Hypothyroidism refers to inadequate production of thyroid hormones, primarily thyroxine and (T4) and triiodothyronine (T3). These hormones are produced by the thyroid gland and play a role in regulating your horse’s metabolism.
Hypothyroidism most commonly occurs in neonatal foals and is rarely observed in adult horses. 
In foals, the condition results in musculoskeletal deformities and, in some cases, thyroid gland enlargement.  When hypothyroidism occurs in adult horses, typical signs include dull coat, lethargy, and poor performance. 
Neonatal hypothyroidism is believed to occur when mares fail to consume sufficient iodine during gestation.  Excess nitrate intake and the consumption of plants that interfere with thyroid function are also proposed causes of the condition. 
Diagnosing hypothyroidism requires testing levels of thyroid hormones including T4 and T3 and carefully considering clinical signs.  A more accurate diagnosis can be made in combination with the thyrotropin-releasing hormone (TRH) stimulation test. 
Treatment of hypothyroidism in adult horses requires identifying and addressing the underlying cause of the condition. There is no treatment for neonatal hypothyroidism. Ensuring optimal nutritional status of pregnant mares during gestation is critical for improving the chance of healthy foals at birth.
What is Hypothyroidism in Horses?
Hypothyroidism is a hormonal imbalance caused by the insufficient production of biologically active thyroid hormone (TH) from the thyroid gland.
When hypothyroidism occurs in horses, the condition most commonly presents in neonatal foals.  Naturally occurring cases of hypothyroidism in horses beyond the neonatal stage are extremely rare.
In the limited case reports of adult horses diagnosed as hypothyroid (in which the condition was not induced), clinical signs primarily included lethargy, exercise intolerance, and poor hair coat. 
Health issues including laminitis and weight gain are sometimes misdiagnosed as hypothyroidism. Measuring low thyroid hormone levels may also be due to other illness and natural variations in thyroid hormone production.
Types of Hypothyroidism
Hypothyroidism in horses can be classified as a primary, secondary, or tertiary disease. Primary hypothyroidism describes diseases involving the thyroid gland itself. 
Experimentally, primary hypothyroidism (diagnosed according to laboratory tests) has been induced in horses that have had their thyroid gland removed (thyroidectomy). The condition has also been induced in horses through the oral administration of the antithyroid medication propylthiouracil (PTU). 
Secondary hypothyroidism describes the abnormal secretion of thyroid-stimulating hormone (TSH) from the anterior pituitary gland in the brain. 
Although not diagnosed in horses, tertiary hypothyroidism describes a condition in which an inadequate amount of thyrotropin-releasing hormone (TRH) is released from the hypothalamus. 
Hypothyroidism in Neonatal Foals
Two thyroid syndromes are recognized in neonatal foals: 
- Congenital hypothyroidism
- Congenital hypothyroidism and dysmaturity syndrome (CHD)
Both syndromes are present at birth and involve thyroid hormone deficiency.
Some researchers believe that congenital hypothyroidism and CHD might be related (depending on the level of, timing, and duration of the thyroid hormone deficiency) rather than distinct disease processes. 
Congenital hypothyroidism can occur with or without visible goitre – an enlargement of the thyroid gland.
Congenital hypothyroidism may result due to a transient or short-lived thyroid hormone deficiency that occurs in pregnant mares during gestation. 
Congenital Hypothyroidism and Dysmaturity Syndrome
CHD is also referred to as TH-musculoskeletal deformity (TH-MSD).  This syndrome was described after a review of cases in western Canada and has also been noted in Europe. 
Although more research is needed, scientist K. Koikkalainen proposes that CHD may result from a chronic state of hypothyroidism in pregnant mares during gestation. 
This results in multiple critical fetal developmental abnormalities due to insufficient thyroid hormone levels. 
Hypothyroidism and Other Health Conditions
Although naturally occurring hypothyroidism is unlikely in horses older than neonates, the condition is often blamed as a cause of laminitis, weight gain, poor performance, and failed lactation in mares. However, other causes of these conditions are more likely. 
According to reports, neither obesity nor laminitis occurred in horses that underwent surgical thyroidectomy or were administered PTU to induce hypothyroidism. 
