Page 929 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

904 CHAPTER 6

VetBooks.ir may not reflect thyroid status. TSH is not currently an endogenous equine TSH assay is not available
commercially. When evaluating the results of thy-
available commercially but TRH is commonly used
to test for PPID and is readily available from com-
pounding pharmacies. Guidelines for performing roid testing, it is important to know the testing
laboratory’s reference intervals, as they can vary
TSH and TRH stimulation tests in adult horses are a great deal between laboratories. Additionally,
shown in Table 6.1. when choosing a diagnostic laboratory, it is impor-
If single point-in-time measurement of thy- tant to verify that the laboratory has validated its
roid hormones is the only option available for assays and established reference intervals in a pop-
the evaluation of thyroid status, measurement of ulation of normal horses.
free fractions of thyroid hormones (alone or in
conjunction with measurement of total amounts) Management
theoretically would provide more useful informa- Options for thyroid hormone supplementation in
tion than measurement of total amounts of thy- adult hypothyroid horses include thyroxine and T
3
roid hormones alone. However, in all but rare (Table 6.2). While 10 mg thyroxine per adult horse
circumstances, total and free thyroid hormone per day is the usual recommended dose, doses of
values mirror each other and the cost of deter- 20–40 mg thyroxine per horse per day have been
mining free hormone concentrations is often not given short term to euthyroid horses without causing
justified. Serum concentrations of free T mea- detectable adverse clinical effects. However, higher
4,
sured by equilibrium dialysis, are superior to doses may produce agitation and hyperexcitability.
other methods. Measurement of serum TSH con- Monitoring serum thyroid hormone concentrations
centrations in single samples could potentially aid and maintaining them in the normal adult range can
in the diagnosis of thyroid status, but currently assure proper dosing.

Table 6.1 Protocol for thyrotropin-releasing hormone stimulation testing

1 Take a control serum sample
2 Administer 1 mg TRH/400–600 kg horse i/v. Administer 0.5 mg to ponies and foals (TRH obtained from a compounding
pharmacy)
3 Take additional serum samples 1, 2 and 4 hours following administration
4 Submit samples for measurement of free T 3 , total T 3 , free T 4 and total T 4 . If a TSH assay becomes available, samples can also be
submitted for TSH measurement
5 TSH should peak after 1 hour and should increase to 2.5–3 times the baseline level after TRH administration
6 Free and total T 3 should double approximately 2 hours following TRH administration
7 Free and total T 4 should increase by at least 1.7 times the baseline levels at 4–6 hours following TRH administration
8 The time frame for increases in thyroid hormones in response to TRH is similar in foals to that in adults, but the amount of
increase is less. In foals less than 3 days of age, total and free T 4 should increase approximately 10% 4 hours after
administration of 0.5 mg TRH. Free T 4 after equilibrium dialysis should increase by approximately 30%. Total and free T 3 should
increase by 40% and 60%, respectively, 2 hours after TRH administration

Table 6.2 Recommended doses of thyroxine and triiodothyronine for adult and neonatal horses

Adults
• L-thyroxine: 20 μg/kg p/o q24 h (approximately 10 mg/adult horse q24 h)
• Triiodothyronine: 1 mg/horse p/o q24 h
Foals
• L-thyroxine: 20–50 μg/kg p/o q24 h. Monitor blood T 4 to avoid overdosing
• Triiodothyronine: 1 μg/kg p/o q24 h