Hypocalcemia Caused by Primary Hypoparathyroidism in a 3-Month-Old Filly

Abstract

A 3-month-old, 160 kg, warmblood filly was referred to the Department of Large Animal Internal Medicine, Ghent University, because of anorexia, weakness, and muscle trembling. One week before admission the foal had shown signs of cervical stiffness. At presentation, the foal had not received any treatment yet. The foal and her dam were kept on a pasture, together with several other horses. Immediately after birth the foal had received antibiotics and antitetanus serum. Anthelminthics were administered at 10 days and 2 months of age. At presentation, the foal showed sweating, stiffness, muscle twitching, and synchronous diaphragmatic flutter. Body condition was normal. Clinical examination identified increased rectal temperature (38.9°C), tachycardia (96 bpm), and normal respiratory rate (24 bpm). Mucous membranes were pink with normal capillary refill time. On auscultation, the heart rhythm was regular and no murmurs were auscultated. Auscultation of thorax and abdomen disclosed no abnormalities. The foal's appetite was decreased. The feces had a normal appearance. On hematology, packed cell volume was 38% (reference range, 35–45%) and white blood cell count was 7,900/μL (reference range, 3,000–7,000/μL). Serum biochemistry disclosed a marked hypocalcemia with an ionized calcium concentration of 0.6 mmol/L (reference range, 1.4–1.7 mmol/L) and a total calcium concentration of 1.35 mmol/L (reference range, 2.5–3.35 mmol/L). Serum total magnesium concentration was low (0.92 mEq/L; reference range, 1.4–1.8 mEq/L), serum sodium concentration was slightly decreased (127 mEq/L; reference range, 135–145 mEq/L), serum potassium concentration was within the reference range (3.7 mEq/L; reference range, 3.5–4.5 mEq/L) and serum phosphorus concentration was markedly increased (12.54 mg/d; reference range, 6–9 mg/dL). There were increases in blood urea (36 mg/dL; reference range, 11–22 mg/dL) and fibrinogen (522 mg/dL; reference range, 100–460 mg/dL) concentrations, and increases in lactate dehydrogenase (693 IU/L; reference range, 246–658 IU/L), aspartate aminotransferase (521 IU/L; reference range, 71–508 IU/L), and creatine kinase (359 IU/L; reference range, 10–146 IU/L) activities. The concentrations of creatinine and activities of total bilirubin, γ-glutamyl transferase, alkaline phosphatase and alanine aminotransferase were within reference ranges. The total protein concentration was within reference range, whereas the serum albumin concentration was decreased (2.8 g/dL; reference range, 3–3.6 g/dL). Thoracic and abdominal ultrasonography disclosed no abnormalities. Serum biochemistry of the mare showed normal ionized calcium, magnesium, and phosphorus concentrations. Total calcium concentration was 2.75 mmol/L (reference range, 2.5–3.35 mmol/L) in serum and 3.3 mmol/L (reference range, 2–3 mmol/L) in milk. Initially, the foal was treated with a calcium and magnesium infusion (100 mL of calcium gluconate [18.7%] and magnesium chloride [6%] solutiona given over a period of 15 minutes through an IV catheter) and antibiotics (160 mg cefquinomeb [4.5%] IV q12h). Overnight, 500 mL of Ringer's solution with 5% glucose and 60 mL of calcium gluconate (18.7%) and magnesium chloride (6%) solutiona were administered every 4 hours. Auscultation of the heart was performed regularly and no dysrhytmias were detected. During the 1st night of hospitalization, the ionized calcium concentration only slightly increased to 0.67 mmol/L. The foal remained lethargic and continued to show synchronous diaphragmatic flutter. On the 2nd day, the filly developed profuse diarrhea and anorexia. Treatment with lactasec (3,000–6,000 IU/kg PO q12h), saccharomyces boulardiid (5 mg/kg PO q12h), and sucralfatee (10–20 mg/kg PO q6h) was initiated. Treatment with antibiotics (160 mg cefquinomeb [4.5%] IV q12h) was continued although bacteriologic culture of the feces did not identify any Salmonella or Clostridium species. Every 2 hours, 300 mL of Ringer's solution with 5% glucose and 80 mL of