NOTES ON EQUINE HYPOTHYROIDISM
EXCERTS FROM: BEECH, J. 1987. EVALUATION OF THYROID, ADRENAL AND PITUITARY FUNCTION. VET CLIN. N. AMER. EQUINE PRACTICE.
Currently, the most reliable parameter of equine thyroid function is total T4. Measurement of total T3 is less reliable because it is affected by changes in binding sites and has less binding affinity. Various drugs such as Anabolic Steroids, Glucocorticoids and Phenylbutazone may compete for binding sites on thyroglobulin and decrease total T4 levels. These drugs must be given at high doses for prolonged periods to have an effect on total T4. Bute at a dose of 2.0 grams BID, intravenously, for 5 days is necessary to reduce total T4, however, levels did not go down to hypothyroid range.
Thyroid tests would be indicated when the following occurs:
~ Poor performance in horses with no apparent physical,
> psychological or systemic abnormality as the cause
~ Horses with idiopathic myopathy or weakness or both
~ Young animals failing to grow, having a decreased appetite,
> an abnormal hair coat and being unusually docile with no
> evidence of infection, cardiovascular defects or other systemic
> cause for the clinical symptoms
~ Mature animals that are dull and have poor hair coats with no
> evidence of other disease
~ Neonatal foals that are idiopathically weak or dull or have
> congenital malformations or neuromuscular incompetence
~ Foals with limb contractures or deviations or both
~ Any animal with an enlarged thyroid gland
~ Any animal that seems abnormally hyper excitable, loses
> conditions and has very prominent eyes
~ Any horse showing unexplained impaired temperature regulation > and increased cold susceptibility
~ Agalactic mares
BET Suggests Testing: All horses in training, all maiden and barren mares, all weanlings and all yearlings
Primary Hypothyroidism: Thyroid is not responding to TSH
Secondary Hypothyroidism: Pituitary does not release TSH when TRF is secreted by hypothalamus
Thyroid neoplasms are usually seen in older horses.
Benign adenomas are most common and may be more prevalent in mares with endemic goiter. Adenomas are usually of no apparent clinical endocrinologic significance.
HYPOTHYROIDISM DOCUMENTED IN FOALS
Signs are variable depending on stage of development at which hypothyroidism occurs.
Postnatal: Limb deformities in foals
Prenatal: Feeding mares excessive kelp may lead to weak foals >>>>>>>>with contracted tendons and long hair.
>>>1. Ruptured common digital extensor tendons
>>>2. Carpal and metacarpophalangeal joint contracture
>>>3. Delayed ossification of tarsal and carpal bones
>>>4. Sometimes mandibular prognathism
Older foals with low serum T3 and T4 and two foals with depressed TSH response and histological changes in the thyroid gland compatible with hypothyroidism also had crushed tarsal bones secondary to delayed ossification.
Agalactic mares have been shown to have lower basal T4 although conversion of T4 to T3 and T4 response to TRH are normal. Circulating T4 is thought to influence milk yield but not the initiation of lactation.
When retesting for T4, sample ~ 24 hours post supplement.
Retest 7 –14 days after starting supplement.