Effects of propylthiouracil and bromocryptine on serum concentrations of thyrotrophin and thyroid hormones in normal female horses.
Authors: Johnson P J, Messer N T, Ganjam V K, Thompson D L, Refsal K R, Loch W E, Ellersieck M R
Journal: Equine veterinary journal
Summary
# Editorial Summary Distinguishing primary thyroid disease from secondary dysfunction arising at the pituitary or hypothalamic level remains a diagnostic challenge in equine practice, limiting the clinical utility of current thyroid testing protocols. Johnson and colleagues investigated whether propylthiouracil (PTU) and bromocriptine (BROM) could reliably model these distinct pathologies by measuring serum T3, T4, reverse T3, and equine TSH (e-TSH) in healthy mares before, at day 14, and day 28 of treatment, plus their response to thyrotropin-releasing hormone (TRH) stimulation. PTU treatment successfully induced a primary hypothyroid state by day 28, with significant suppression of T3, T4, and rT3 coupled to elevated e-TSH and blunted TRH-stimulated hormone responses; bromocriptine, conversely, produced no measurable effects on any thyroid parameter or TRH responsiveness. These findings establish PTU as a valid experimental model for primary hypothyroidism in horses and suggest that e-TSH assay warrants further clinical investigation for differential diagnosis, though practitioners should note the substantial inter-individual variability in response to oral PTU administration. The marked failure of bromocriptine to induce secondary hypothyroidism indicates that alternative pharmacological approaches or diagnostic protocols are needed to characterise pituitary-origin thyroid dysfunction in horses.
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Practical Takeaways
- •PTU can be used experimentally to model primary hypothyroidism in horses, helping veterinarians understand thyroid gland disorders versus central (pituitary/hypothalamic) dysfunction
- •e-TSH measurement may become a valuable diagnostic tool to differentiate between primary and secondary thyroid disease in clinical practice
- •Bromocriptine is not suitable as an experimental model for secondary hypothyroidism in horses, limiting its utility for research or diagnostic purposes
Key Findings
- •Propylthiouracil (PTU) treatment for 28 days significantly reduced serum T3, T4, and rT3 concentrations while increasing e-TSH in horses (P < 0.05)
- •Bromocriptine (BROM) treatment showed no measurable effect on serum thyroid hormone or e-TSH concentrations
- •PTU treatment decreased the percentage increment of T4, T3, and e-TSH response to TRH stimulation, but BROM did not affect TRH response
- •e-TSH assay shows promise for clinical diagnosis of equine thyroid axis dysfunction and deserves further investigation