calcium gluconate (18.7%) and magnesium chloride (6%) solutiona were given. During the 2nd day, the ionized calcium concentration increased from 0.78 to 1.02 mmol/L, while the serum total magnesium concentration increased from 0.92 to 1.22 mEq/L. Unfortunately, the foal also developed mild metabolic acidosis (pH 7.32; reference range, 7.35–7.45; bicarbonate, 20 mmol/L; reference range, 22–26; PCO2 40 mmHg; reference range, 40–50; base excess −5.3 mEq/L; reference range, −5 to +5) and hypokalemia (2.8 mEq/L; reference range, 3.5–4.5 mEq/L). Supplementation with sodium bicarbonate (21,370 mg) and potassium chloride (50 mEq twice over 24 hours) IV was initiated. Serum parathyroid hormone (PTH) concentration, measured by a human 2-site immunochemiluminometric assay (intact PTH),f was below the detection limit. The low PTH concentration in the face of severe hypocalcemia suggested a diagnosis of hypoparathyroidism. On the 3rd day, the ionized calcium concentration remained low (1.04 mmol/L). Calcium was supplemented by means of 100 mL of calcium gluconate (18.7%) and magnesium chloride (6%) solutiona added to 333 mL of Ringer's solution with 5% glucose given every 2–4 hours. The ionized calcium concentration increased from 1.04 mmol/L in the morning of the 4th day to 1.28 mmol/L on the 5th day. On the 6th day, the ionized calcium concentration stabilized between 1.2 and 1.3 mmol/L (Table 1). Intravenous calcium supplementation was discontinued and oral treatment with calcium carbonate (15 g PO q8h) was started. On day 7, diarrhea ceased and IV fluid therapy was discontinued. Treatment with antibiotics (160 mg cefquinomeb [4.5%] IM q12h), lactasec (3,000–6,000 IU/kg PO q12h), saccharomyces boulardiid (5 mg/kg PO q12h), sucralfatee (10–20 mg/kg PO q6h) and calcium carbonate (15 g PO q8h) was continued. On day 9, antibiotic and antidiarrheal treatment was discontinued and only calcium carbonate treatment (15 g PO q12h) was continued. On day 11, serum PTH concentration was again measured in the filly. After collection and clotting, blood was cold centrifuged. Serum samples were frozen (−18°C) and shipped on ice for analysis. Serum intact PTHf in the filly was 8.4 pg/mL (reference range, 1.1–208 pg/mL by a human 2-site immunochemiluminometric assay1; 7.4–82.7 pg/mL in adult horses by a human intact PTH assay2; 10.2–96.5 pg/mL in adult horses by a rat amino-terminal assay2; 9.2 ± 1.8 pg/mL in foals of 90-days-old by a human intact PTH assay,3 and 32.8 ± 8.7 pg/mL in foals of 90 days old by a human whole PTH assay3; 1,100–6,400 pg/mL by a human intact assay4). Urinalysis and fractional excretion of calcium, phosphorus, sodium, potassium, and chloride, performed on day 13, were normal. On day 17, a PTH stimulation test2 was performed in the filly and in 1 control horse. By inducing hypocalcemia, PTH response to changes in blood ionized calcium was assessed. In brief, hypocalcaemia was induced by IV infusion of disodium ethylenediaminetetraacetic acid (EDTA) during 30 minutes. The dose of EDTA was progressively increased each 5 minutes to achieve a linear decrease in blood calcium (0–5 minutes: 30 mg/kg/h, 5–10 minutes: 40 mg/kg/h, 10–15 minutes: 60 mg/kg/h, 15–20 minutes: 90 mg/kg/h, 20–25 minutes: 120 mg/kg/h, and 25–30 minutes: 180 mg/kg/h). Blood samples were drawn before starting the infusion and then every 5 minutes. In these samples blood ionized calcium and PTH concentrations (intact PTH)f were measured. In the control horse the induced hypocalcaemia resulted in a strong increase in the PTH concentration, whereas in the foal's serum, PTH remained undetectable despite low ionized calcium concentration (Fig 1). This EDTA infusion test confirmed the diagnosis of primary hypoparathyroidism. Serum parathyroid hormone (PTH) concentration and serum ionized calcium concentration in the filly and in 1 healthy control horse after modifying blood ionized calcium concentration (infusion of disodium EDTA). On day 19, the foal was discharged. Over the next 2 months, the foal was alternated between box and pasture together with her dam. The filly was