This hypothesis has not been investigated in horses with trigeminal‐mediated headshaking. Objective To investigate head‐shaking behavior in affected horses after IV magnesium sulfate infusion. Animals Six geldings with trigeminal‐mediated headshaking. Methods Prospective randomized crossover study. Horses were controlled for diet and infused IV with 5% dextrose solution (DS; control solution at 2 mL/kg body weight [BW]) and MgSO4 50% solution (MSS at 40 mg/kg BW). Head‐shaking behavior was recorded at times T0 (baseline, before infusion) and T15, T30, T60, and T120 minutes post‐infusion. Venous blood variables such as pH, HCO3−, standard base excess (SBE), Na+, Cl−, K+, Ca2+, Mg2+, total magnesium (tMg), glucose, and lactate were measured; strong ion difference (SID) and anion gap (AG) were calculated for each time point. Results Blood variables including pH, Na+, Cl−, K+, SID, AG, lactate, Ca2+, tMg, and Mg2+ had significant changes with MSS as compared to DS treatment. Glucose, SBE, and HCO3− did not have significant changes. A 29% reduction in head‐shaking rate occurred after MSS treatment but no change occurred after DS treatment. Conclusions and Clinical Importance Administration of MSS IV increased plasma total and ionized magnesium concentrations and significantly decreased head‐shaking behavior in horses with trigeminal‐mediated headshaking.