Open AccessThyroid‐Stimulating Hormone in Adult Euthyroid and Hypothyroid Horses
Author(s) -
Breuhaus Babetta A.
Publication year - 2002
Publication title -
journal of veterinary internal medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.356
H-Index - 103
eISSN - 1939-1676
pISSN - 0891-6640
DOI - 10.1111/j.1939-1676.2002.tb01615.x
Subject(s) - propylthiouracil , medicine , endocrinology , euthyroid , triiodothyronine , hormone , thyroid , free thyroxine , thyroid stimulating hormone , antithyroid agent , thyroid function , graves' disease
The purpose of this study was to validate a thyroid‐stimulating hormone (TSH) assay in a model of equine hypothyroidism. Thyrotropin‐releasing hormone (TRH) stimulation tests were performed in 12 healthy adult mares and geldings, aged 4 to greater than 20 years, before and during administration of the antithyroid drug propylthiouracil (PTU) for 6 weeks. Serum concentrations of equine TSH, total and free thyroxine (T 4 ), and total and free triiodothyronine (T 3 ) were measured. Before PTU administration, mean ± standard deviation baseline concentrations of TSH were 0.40 ± 0.29 ng/mL. TSH increased in response to TRH, reaching a peak concentration of 0.78 ± 0.28 ng/mL at 45 minutes. Total and free T 4 increased from 12.9 ± 5.6 nmol/L and 12.2 ± 3.5 pmol/L to 36.8 ± 11.4 nmol/L and 23.1 ± 5.9 pmol/L, respectively, peaking at 4–6 hours. Total and free T 3 increased from 0.99 ± 0.51 nmol/L and 2.07 ± 1.14 pmol/L to 2.23 ± 0.60 nmol/L and 5.78 ± 1.94 pmol/L, respectively, peaking at 2–4 hours. Weekly measurements of baseline TSH and thyroid hormones during PTU administration showed that total and free T 3 concentrations fell abruptly and remained low throughout PTU administration. Total and free T 4 concentrations did not decrease dramatically until weeks 5 and 4 of PTU administration, respectively. A steady increase in TSH concentration occurred throughout PTU administration, with TSH becoming markedly increased by weeks 5 and 6 (1.46 ± 0.94 ng/mL at 6 weeks). During weeks 5 and 6 of PTU administration, TSH response to TRH was exaggerated, and thyroid hormone response was blunted. Results of this study show that measurement of equine TSH in conjunction with thyroid hormone measurement differentiated normal and hypothyroid horses in this model of equine hypothyroidism.