Research indicates that thyroid hormone supplementation might accelerate weight loss and improve insulin sensitivity, which could reduce the risk of laminitis. 
A study in horses with normal thyroid function that were administered levothyroxine (synthetic T4 hormone) at an increasing rate of 24 to 96 mg/day over two-week intervals lost weight and had improved insulin sensitivity. 
Hypothyroidism and Thyroid Tumors
Thyroid gland tumors (goitres) are frequently associated with thyroid dysfunction in humans and other species.
In horses, goitres are most likely to affect senior horses rather than younger animals. Goitres are not typically associated with hypothyroidism in horses beyond neonatal age. 
Equine Thyroid Gland
The thyroid gland is responsible for thyroid hormone production. This gland consists of two lobes and straddles the windpipe (trachea) in the throat.
The gland is primarily composed of thyroid follicular cells.
Although not typically visible from the outside of the body, the thyroid gland can be palpated and is approximately the size of an egg.
The thyroid gland synthesizes and secretes two iodine-containing hormones: Thyroxine or tetraiodothyronine (T4) and triiodothyronine (T3). 
Triiodothyronine (T3) is the primary active form responsible for the physiological effects of thyroid hormones, while thyroxine (T4) acts as a precursor to other hormones and gets converted into T3 in many cells of the body. 
Thyroid hormones play diverse roles in the body’s functions. They are necessary for cellular growth as well as the metabolism of fats and carbohydrates. 
Despite this, the thyroid gland and the hormones it produces are not essential for life.  However, proper thyroid gland function is critical in pregnant mares as it is required for the normal development of the skeletal and nervous systems in developing foals.
The effects of hypothyroidism on developing foals are far more severe than those observed in adult horses. 
Thyrotropin thyroid-stimulating hormone (TSH) is secreted from the anterior pituitary gland and promotes the release of T4 and T3 hormones from the thyroid gland. 
The hypothalamus of the brain regulates thyrotropin-releasing hormone (TRH) to influence the pituitary gland’s secretion of TSH. 
Iodine and Thyroid Function
To produce thyroid hormones, the thyroid gland requires iodine – a trace mineral. Dietary iodine is converted into iodide in the intestinal tract and quickly absorbed. 
The thyroid gland absorbs iodide from the blood and stores it as colloid, a substance containing thyroid hormones and proteins. 
When T3 and T4 enter the bloodstream, most of it binds to one of several proteins, such as albumin, in the blood plasma. 
Bound T3 and T4 provide a reserve of available thyroid hormones circulating in the bloodstream. Free or unbound thyroid hormones are biologically active. 
Recommended Iodine Intake
The recommended daily iodine intake for adult horses is 0.007 mg/kg of body weight. For a typical 500 kg (1100 lb) horse, this is 3.5 mg of iodine per day.
On a dietary concentration basis this is equivalent to 0.35 mg/kg of dry matter in the diet. In the last semester of pregnancy, iodine should be increased to 0.4 mg/kg dry matter in mares. 
Both insufficient and excess iodine intake can cause thyroid dysfunction, therefore care must be taken when feeding iodine. It is recommended to feed iodine as a component of a complete vitamin and mineral supplement like Mad Barn’s Omneity rather than as a single ingredient such as from an iodine-rich supplement like kelp meal.
Signs of hypothyroidism vary depending on the age and development of the affected horse. The condition produces more serious symptoms in neonatal foals than older horses.
Horses Older than Neonatal Age
Clinical signs of hypothyroidism in horses older than neonatal age have been described in experimentally induced cases in which horses have had their thyroid gland removed.
A study of yearlings that had their thyroid gland removed showed the following signs: 
- Failure to grow to a normal height
- Cold intolerance and hypothermia
- Rough and dull coats
- Swelling in the hind limbs
- Delayed closure of the growth plates at the end of long bones
- Delayed eruption of the incisor teeth
Congenital Hypothyroidism in Foals
Signs of congenital hypothyroidism in foals can range from mild to severe. Typical symptoms of the condition include:
- Weakness at birth
- Absent or abnormal sucking reflex
- Hypothermia (low body temperature)
- Multiple musculoskeletal abnormalities such as flexural deformities, ruptured common digital extensor tendons, and incomplete formation of the knee (carpal) and hock (tarsal) bones 
Foals with congenital hypothyroidism may present with or without a visible goitre and an accumulation of colloid in the gland. 
