Thyroid function and dysfunction in term and premature equine neonates

Diagnosis and management of thyroid disorders and thyroid hormone supplementation in adult horses and foals

Equine thyroid disorders pose a diagnostic challenge in clinical practice because of the effects of nonthyroidal factors on the hypothalamic-pituitary-thyroid axis, and the horse's ability to tolerate wide fluctuations in thyroid hormone concentrations and survive without a thyroid gland. While benign thyroid tumours are common in older horses, other disorders like primary hypothyroidism or hyperthyroidism in adult horses and congenital hypothyroidism in foals are rare. There is a common misunderstanding regarding hypothyroidism in adult horses, especially when associated with the clinical profile of obesity, lethargy, and poor performance observed in dogs and humans. Low blood thyroid hormone concentrations are often detected in horses as a secondary response to metabolic and disease states, including with the nonthyroidal illness syndrome; however, it is important to note that low thyroid hormone concentrations in these cases do not necessarily indicate hypothyroidism. Assessing equine thyroid function involves measuring thyroid hormone concentrations, including total and free fractions of thyroxine (T4) and triiodothyronine (T3); however, interpreting these results can be challenging due to the pulsatile secretion of thyroid hormones and the many factors that can affect their concentrations. Dynamic testing, such as the thyrotropin-releasing hormone stimulation test, can help assess the thyroid gland response to stimulation. Although true hypothyroidism is extremely rare, thyroid hormone supplementation is commonly used in equine practice to help manage obesity and poor performance. This review focuses on thyroid gland pathophysiology in adult horses and foals, interpretation of blood thyroid hormone concentrations, and evaluation of horses with thyroid disorders. It also discusses the use of T4 supplementation in equine practice.

Additional effects using progestins in mares on levels of thyroid hormones and steroids in neonates

The risk of pregnancy loss in mares leads to the use of exogenous hormones to help pregnancy maintenance. The objective was to evaluate the proportion of thyroid hormones and steroids in neonates, in the following postpartum period, born to mares fed with synthetic progesterone and to verify the existence of a correlation between the level of progesterone between mother and neonate. Twenty-seven mares and their foals were used. The animals were divided into 5 experimental groups: group 1 (control, without hormonal supplementation), group 2 (random samples fed to 120 days of pregnancy with long-term progesterone), group 3 (mares fed with short-term progesterone as of 280.º day of pregnancy), group 4 (mares fed with long-term progesterone as of 280.º day of pregnancy) and group 5 (mares fed with synthetic hormone [altrenogest] as of 280.º day of pregnancy). The animal's blood collection took place immediately after parturition for the hormonal measurement. The hormones measured in neonates were total T3, free T4, TSH, progesterone and cortisone. In mares, only levels of progesterone. The groups of neonates showed no difference on levels of total T3, free T4, TSH and progesterone. There was no difference on levels of progesterone in mares among the groups. Neonates from groups 4 and 5 had higher and lower cortisone levels, respectively. No neonate showed clinical change. There was also no correlation between levels of progesterone in mares and foals. Thus, hormonal supplementation with long-term progesterone as of 280 days of pregnancy leds to an increase in the neonate's cortisone levels, in the meantime, supplementation with altrenogest as of 280 days of pregnancy caused a decrease on cortisone levels in foals, despite clinical signs have not been observed on these animals.

Can Endocrine Dysfunction Be Reliably Tested in Aged Horses That Are Experiencing Pain?

