The thyroid gland in horses is an endocrine organ responsible for producing hormones that regulate metabolism, growth, and development. Located in the neck region, it synthesizes thyroxine (T4) and triiodothyronine (T3), which influence various physiological processes. The gland's function is essential for maintaining metabolic homeostasis and supporting normal growth patterns. Abnormalities in thyroid function can lead to a range of metabolic disorders in horses. This page compiles peer-reviewed research studies and scholarly articles that explore the anatomy, physiology, and clinical implications of thyroid gland function in equine health.
Morris DD, Garcia M.Adult horses showed a mild diurnal variation in equine plasma thyroxine (T4) concentrations, but not triiodothyronine (T3). Plasma T4 concentrations tended to be higher between 5 PM and 8 PM than at 8 AM. Increases in plasma T4 and T3 were similar in adult healthy horses given 5, 10, or 20 IU of thyroid-stimulating hormone (TSH). The T4 peaked at approximately twice (2.0 +/- 0.4 times) as high as the base line at 6 to 12 hours after the TSH was given. The greatest change from base line T3 occurred at 1 to 3 hours after the TSH was given, but the magnitude of increase was widely variable (4.36 ...
Argiroudis SA, Kent JE, Blackmore DJ.The isoenzymes of creatine kinase have been measured in serum and selected tissues from horses. The distribution followed that reported in other species in that the MM dimer of the isoenzyme was present in voluntary and non-voluntary muscle, thyroid, liver, spleen, lung and intestine. The BB dimer of the isoenzyme was predominant in brain, pancreas, kidney, intestine, lung, spleen, liver and thyroid. In contrast, in 4 hearts examined less than 1.5 per cent of the total creatine kinase activity was attributable to the MB form of the isoenzyme. The MB isoenzyme was, however, present in intestine...
Caple IW, Bourke JM, Ellis PG.The calcium and phosphorus nutrition of thoroughbred racehorses was assessed by analysis of serum and urine samples collected from 90 horses in 1975 and 139 horses in 1980-81 at racetracks in Melbourne. Horses that were excreting greater than 15 mumole Ca/mosmole and which had a calcium to creatinine clearance ratio greater tha 2.5% were considered to have adequate Ca intake. Horses that were excreting greater than 15 mumole P/mosmole and which had a phosphorus to creatinine clearance ratio greater than 4% were considered to have excessive phosphorus intake. Sixty-percent of the horses sampled...
Chen CL, Riley AM.Serum thyroxine (T-4) and triiodothyronine (T-3) concentrations were assayed in neonatal foals (1.5 to 4 months) and mature horses (2 to 25 years old) by a modified radioimmunoassay procedure. Blood was collected from 52 clinically healthy foals and horses of various breeds (Thoroughbreds, Quarter Horses, American Saddle Horses, and a single cross-bred horse). Neonatal foals had high serum concentrations of T-4 (mean, 4.02 microgram/dl) and T-3 (192.9 ng/dl) as compared with the values in mature horses (T-4, mean of 1.76 microgram/dl; T-3, mean of 98.69 ng/dl). Stallions had slightly higher T-...
Doige CE, McLaughlin BG.Hyperplastic goitre was observed in seven newborn foals. Several were weak at birth and died in the first 48 hours of life. Only one foal had myxedema and only three of the seven had obvious enlargement of the thyroid at necropsy. It is suggested that the goitre observed was caused by a dietary deficiency of iodine.
Blähser S.In the thyroid glands of the horse, pig, deer, mole, and rat, C-cells could be demonstrated by means of the immunocytochemical PAP-technique using rabbit antisera against human calcitonin. Only in ruminants, the cross-reaction between the intracellularly stored antigen and the antibodies used appeared to be incomplete.
Verhaegen M, Sand G.The distribution of protein phosphokinase (EC 2.7.1.37) activities has been established in horse thyroid nuclei. The presence of several enzyme activities has been demonstrated, two of which are clearly distinct. The first one acts on histone as substrate and is activated by cyclic AMP. Physico-chemical properties of this nuclear cyclic AMP-dependent histone kinase and of the cytosol histone kinase are different, demonstrating the absence of a contamination from the cytosol. The second enzyme acts on casein as substrate and is not stimulated by cyclic AMP POR CYCLIC GMP. The findings are consi...
Ganjam VK, Kenney RM.Total androgens, testosterone and total oestrogens were measured in twenty-one intact, nine unilaterally cryptorchid, three bilaterally cryptorchid stallions and four geldings. Total oestrogens were significantly higher (P less than 0-005) and total androgens significantly lower (P less than 0-05) in the bilateral cryptorchid compared to other groups. There was a significant (P less than 0-025) day and night variation in total androgen levels. Thyroidectomized and intact animals showed a marked decrease in total androgen as well as testosterone levels during the winter period thus showing an e...
