Effects of propylthiouracil and bromocryptine on serum concentrations of thyrotrophin and thyroid hormones in normal female horses.
BACKGROUND
There exists a need for better diagnostic tests to characterise thyroid disease in horses. Currently available diagnostic tests fail to differentiate between thyroid gland disorders and thyroid abnormalities resulting from pituitary or hypothalamic problems.


OBJECTIVE
To evaluate the effects of treatment with propylthiouracil (PTU) and bromocryptine (BROM) on serum concentrations of triiodothyronine (T3), thyroxine (T4), reverse T3 (rT3) and equine thyroid-stimulating hormone (e-TSH, thyrotrophin) in mature horses.


METHODS
Healthy mature horses were treated using either PTU or BROM for 28 days. The effect of treatment on the thyroid axis was assessed by measuring T3, T4, rT3 and e-TSH before and at +14 and +28 days. The effect of PTU and BROM on the response of T3, T4, rT3 and e-TSH to thyrotrophin-release hormone (TRH) administration was also assessed before and at +14 and +28 days of treatment.


RESULTS
Treatment with PTU led to a significant reduction in serum concentrations of T3, T4 and rT3 on Day 28 and increase of e-TSH on Day 28 (P < 0.05). Treatment with BROM did not cause any measurable effect on serum concentrations of T3, T4, rT3 or e-TSH. The percentage increment by which serum concentration of T4, T3 and e-TSH increased following stimulation with TRH was decreased by treatment with PTU for 28 days (P < 0.05) but were not affected by treatment with BROM for 28 days.


CONCLUSIONS
These results suggest that 1) treatment with PTU may be used in horses as a model of primary hypothyroidism; 2) the use of BROM as a model of secondary hypothyroidism in horses is not supported; and 3) e-TSH assay deserves further investigation for the clinical diagnosis of thyroid axis dysfunction in horses.


CONCLUSIONS
Propylthiouracil effectively causes primary hypothyroidism. There is substantial variability between horses with respect to their sensitivity to this substance when administered orally. Further studies pertaining to the characterisation of equine thyroid disorders are warranted and the use of both PTU for the experimental induction of primary hypothyroidism and e-TSH for the diagnostic characterisation of thyroid disorders in horses should be considered.
Johnson
P J
PJ
Department of Veterinary Medicine and Surgery, College of Veterinary Medicine, University of Missouri, Columbia, Missouri 65211, USA.
Messer
N T
NT
Ganjam
V K
VK
Thompson
D L
DL
Refsal
K R
KR
Loch
W E
WE
Ellersieck
M R
MR
D000818:Animals; D013956:Antithyroid Agents / Q000494:pharmacology*; D001971:Bromocriptine / Q000494:pharmacology*; D004195:Disease Models, Animal; D005260:Female; D006727:Hormone Antagonists / Q000494:pharmacology; D006734:Horse Diseases / Q000097:blood / Q000139:chemically induced; D006736:Horses / Q000097:blood*; D007037:Hypothyroidism / Q000097:blood / Q000139:chemically induced / Q000662:veterinary; D011441:Propylthiouracil / Q000494:pharmacology*; D013963:Thyroid Hormones / Q000097:blood*; D013972:Thyrotropin / Q000097:blood* / Q000187:drug effects; D013974:Thyroxine / Q000097:blood / Q000187:drug effects; D014284:Triiodothyronine / Q000097:blood / Q000187:drug effects
D013956:Antithyroid Agents; D006727:Hormone Antagonists; D013963:Thyroid Hormones; D014284:Triiodothyronine; D001971:Bromocriptine; D011441:Propylthiouracil; D013972:Thyrotropin; D013974:Thyroxine