Simple Summary

Pituitary pars intermedia dysfunction (PPID) is an endocrine (secreting internally) disease of aged horses and ponies. An enlargement (hyperplasia) of the pars intermedia of the pituitary gland leads to an increased secretion of hormones, including adrenocorticotropic hormone (ACTH). The main tests for a diagnosis of PPID are the measurement of basal ACTH and the thyrotropin-releasing hormone (TRH) stimulation test, where TRH stimulates the secretion of ACTH. Since pain can also lead to elevated concentrations of ACTH, it is unclear whether horses with pain can be tested for PPID correctly. The aim of the present study was to find out whether pain caused a marked increase of ACTH can lead to a false positive result in the diagnosis of PPID. Therefore, we examined fifteen horses treated for different pain conditions, which also served as their own controls as soon as they were pain-free again. The ACTH and cortisol were measured before and after the TRH stimulation test. There was no significant difference in the ACTH concentration in horses with pain and the controls, between different pain intensities or between disease groups. Thus, measuring the basal ACTH concentration and performing the TRH stimulation test for the diagnosis of PPID seem to be possible in horses with a treated low to moderate pain condition.

Abstract

The aim of the present study was to evaluate (i) the effects of different intensities and types of treated pain on the basal concentrations of adrenocorticotropic hormone (ACTH) and cortisol, and (ii) the thyrotropin-releasing hormone (TRH) stimulation test, to determine whether treated pain caused a marked increase of ACTH, which would lead to a false positive result in the diagnosis of pituitary pars intermedia dysfunction (PPID). Methods: Fifteen horses with treated low to moderate pain intensities were part of the study. They served as their own controls as soon as they were pain-free again. The horses were divided into three disease groups, depending on their underlying disease (disease group 1 = colic, disease group 2 = laminitis, disease group 3 = orthopedic problems). A composite pain scale was used to evaluate the intensity of the pain. This pain scale contained a general part and specific criteria for every disease. Subsequently, ACTH and cortisol were measured before and after the intravenous application of 1 mg of TRH. Results: There was no significant difference in the basal or stimulated ACTH concentration in horses with pain and controls, between different pain intensities or between disease groups. Descriptive statistics, however, revealed that pain might decrease the effect of TRH on the secretion of ACTH. There was an increase of ACTH 30 min after TRH application (p = 0.007) in the treated pain group, but this difference could not be statistically confirmed. Measuring the basal ACTH concentration and performing the TRH stimulation test for the diagnosis of PPID seem to be possible in horses with low to moderate pain.

C-terminal telopeptide of type I collagen, osteocalcin, alkaline phosphatase, and parathyroid hormone in healthy and hospitalized foals

Hypocalcemia is a common finding in critically ill equine patients. Parathyroid hormone (PTH) helps to maintain calcium homeostasis in hypocalcemic patients by promoting renal calcium reabsorption and bone resorption. Increased serum PTH concentrations have been reported in critically ill people and animals, including horses and foals. It is unknown whether increased secretion of PTH is associated with markers of bone turnover in hospitalized foals. The goals of this study were to measure markers of bone resorption (C-terminal telopeptide of type I collagen [CTX-I]) and bone formation (osteocalcin [OCN]; alkaline phosphatase [ALP]) and to determine their association with PTH concentrations, disease severity, and mortality in hospitalized foals. This prospective, multicenter, cross-sectional study was conducted on 75 newborn foals ≤3 d old divided into hospitalized (n = 65; 41 septic; 24 sick nonseptic) and healthy (n = 10) groups. Blood samples were collected on admission to measure serum CTX-I, OCN, and PTH concentrations and ALP activity. Data were analyzed by nonparametric methods and univariate logistic regression. Serum CTX-I and PTH concentrations were significantly higher, whereas OCN concentrations were lower, in septic compared with healthy foals (P < 0.05). Serum ALP activity was not different between groups; however, it was lower in hospitalized and septic foals with low OCN concentrations (P < 0.05). In hospitalized foals, PTH concentrations were positively correlated with CTX-I concentrations and inversely associated with ALP activity (P < 0.05). High CTX-I and low OCN concentrations were associated with disease severity (P < 0.05). Hospitalized nonsurviving foals had significantly lower OCN concentrations compared with survivors (P < 0.05), but CTX-I concentrations were not associated with survival. Hospitalized foals with PTH concentrations >12.4 pmol/L were more likely to die (OR = 1.5; 95% CI = 1.1-4.16; P < 0.05). Elevated PTH and CTX-I together with reduced OCN concentrations and ALP activity in sick foals indicates that bone resorption is increased during critical illness, which may be a compensatory mechanism to correct hypocalcemia or reflect a response to systemic inflammation and metabolic imbalances. Bone resorption could negatively impact skeletal development in the growing foal. Low OCN and high PTH concentrations were predictors of nonsurvival in hospitalized foals.