Lowe JE, Baldwin BH, Foote RH, Hillman RB, Kallefelz FA.Three Quarter-horse stallions were surgically thyroidectomized at 18 months of age. Weekly semen collections were made from these and three control stallions from 25 to 39 months of age. Although the induced state of hypothyroidism in young stallions has a depressing effect on libido and general animation and can be corrected by thyroprotein supplementation, semen characteristics, testicular histology and fertility are not affected by the operation.
Irvine CH, Evans MJ.Total tyhroxine (TT4), free thyroxine (FT4), total triiodothyronine (TT3) and free triiodothyronine (FT3) concentrations in foal umbilical cord blood were respectively 14, 5 7 and 3 times the concentrations of these hormones in adult horse blood. The TT4 levels in foals declined rapidly to reach adult concentrations by Day 16 and FT4 levels declined steadily during the first 3 months of life. Foal TT3 levels rose during the first 10 hr after birth and thereafter declined, although they were still X2-5 higher than adult levels at 3 months of age. Levels of FT3 similarly increased after birth be...
Drew B, Barber WP, Williams DG.On a thoroughbred stud four foals were born with greatly enlarged thyroids and leg weakness. Two foals died within 18 hours of birth, the others subsequently recovered. An enlarged thyroid was also evident in one of the resident mares. The thyroids from the dead foals were hyperplastic. Feed analyses showed that the mares had an iodine intake of about 83 mg daily, 8-8 ppm of the dietary dry matter, due almost entirely to the high iodine content of a proprietary compound horse nut which had been fed at the daily rate of 12 lb per head. It was concluded from the histology of the thyroids, the hi...
Miquelis R, Simon C.1. The subcellular particles of horse and rat thyroids were fractionated in a B XIV zonal rotor on a non-linear gradient of Ficoll after labelling with radioactive iodine in vitro (horse) or in vivo (rat). In the horse, the resulting fractions were analysed for radioactive iodine, protein and enzymes representative of certain subcellular particles. In the rat, iodine turnover and thyrotrophin stimulation were studied. 2. The population of iodinated particles could be subdivided into three main classes, characterized by differences in beta-galactosidase and acid phosphatase content and position...
Kallfelz FA, Lowe JE.Three thyroid function tests were carried out on ten normal yearling horses. These were the T-4 test which allows the determination of total serum thyroxine and FBI, the labelled triiodothyronine resin sponge uptake test (T-3 test), and the chemical determination of FBI. The T-4 test appeared to be superior to the chemical determination of FBI in assessing thyroid function in the horse. The value of the T-3 test could not be assessed.--BMW.
Breuhaus BA.This study was performed to determine whether anhidrotic horses have altered thyroid function compared with horses that sweat normally. Objective: Anhidrotic horses have normal thyroid function. Methods: Ten client-owned horses with clinical signs of anhidrosis were paired with 10 horses living in the same environment that had normal sweat production. Methods: Horses were diagnosed as having normal sweat production or being anhidrotic based on responses to intradermal injections of terbutaline and physiologic responses to lunging exercise. Control horses were selected from the same environment...
Frank N, Sojka JE, Latour MA.Blood lipid and lipoprotein concentrations were measured and compared between euthyroid and thyroidectomized mares on low-fat or high-fat diets to test the hypothesis that hypothyroidism alters the blood lipid response to higher dietary fat intake. Four healthy adult mares and four adult mares that had been thyroidectomized 3 to 6 mo earlier were placed on low-fat or high-fat diets according to a replicated 2 x 2 Latin square design consisting of two 5-wk feeding periods separated by a 2-wk washout interval. Plasma lipid concentrations were measured at 0, 3, 4, and 5 wk, and plasma lipase acti...
Meredith TB, Dobrinski I.To determine whether thyroid function was associated with pregnancy status in broodmares. Methods: Prospective study. Methods: 79 Thoroughbred and Standardbred broodmares between 2 and 22 years old. Methods: Serum triiodothyronine (T3) concentration was measured before and 2 hours after i.v. administration of thyrotropin releasing hormone (TRH), and serum thyroxine (T4) concentration was measured before and 4 hours after TRH administration. Pregnancy status was monitored by means of transrectal ultrasonography beginning 16 days after ovulation. Results: Baseline T3 and T4 concentrations varied...