supplemented with calcium carbonate (20 g PO q12h) and received a concentrateg (1 kg per day) with an adapted calcium/phosphorus concentration and ratio (calcium [4.6%]/phosphorus [2%]) together with good quality hay ad libitum. At the end of these 2 months, the filly was reexamined in the clinic. The foal was healthy (230 kg) and there were no abnormalities on general examination. However, the ionized calcium concentration remained low (1.13 mmol/L) and serum PTH concentration was undetectable. Therefore, the foal was supplemented with 2.5 mL vitamin AD3Eh IM. At the same time, the filly was weaned and the supplementation with vitamin AD3Eh (50,000 IE vitamin D3 [1 mL] per 100 kg every 2–3 months) was continued, together with calcium carbonate (80–100 g/day), adapted concentrateg (1 kg/day), and alfalfa hay (5 kg/day). On day 139 after 1st admission, the serum-ionized calcium concentration was 1.42 mmol/L and the foal was fit. Nine months after her discharge, the filly is still healthy and without any clinical signs (Table 1). In this filly, the findings of hypocalcemia, hyperphosphatemia, hypomagnesemia, and the low serum PTH concentrations indicated the presence of hypoparathyroidism. In such cases, differentiation between primary hypoparathyroidism and secondary hypoparathyroidism should be made. Primary hypoparathyroidism is caused by a dysfunction of the parathyroid cells, resulting in an impaired synthesis and secretion of PTH, whereas secondary hypoparathyroidism is a functional disorder, seen in correlation with sepsis, endotoxemia, hypomagnesemia, or hypercalcemia. In horses, pseudohypoparathyroidism, resulting from PTH resistance at the level of the target cells, has not been described.5,6 In the present case, pseudohypoparathyroidism was excluded because the PTH concentrations were extremely low. The diarrhea and endotoxemia in this filly may have contributed to the initial hypocalcemia (impairment of the parathyroid glands by inflammatory mediators), but hypocalcemia persisted after resolution of the diarrhea. Also, hypomagnesemia as a cause of parathyroid dysfunction seemed unlikely because the administration of magnesium chloride did not result in an increased serum PTH concentration. Consequently, the hypomagnesemia initially seen in the foal is believed to be a consequence of the hypoparathyroidism (as PTH stimulates magnesium reabsorption from the kidney, intestine, and bone) rather than a cause of it (because magnesium is required for the release of PTH from the chief cells). Primary hypoparathyroidism, caused by dysfunction of the parathyroid chief cells responsible for the secretion of PTH, is a rare cause of hypocalcaemia that has only been described in adult horses.4,7 The parathyroid glands in horses consist of the internal, external, and in some cases accessory glands. They can be found adjacent to the thyroid glands and along the trachea.8,9 On necropsy, it is often difficult to localize the parathyroid glands and they can easily be confused with lymph nodes. Parathyroid hormone is a polypeptide of 84 amino acids secreted by the parathyroid chief cells. It plays a crucial role in the regulation of calcium and phosphorus concentration in extracellular fluid.5,6 Hypoparathyroidism is characterized by hypocalcemia, hyperphosphatemia, and hypomagnesemia. The lack of PTH usually is associated with increased urinary excretion of calcium and decreased urinary excretion of phosphorus. In this filly, the urinary excretion values were normal. Unfortunately, these measurements were done once the foal stabilized and no urinary measurements were done on admission. The measurement of parathyroid hormone concentration is a valuable diagnostic tool for the detection of parathyroid gland disorders. In the circulation, intact active PTH hormone as well as incomplete inactive fragments (carboxyl-terminal, amino-terminal, and intermediate segments)2,3,10 can be detected. In our case, plasma PTH concentration was measured by a human solid-phase, 2-site chemiluminescent enzyme-labeled immunometric assay detecting intact, biological