Serum cortisol and thyroid hormone concentrations and survival in foals born from mares with experimentally induced ascending placentitis

Background

There are few publications on occurrence of nonthyroidal illness syndrome in foals and on the prognostic value of cortisol and thyroid hormone (TH) concentrations in newborn foals.

Objectives

To determine serum cortisol and TH concentrations (total and free thyroxine: T₄ and _FT₄; total and free triiodothyronine: T₃ and _FT₃) in foals born from mares with placentitis, to determine their association with survival, and their use as prognostic markers.

Animals

A cohort of 29 newborn foals comprising 5 Control, 14 Low‐risk, and 10 Sick foals were evaluated over the first week of life.

Methods

In this prospective study foals born to mares with experimentally‐induced placentitis were assigned to Low‐risk or Sick groups while foals born to control mares were classified as Control based on clinical findings. Foals were also classified as Term (n = 13), Dysmature (n = 7), or Premature (n = 9), and survival rate was recorded. Serum cortisol and TH hormone concentrations were measured at 0, 12, 24, 48, and 168 hours of life.

Results

Sick non‐surviving foals had lower (P < .05) T₃ : cortisol ratio at 12 (3.68 ± 1.06 versus 18.58 ± 2.78), 24 (5.47 ± 2.34 versus 23.40 ± 3.82), and 48 (10.47 ± 6.29 versus 26.6 ± 2.90) hours of life when compared to Sick surviving foals and lower (P < .05) T₄ : cortisol ratio at 12 (75.12 ± 21.71 versus 414.47 ± 58.47) and 24 hours (127.83 ± 55.21 versus 430.87 ± 80.31) after birth than Sick surviving foals.

Conclusion and Clinical Importance

Placental infections can impair fetal thyroid function. Low T₃ : cortisol and T₄ : cortisol ratios seem to be good prognostic markers in newborn foals.

Clinical Pathology in the Foal

The dynamic physiologic changes and unique diet during the neonatal period contribute to key differences in clinicopathologic test results of healthy foals relative to healthy adult horses. When reporting results, most diagnostic laboratories only provide reference intervals for mature horses. Thus, failure to recognize the unique differences that occur in foals relative to adult horses can lead to erroneous interpretation of neonatal clinical pathologic values. Thus, the main objective of this article was to review distinct features of common clinicopathologic tests in foals, relative to mature horses.

A Field Study of Serum, Colostrum, Milk Iodine, and Thyroid Hormone Concentrations in Postpartum Draft Mares and Foals

Iodine, thyroxine (T4) and triiodothyronine (T3) are required for normal fetal growth, maturation, and neonatal survival. There is a lack of robust information on iodine levels found in colostrum, milk, and serum of mares and foals after a healthy pregnancy. Our objective was to characterize colostrum, milk, and serum iodine levels in healthy postpartum mares and foals (n = 10) and explore relationships with thyroid hormone concentrations. Colostrum, milk, and jugular blood samples from draft breed mares and foals with an estimated average iodine daily intake of 39 mg per mare during pregnancy were obtained at Day 0 (foaling date) and/or 10 days later. Parameters studied were (1) mare basal concentrations of serum: TT3, TT4, and iodine; (2) iodine in colostrum at Day 0 and milk iodine (Day 10); and (3) foal basal: TT3, TT4, and serum iodine (Days 0 and 10). Median ± median error colostrum iodine levels (165 ± 15.1 μg/L) were higher than milk (48 ± 5.6 μg/L; P = .007) levels. Median ± median error foal serum iodine (268.5 ± 7.6 μg/L), TT4 (1,225 ± 47.8 nmol/L), and TT3 (14.2 ± 1.1 nmol/L) at foaling date were higher than at 10 days (serum iodine: 70 ± 3.6 μg/L; TT4: 69.6. ± 20.4 nmol/L; and TT3: 5.4 ± 0.3 nmol/L). In conclusion, equine mammary tissue concentrates iodine beyond plasma levels, making colostrum and milk a significant source of iodine. Foal serum iodine levels are high in the neonatal period and are positively correlated with TT4, which is important for neonatal adaptation.