Caple IW, Bourke JM, Ellis PG.The calcium and phosphorus nutrition of thoroughbred racehorses was assessed by analysis of serum and urine samples collected from 90 horses in 1975 and 139 horses in 1980-81 at racetracks in Melbourne. Horses that were excreting greater than 15 mumole Ca/mosmole and which had a calcium to creatinine clearance ratio greater tha 2.5% were considered to have adequate Ca intake. Horses that were excreting greater than 15 mumole P/mosmole and which had a phosphorus to creatinine clearance ratio greater than 4% were considered to have excessive phosphorus intake. Sixty-percent of the horses sampled...
Breuhaus BA.To determine whether ingestion of fescue seed infected with the endophyte Neotyphodium coenophialum would alter thyroid function in adult horses. Methods: Original study. Methods: 4 adult mares that were not pregnant and 6 adult geldings. Methods: Thyrotropin releasing hormone stimulation tests were performed while horses received a standard diet and after infected seed (2.3 kg/d [5 lb/d]) had been fed for 1 and 2 months. Serum prolactin concentrations were measured to verify endophyte absorption. Results: Serum prolactin concentrations indicated that at least 8 of 10 horses absorbed the endop...
Viana GF, Midon M, Fonseca MW, Hataka A, Carandina L, Bueno LMC, Puoli Filho JNP, Machado VMV.In humans and pets, magnetic resonance imaging (MRI) provides important adjunctive anatomic information about the thyroid and its adjacent structures. To the best of our knowledge, however, no studies have been reported regarding the use of MRI for thyroid evaluation in horses. The aim of this study was to obtain qualitative and quantitative parameters of healthy thyroid gland in adult horses by MRI. Eleven horses were submitted to thyroid MRI evaluation. The healthy thyroid of these animals was ensured by normal examinations performed before (free thyroxine and ultrasound) and after (cytology...
Nieth J, Köhler K, Wehrend A.Two cases of an innate hyperplastic goiter in foals as well as the sonographic evaluation of the hyperplastic gland are presented. One foal displayed skeletal deformities in the form of a mandibular prognathism and forelimb contractures in addition to the swollen thyroid gland. Because of a poor prognosis, the animal was euthanized. The second foal was premature and displayed respiratory signs. Under symptomatic therapy, the goiter regressed within a few weeks.
Dalefield RR, Palmer DN, Jolly RD.Lipofuscin accumulation and other histological changes in thyroid tissue, previously reported to be age-related, were studied in 31 horses aged up to 35 years. The number of lipofuscin granules relative to thyrocytes increased from birth to 5 years of age. There was a wide individual variation in the number of lipofuscin granules in thyrocytes in mature horses, but this was not directly related to age. Several abnormalities were identified in thyroid colloid. The prevalence of spherites, lipofuscin granules, nucleated cells and shreds of colloid increased with age, but the prevalence of calciu...
Costello J, Firshman AM, Brown JC, Maher M, Tadros EM.A 16-year-old American Paint Horse gelding was presented for evaluation of weight loss and high serum thyroid hormone concentrations resulting from a functional thyroid adenoma. The horse showed no response to a thyrotropin-releasing hormone (TRH) stimulation test. Clinical signs resolved following surgical removal of the adenoma. . Un cheval American Paint Horse hongre âgé de 16 ans fut présenté pour évaluation à la suite d’une perte de poids et d’une concentration sérique élevée d’hormone thyroïdienne résultant d’un adénome thyroïdien fonctionnel. Le cheval ne démontrai...
Osame S, Ichijo S.Seven cases of thoroughbred foals kept on a farm in the Hidaka district of Hokkaido showed bilateral thyroid enlargement from 3 or 4 months after birth. Their serum T4 levels were remarkably low, whereas T3 levels were in the normal range. Serum T4 levels were also low in the other clinically normal foals and their dams kept together. Since the iodine content in pasture soil and grass was very low, iodine deficiency was supposed to be the cause of the disease.
Lowe JE, Baldwin BH, Foote RH, Hillman RB, Kallefelz FA.Three Quarter-horse stallions were surgically thyroidectomized at 18 months of age. Weekly semen collections were made from these and three control stallions from 25 to 39 months of age. Although the induced state of hypothyroidism in young stallions has a depressing effect on libido and general animation and can be corrected by thyroprotein supplementation, semen characteristics, testicular histology and fertility are not affected by the operation.
Held JP, Patton CS, Toal RL, Geiser DR.A thyroid carcinoma was diagnosed in a 14-year-old competitive trail horse with a 3-month history of work intolerance. Abnormal findings included low base-line triiodothyronine (T3) and thyroxine (T4) values, a large thyroid gland and decreased work tolerance. Nuclear medicine scanning revealed displacement of the right thyroid gland by a mass. Needle biopsy of the mass revealed neoplastic changes compatible with thyroid carcinoma. After removing the tumor surgically, T3 and T4 values returned to normal. Subsequently, the horse was able to compete successfully. Horses with work intolerance com...
