1. General Considerations

   1. Naturally occurring hypothyroidism is rare in large animals; nevertheless, hypothyroidism is over-diagnosed in the horse.
   2. Hypothyroidism is characterized by a plethora of cutaneous and non-cutaneous clinical signs associated with a deficiency of thyroid hormone activity.

2. Cause and Pathogenesis

   1. Hypothyroidism may occur naturally or may be iatrogenic.
   2. It has been produced experimentally in horses, cattle, goats and sheep.
   3. Naturally occurring hypothyroidism caused by dietary iodine deficiency and goiter has been recognized in all large animal species.
   4. Hereditary, congenital goiter and hypothyroidism has been described in Merino sheep, Afrikaner cattle, and Saanen-Dwarf crossbred goats.
   5. Congenital hypothyroidism and dysmaturity syndrome is a thyroid disorder that affects neonatal foals and is characterized by hyperplasia of the thyroid glands, increased gestational length and various musculoskeletal abnormalities. The cause is not known, but excess nitrate in the pregnant mare diet, deficiency or excess of iodine in the pregnant mare diet, and toxic substances (e.g., infectious agents, fungal toxins) have been implicated.
   6. Anecdotal or poorly documented reports in the literature suggest that naturally occurring hypothyroidism in adult horses may be responsible for laminitis, infertility, anhidrosis, alopecia, anemia, myopathy, and osteodystrophy. However, accumulated experience throughout the recent years have indicated that hypothyroidism is not the cause of these disorders in most cases and the low thyroid hormone found in some horses is due to non-thyroidal illness.

3. Clinical Signs

   1. Animals with hypothyroidism and goiter associated with dietary iodine deficiency or defective thyroglobulin synthesis are usually born weak and die within a few hours to a few weeks after birth.
   2. The hair coat varies from short and fuzzy to completely absent.
   3. The skin is often thickened and puffy as a result of myxedema, especially on the head, ear, and extremities.
   4. Horses with congenital hypothyroidism and dysmaturity syndrome are weak and hypothermic at birth and show signs of dysmaturity such as soft and silky hair coat, domed forehead, and floppy ears. Foals are born with various musculoskeletal abnormalities that include mandibular prognathism, flexural limb deformities, rupture of the common digital extensor tendons, incomplete abdominal wall closure, and incomplete closure of the tarsal and carpal cuboidal bones.
   5. Merino sheep with hereditary hypothyroidism may develop lustrous, fine, straight (silky) wool.
   6. Saanen-Dwarf crossbred goats with hereditary goiter exhibit retarded growth, decreased ruminations with a tendency for recurrent bloat, thick and scaly skin, and a sparse hair coat.
   7. In cattle, surgical thyroidectomy resulted in retarded growth, lower weight gain per day, calmer attitude, lower body temperature, heart rate and respiratory rate, decreased ruminal contractions with a tendency for recurrent bloat, thickened, wrinkled, and myxedematous skin, and a hair coat that was longer, duller, and thinner than normal.
   8. In horses, surgical thyroidectomy resulted in retarded growth, sensitivity to cold (shivering), lethargy, decreased rectal temperature, decreased heart rate, delayed epiphyseal plate closure time, delayed eruption of incisor teeth, decreased feed consumption, decreased weight gain, dull and rough hair coat, delayed shedding of winter coat, and edema of the face and distal limbs.

4. Diagnosis

   1. Diagnosis is based on a characteristic history, typical clinical signs, and compatible results of skin biopsy, and TRH or TSH stimulation test.
   2. Basal serum total T4 concentration has to be interpreted with caution if used to diagnose hypothyroidism because in dogs, non-thyroidal illnesses and drugs (especially glucocorticoids, potentiated sulfas and phenylbutazone) can lower basal total T4 concentration.
   3. TSH stimulation test is the gold standard diagnostic test but bovine TSH can be difficult to acquire and the recombinant human TSH (rTSH) has not been validated in animals other than dogs. In addition, it is very expensive.
   4. Horses with congenital hypothyroidism and dysmaturity syndrome may have normal T4 and T3 concentrations but the response to the TSH stimulation test is consistently decreased.
   5. A commonly used TSH protocol in horses is the following (be aware that TSH may only be available for research purposes):
      1. Collect samples for basal T4 determination before and 6 to 12 hours after the intramuscular injection of 5 IU of bovine TSH or,
      2. Collect samples for basal T4 determination before and 4 hours after the intravenous injection of 5 IU of bovine TSH.
      3. The post-TSH T4 concentration in euthyroid horses should be at least double and should fall within or above the normal range for the laboratory assaying the sample.
   6. The TRH response test was reported to be a valuable indicator of thyroid function in horses.
   7. In normal horses, TRH given intravenously at the dose of 0.5 to 1 mg caused maximal elevations in serum T4 and T3 concentrations within 4 to 2 hours, respectively, after injection. The post-TRH T4 and T3 concentrations, typically, at least double at 2 or 4 hours.

5. Treatment

   1. A study suggested that 10 mg of sodium levothyroxine (T4), given orally, once daily was effective therapy in horses.
   2. There is no effective treatment for congenital hypothyroidism and dysmaturity syndrome and most foals die or are euthanized shortly after birth. The ones that survive tend to develop various orthopedic disorders and rarely have a normal athletic career.

References

Bertin F-R, Frank N, Breuhaus BA et al. Diagnosis and management of thyroid disorders and thyroid hormone supplementation in adult horses and foals. Equine Vet J 2023; DOI: 10.1111/evj.13981

Breuhaus BA. Disorders of the equine thyroid gland. Vet Clin North Am Equine Pract  2011; 27: 115-128.

Koikkalainen K, Knuuttila A, Karikoski N et al. Congenital hypothyroidism and dysmaturity syndrome in foals: First reported cases in Europe. Equine Vet Educ  2014; 26: 181-189.

Peterson ME. Hypothyroidism in animals. Merck Vet Manual. Acessed 10/14/2023.

Scott DW.  Endocrine diseases.  In: Large Animal Dermatology. 1st ed., 1988; 374-386.