Serum Thyroid Hormone and Thyrotropin Concentrations in Adult Horses as They Age

With more horses remaining active longer in life, it is important to characterize changes that occur normally with aging, so that these can be differentiated from development of disease. The objective of the study was to test the hypotheses that geriatric horses have lower circulating concentrations of thyroid hormones and/or higher serum thyrotropin (TSH) concentrations compared to younger horses. Serum thyroid hormone and TSH concentrations from 71 normal, healthy horses that had participated in prior research projects were analyzed for effects of age, sex, and season when samples were obtained. All samples had been assayed in the same previously validated radioimmunoassays. There were no differences in serum concentrations of thyroid hormones or TSH by sex or season. Serum total thyroxine (T4) was greater in 3- to 6-year-old horses compared to all other age groups and was negatively correlated with age. There were no differences among age groups for free T4 and total and free tri-iodothyronine (T3). Serum TSH concentration was significantly greater in old horses (≥15 or ≥20 years) compared to young (3-10 years) and intermediate (11-14 years) age groups. Serum TSH was positively correlated with age. There were no significant differences in thyroid hormone responses to thyrotropin releasing hormone (TRH) among young, intermediate, or old horses. However, the TSH response to TRH was significantly different in both groups of older horses compared to intermediate and young horses. Serum total thyroxine concentrations decrease and serum TSH concentrations increase in horses as they age, with no changes in free T4 or T3.

[Congenital goiter in the neonatal foal. Two case reports. Zwei Fallberichte]

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.

The outcome of the first stages of pregnancy on mares' bloodstream thyroid hormones

The objective of this study was to compare in detail the total and free iodothyronines' pattern of mares from the first ovulation of the year over an extended period of 12 weeks. A total of 20 mares were used in the study. The mares were classified into two groups: mares mated at the ovulation (n = 10) used as observational group and mares unmated at the ovulation (n = 10) used as control group. Serum total and free triiodothyronine (T3, fT3) and thyroxine (T4, fT4) levels were measured in baseline conditions at the first ovulation of year and once a week until 12 weeks later. For the experimental group, the first week of postovulation mating was considered as the first week of gestation. One-way analysis of variance showed a significant effect of time over 12 weeks for T3 (F = 2.44; P = 0.007) in pregnant mares, with the higher levels at the seventh and 12th weeks (P < 0.05) than baseline values, and for fT3 (F = 2.36; P = 0.009), with the higher levels at the 11th week (P < 0.05) than baseline values. Two-way analysis of variance showed a significant pregnancy effect compared with nonpregnancy stage for T3 (F = 15.82; P = 0.009), with the higher levels at the seventh and 12th weeks (P < 0.05) of pregnancy than that in nonpregnant values. Thus, it appears that, under similar environment, management and nutritional regime, the first trimester of pregnancy plays a dynamic role on the thyroid patterns by their anabolic activity; therefore, significant effects of time points on the T3 and fT3 concentrations probably may contribute to the control of early embryonic growth and development, before the onset of fetal thyroid activity. Considerable additional research, outwith the aim of this study, will be required to elucidate the mechanisms by which gestational age affects the physiological thyroid function in mares and/or fetus ratio in the first pregnancy stage.