Adequacy of equine thyroid function testing in mares

Abstract

The gold standard for thyroid function in horses is a thyroid releasing hormone (TRH) response test. We hypothesized that published reference values for thyroxine (T4), triiodothyronine (T3) would apply to mares in an iodine deficient zone. Our objective was to evaluate basal thyroid hormone levels and TRH responses in twenty-seven healthy Quarter Horse type mares fed grass hay, oats, with free access to cobalt iodine salt blocks and mean ± SD age of 9.2 ± 4.4 years, and weight of 527 ± 65.1 kg. Four mares were 10 to 35 days pregnant. Physical examinations were within normal limits including palpation and ultrasound of the thyroid gland. TRH response tests were performed mornings from July to September. A basal jugular blood sample (0 h) was obtained, TRH (1 mg IV, Sigma) was administered, and additional samples obtained at 2 h for T3 and 4 h for T4. Definitions: Fold change (FC) post-TRH level / basal level; Euthyroid basal T4 (12.87-38.6 nmol/L) and T3 (0.41-1.1 nmol/L) within reference ranges of Animal Health Diagnostic Lab, Ithaca, NY and TRH T4 and T3 responses with a FC >2. The Post TRH FC reference ranges for T4 and T3 reported by Breuhaus (2018) were also evaluated (T4 FC 1.3-3.8; T3 FC 1.1-10). Serum was separated and stored at -20C. A single assay was performed to measure T3 and T4 using the Siemen's Immulite method (PDS, Saskatoon, SK). Data were evaluated using Shapiro Wilk, Kruskal Wallis and Spearman Rank Correlation at P < .05. Low basal levels were measured in three mares for T3 (11%), and two for T4 (7.4%). Ten mares had T3 levels with a FC<2 (37%), and two had FC<1.1 (7.4%). Regarding T4 twenty-two mares (81%) had a FC<2 and ten had a FC <1.3 (37%). The FC was below the reference range for both T3 and T4 in two mares. There were significant correlations between basal T3 and T4 (R=0.47 P=.012) and between T3 FC and T4 FC (R=0.64, P=.0001), and no effect of pregnancy. We concluded that basal T3 and T4 values were too variable to assess thyroid function, as mares with low basal values usually had a FC>2. Using FC >2 as an adequate TRH response is problematic as a FC<2 was identified in 37% of T3 and 81% of T4 mares. There were 7.4% and 37% of mares below the low end of the FC reference for T3 and T4 respectively. These low values may indicate that the mares were hypothyroid, which is often due to underlying iodine insufficiency. Iodine status and its relationship to the thyroid response to TRH should be further evaluated due to iodine's central role in equine health.