Löhr CV, Polster U, Kuhnert P, Karger A, Rurangirwa FR, Teifke JP.Actinobacillus suis-like organisms (ASLOs) have been isolated from the genital, respiratory, and digestive tracts of healthy adult horses, horses with respiratory disease, and septic foals. Two foals with congenital hypothyroidism-dysmaturity syndrome from separate farms developed ASLO infection. At necropsy, both had contracted carpal flexor tendons, thyroid hyperplasia, and thrombotic and necrotizing mesenteric lymphangitis and lymphadenitis; one foal also had mandibular prognathism. Numerous ASLOs were isolated from tissues from both foals, including intestine. Biochemical testing and mass ...
Miquelis R, Simon C.1. The subcellular particles of horse and rat thyroids were fractionated in a B XIV zonal rotor on a non-linear gradient of Ficoll after labelling with radioactive iodine in vitro (horse) or in vivo (rat). In the horse, the resulting fractions were analysed for radioactive iodine, protein and enzymes representative of certain subcellular particles. In the rat, iodine turnover and thyrotrophin stimulation were studied. 2. The population of iodinated particles could be subdivided into three main classes, characterized by differences in beta-galactosidase and acid phosphatase content and position...
Card CE, Manning ST.Fetal maturation and equine parturition are not understood fully, although the adrenal and thyroid glands are thought to have regulatory roles. Thyroidectomized equine fetuses undergo prolonged gestation, and spontaneous diseases such as thyroid musculoskeletal disease and gestational fescue endophyte exposure are also associated with delayed parturition. Thyroid musculoskeletal disease is characterized by: histologically hyperplastic thyroid glands, chondro-osseous dysplasia and dysgenesis, angular limb deformity, low resting thyroxine and triiodothyronine concentrations, and lack of response...
Smith JJ, Embertson RM.To compare racing performance before and after sternothyroideus myotomy, staphylectomy, and oral caudal soft palate photothermoplasty in Thoroughbred racehorses with intermittent dorsal displacement of the soft palate (DDSP). Methods: Retrospective study. Methods: Thoroughbred racehorses (n=102) with DDSP. Methods: Retrieved data included signalment, primary complaint, and upper respiratory tract endoscopic finding. Lifetime race records were compared for earnings per start before and after surgery, days to 1st start, and races won postoperatively. Results: Comparing mean earnings per start fo...
Saadi A, Dalir-Naghadeh B, Akbari H, Rashedi M, Mohammadi R.An 11-year-old Hanoverian gelding used for jumping was evaluated for gait abnormalities and hoof problems in the hindlimbs. Clinical examinations revealed signs consistent with shivers. A thyroid gland enlargement was noticed, baseline serum thyroid hormone (TH) concentrations were low, and a low response to thyrotropin-releasing hormone administration was observed. Hypothyroidism was suspected. The horse was treated with levothyroxine for 1 year. TH concentrations returned to the normal range by week 4 of treatment. Thirty weeks after the initiation of levothyroxine therapy, the gait abnormal...
Alexander SL, Irvine CH, Evans MJ.We used our nonsurgical technique for collecting pituitary venous blood to relate the dynamics of thyrotrophin-releasing hormone (TRH) secretion to the secretion patterns of both prolactin and thyrotrophin in periovulatory mares, either euthyroid (n = 5) or made hypothyroid by treatment with propyl-thiouracil (n = 5). Pituitary venous blood was collected continuously and divided into 1-min aliquots for 4 h. To test the effect of dopamine on the relationship between secretion patterns, sulpiride, a selective D2 receptor antagonist, was given i.m. after 2 h of sampling. Thorough testing of the m...
Lowe JE, Foote RH, Baldwin BH, Hillman RB, Kallfelz FA.Three Quarter-horse mares were thyroidectomized at about 1.5 years of age. Three similar intact mares served as controls. The study continued through two breeding seasons. The thyroidectomized mares were lethargic, rear limbs were oedematous and hair coats were coarse. They displayed a tranquil oestrous behaviour when exposed to a stallion and were only mildly antagonistic when not in oestrus. Length of oestrous cycles varied but most often they were 19-24 days long. Duration of oestrus (mean +/- s.e.m.) for the control and thyroidectomized mares was 12.9 +/- 2.9 and 11.7 +/- 2.2 days respecti...