Thyroid hormone levels can either be low or normal when compared with foals of the same age. 
CHD in Foals
Similar to foals with congenital hypothyroidism, foals affected by CHD also have musculoskeletal abnormalities. 
Foals with CHD have a prolonged gestation and a high incidence of protruding lower jaw (mandibular prognathism). Microscopic thyroid tissue analysis indicates the gland becomes enlarged (hyperplasia) at some point in time. 
In foals with CHD, thyroid hormone levels can be either low or normal when compared with foals of the same age. 
Causes of Hypothyroidism in Horses
Causes of Primary Hypothyroidism
Primary hypothyroidism in horses older than neonatal age can result if iodine intake is insufficient.
Other natural causes of primary hypothyroidism in older horses are unknown (idiopathic).  Hypothyroidism may also be medically induced by thyroid removal or PTU administration.
While known to occur commonly in other species, autoimmune thyroid disease (Hashimoto’s disease) has not been observed in horses. 
Congenital Hypothyroidism and CHD in Foals
In neonatal foals, primary hypothyroidism may be related to the mare’s nutritional intake of iodine during gestation. 
Potential risk factors for the development of congenital hypothyroidism or CHD in foals include:
- Ingestion of plants (goitrogenic plants) that inhibit thyroid gland function 
- Consumption of goitrogenic compounds that inhibit thyroid hormone synthesis (i.e. nitrates) 
- Consuming excessive or insufficient amounts of iodine 
- Low dietary selenium intake 
- Excess dietary nitrate intake 
No specific breed or sex of foals appear to be at higher risk for CHD, so a genetic cause is unlikely. 
Causes of Secondary Hypothyroidism
Secondary hypothyroidism occurs when the pituitary gland fails to secrete an adequate level of thyroid-stimulating hormone (TSH) from the anterior pituitary gland. 
Horses beyond neonatal age may develop hypothyroidism due to equine metabolic syndrome (EMS) or pituitary pars intermedia dysfunction (PPID), previously known as Cushing’s syndrome, due to excess steroid hormones (glucocorticoids) in the body. 
Various factors can cause measured thyroid hormone values to be low, even when thyroid function is normal. Some of these include: 
- Medications such as phenylbutazone, corticosteroids, salicylates, and the hormone estrogen; these substances influence the amount of thyroid hormone that binds to proteins in the body
- Strenuous exercise
- Diets high in energy, protein, zinc, or copper
- Nonthyroidal illnesses
Hypothyroidism in horses can be difficult to diagnose due to: 
- Nonspecific clinical signs
- A lack of validated diagnostic testing strategies for horses
- Inaccessibility of testing compounds and the high cost of them
- Multiple factors that affect thyroid hormone levels in horses
Ruling Out Other Conditions
As previously mentioned, hypothyroidism that occurs in horses beyond the neonatal age usually has an underlying cause. Before hypothyroidism can be diagnosed, a veterinarian must rule out other health conditions as an underlying cause.
A full physical examination should be performed to assess overall health when determining if hypothyroidism is present.
Hypothyroidism may be suspected as a cause of health and performance issues when other factors are involved. For example:
- Poor performance may be caused by respiratory, heart, and skeletal issues.
- A dull and coarse coat and dry skin may be caused by dietary imbalances, time of year, senior age, and pituitary pars intermedia dysfunction (PPID).
- Excess weight, cresty neck, and fatty deposits on the body may result from PPID, overfeeding, and lack of exercise.
- Chronic laminitis may result from PPID, poor hoof care, and insulin resistance.
Laboratory testing is helpful for evaluating cases of suspected hypothyroidism to indicate if the concentration of one or more thyroid hormones is outside a normal range.
Your veterinarian will determine if thyroid hormone levels are abnormal and if another factor such as physiological status, diet, medication, or an underlying health condition is influencing them.
Measuring T3 and T4 Hormone Levels
T4 and T3 are commonly measured thyroid hormones and provide useful information on the health of the thyroid gland. 
