|INSULIN||2 - 25 µI.U./mL|
|CORTISOL (RESTING)||20 – 90 ng/mL|
|CORTISOL (POST DEX)||Less than 10 ng/mL|
|CORTISOL RHYTHM||Greater than 30%|
|ACTH||Less than 30 pg/mL Mid Nov - Mid July. Less than 100 pg/ml Mid July to Mid Nov.
|Days Post Ovulation||ng/mL|
|0||Less than 1.0|
|2||1.0 - 3.0|
|5 - 12||4.0 - 10.0|
|15 - 18||3.0 - 10.0|
|21||3.0 - 10.0|
|25 - 30||2.5 - 10.0|
|45||4.0 - 10.0|
|60||4.0 - 15.0|
|80 - 315||5.0 - 10.0|
|316 to term||20.0 - 40.0'|
|Type of Horse||TOTAL T4 (ng/mL)|
|Stallions||7 – 25|
|Horses in training||12 – 25|
|Broodmares, open||12 – 25|
|First Trimester||12 – 25|
|Second Trimester||9 – 25|
|Third Trimester||7 – 25|
|Normal 2-day-old foal||150+|
|Sucklings, in June:||MEAN|
|Less Than One Month||41.5|
|Weanlings, in September||25.0|
|Yearlings, in February||21.0|
When retesting for TOTAL T4, sample ~24 hours post thyroid supplement dose. Retest 7-14 days after starting supplement.
NOTES ON HYPOTHYROIDISM
EXPERTS 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:
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.
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.
|MONTH||SCROTAL WIDTH |
|TOTAL T4 |
A. Total Estrogens levels less than 125 pg/mL are abnormal, at any time of the year. The most advanced cases of testicular degeneration have levels less than 100 pg/mL concomitant with levels of FSH exceeding 25 ng/mL.
B. High levels of LH are also present in many cases of testicular degeneration.
C. Insulin levels exceeding 80 µI.U./mL are present in cases of pituitary adenomas. Cushing’s screen would be suggested here, especially if horse is aged.
D. Low Testosterone levels (<500 pg/mL) and/or low Total Estrogens levels (<150 pg/mL) concomitant with normal FSH and LH levels are often present in stallions with low libido.
|DAYS||PROGESTAGENS (ng/mL)||TOTAL ESTROGENS (pg/mL)||TOTAL T4 (ng/mL)|
|0 days post-ovulation||<1.0||less than 200||12.0-25.0|
|2 days post-ovulation||1.0-3.0||less than 200||12.0-25.0|
|5-90||4.0-10.0|| less than 200||12.0-25.0|
|100-110||4.0-10.0||less than 200||12.0-25.0|
|150-320||4.0-10.0||greater than 1000||7.0-25.0|
|post foaling||<20.0||less than 200||12.0-25.0|
NOTE: Normal Total Estrogens in donkeys are approximately 20% lower than Normal Total Estrogens in horses.