active PTH. This technique previously has been validated in horses.1 In this foal, the low basal PTH concentration and abnormal response to induced hypocalcemia were suggestive of primary hypoparathyroidism. The very low blood PTH concentration concurrent with a low ionized calcium concentration indicated an inappropriate response of the parathyroid glands. Different causes of primary hypoparathyroidism have been reported in dogs, cats, and humans such as iatrogenic damage or surgical removal of the gland during thyroidectomy, parathyroid agenesis, idiopathic hypoparathyroidism, lymphocytic parathyroiditis, autoimmune disorders, or neoplasia.11–18 In humans, several mutations or deletions affecting genes responsible for the biosynthesis of PTH and the development of the parathyroid glands have been described. In these cases, hypoparathyroidism can be manifested as an isolated pathology or as part of a complex genetic syndrome.18 The exact cause of the primary hypoparathyroidism in this filly is unclear. Ultrasonography and biopsies of the parathyroid glands were not performed because the glands are small and difficult to localize in horses. The observed clinical signs (eg sweating, stiffness, muscle twitching, and synchronous diaphragmatic flutter) can be explained by increased neuromuscular excitability resulting from profound hypocalcemia.5,6 The profuse diarrhea starting on the 2nd day of hospitalization could have been the result of abnormal bacterial flora after anorexia. However, 12% of dogs with primary hypoparathyroidism developed diarrhea.16 Replacement of the deficient hormone is the ideal treatment for primary hypoparathyroidism. Nevertheless, even in humans, clinical experience with exogenous administration of the hormone is limited.12,18 Therefore, in humans and small animals, life-long oral treatment with vitamin D analogues and calcium is used.11–18 In dogs, calcitriol (1,25 dihydroxy-cholecalciferol), which does not require renal or hepatic metabolism and has a short onset of action, is the treatment of choice.16 Use of this product in horses would be expensive. In horses with hypoparathyroidism, successful use of dihydrotachysterol has been described.4 In this case, cost considerations precluded oral administration of dihydrotachysterol. The foal was supplemented with a combination preparation of vitamin A, D3, and E IM. Calcium was supplemented by calcium carbonate or limestone, which is inexpensive and has good bioavailability. Alfalfa hay has been given to horses with hypocalcemia, but it contains oxalates. These substances can impair calcium absorption in the intestinal tract by chelating calcium ions.6 For that reason, preference is given to normal good quality hay. The overall goal of treatment is to maintain calcium concentrations in the low-normal range. In cats and dogs, if treated appropriately, primary hypoparathyroidism has an excellent prognosis.11,16 In 3 cases of adult horses with primary hypoparathyroidism,4,7 2 survived and were able to return to training. In this present case, because of the young age, the impact of calcium deficiency on the skeletal integrity cannot be overlooked because hypocalcemia can lead to osteopenia in young developing animals.19 Knowing this, it is impossible to predict the long-term prognosis, for sport or breeding, for our foal. a Calcium gluconate 18.7%+ magnesium chloride 6% (Calcii Borogluconas), Eurovet Animal Health BV, Bladel, the Netherlands b Cefquinome (Cobactan IV/IM 4.5%), Intervet, Schering-Plough Animal Health, Brussels, Belgium c Lactase (Lactase), Deba Pharma NV, Wevelgem, Belgium d Saccharomyces boulardii (Enterol 250 mg), Biodiphar NV, Brussels, Belgium e Sucralfate (Ulcogant), Merck KGaA, Darmstadt, Germany f Intact PTH, Immulite 2000, Siemens, Los Angeles, CA g Concentrate Lannoo Ovation Evolution, Voeders Lannoo-Martens, Hansbeke-Nevele, Belgium h Duphafral AD3E (Vitamine AD3E), Fort Dodge Animal Health, Kelmis, Belgium The authors thank L. Van Campe (CRI Labo) for analyzing the blood samples and J. Vandermeiren for performing the food analysis.