T4 is the dominant form of thyroid hormone circulating in the blood. Most T3 is produced when one iodine atom is removed from T4 by the body’s tissues (deiodination). 
Both T3 and T4 are bound to proteins in the blood. Only unbound/free hormones, such as free T3 (fT3), are biologically active. 
Measuring levels of fT3, total T3, and T4 can provide a more accurate indication of thyroid status than measuring only T3 and T4, if factors such as drug administration, supplements, or diet are affecting total T3 and T4 levels. 
Thyroid Releasing Hormone (TRH) Stimulation Test
The thyrotropin-releasing hormone (TRH) stimulation test is useful for evaluating the function of the complete thyroid hormonal axis, which includes the hypothalamus in the brain, pituitary gland, and thyroid gland. This test can aid in the diagnosis of hypothyroidism. 
However, the TRH test is not frequently performed in ambulatory practice as the test is costly and requires collecting multiple blood samples over several hours.
TRH testing measures thyroid hormones before and two and four hours after the injection of TRH. Horses with normal thyroid function will have at least double T3 and T4 levels at two to four hours after TRH injection. 
A lack of response to the TRH test may indicate an abnormally functioning thyroid hormonal axis.
Tests for cholesterol levels and red blood cell count are of limited use in diagnosing hypothyroidism. 
High cholesterol (hypercholesterolemia) and a low red blood cell count (anemia) may be present in horses with hypothyroidism.  However, these abnormalities don’t always occur in conjunction with hypothyroidism. 
Primary Hypothyroidism in Adult Horses
Adult horses with hypothyroidism are typically treated with iodine supplements and/or synthetic thyroid hormone medications to correct low levels of thyroid hormones.
Horses with hypothyroidism that are deficient in iodine can be treated with an iodinated protein product (typically iodinated casein) to improve the levels of this mineral.
Thyroid Hormone Replacement
In experimentally induced hypothyroidism via thyroidectomy, thyroid hormone replacement therapy reverses clinical signs of the condition. 
Thyroid hormone replacement therapies consist of purified natural thyroid hormones or synthetic thyroxine (levothyroxine, L-thyroxine). 
To ensure proper dosing, serum TH measurements should be taken frequently by your veterinarian. When discontinuing therapy, horses should be gradually weaned off replacement hormones to enable the body to adapt.
Research shows that synthetic thyroxine administered at a dose of 20 mcg/kg (10 mg in a 500-kg horse) sustains normal T3 and T4 concentrations for 24 hours. 
Congenital Hypothyroidism and CHD in Foals
Thyroid hormone supplements does not reverse the musculoskeletal changes in neonatal foals affected by congenital hypothyroidism or CHD, so they are not typically administered. 
However, veterinarians may recommend treating some foals that are hypothyroid at birth with thyroid hormone supplementation and providing supportive interventions, such as splints and restricted exercise. 
Dietary imbalances should be corrected in mares that have delivered foals with congenital hypothyroidism or CHD. 
Horses with non-thyroidal illness syndrome caused by endocrine diseases including PPID and EMS have low thyroid hormone concentrations that may lead to the misdiagnosis of primary hypothyroidism. 
In these cases, treatment of the underlying endocrine disease is necessary, rather than administering the horse with thyroid hormones.
Horses with PPID are treated with the medication pergolide. Horses with EMS typically require diet adjustments and, in some cases, medications including metformin and levothyroxine.
The prognosis for adult horses and foals with hypothyroidism is guarded. In adult horses, the prognosis depends on the underlying factors contributing to low thyroid hormone levels.
In foals, the outcome is determined by the severity and reversibility of musculoskeletal or developmental issues.
For foals with CHD, the prognosis is typically poor as most die or are euthanized within a few days post-birth. Foals that do survive may have a range of musculoskeletal issues that result in unsoundness. 
Foals with incomplete skeletal development have an increased risk of developing collapse of the tarsal bones. 
Providing pregnant mares with optimal iodine intake and balanced nutrition during gestation is essential for preventing neonatal hypothyroidism.
Ensuring adult horses have an adequate iodine intake can help to prevent the development of hypothyroidism due to deficiency.
Horses with endocrine disturbances including EMS and PPID require treatment to help prevent the condition.
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