Thyroid function, diseases, and tests in domestic ruminants

Abstract

While thyroid physiology has been studied in domestic ruminants, many uncertainties remain. In fact, this metabolism is rarely assessed in routine veterinary practice, and diseases of the thyroid gland or its metabolism are poorly documented in domestic ruminants. This scoping review aims to summarize current knowledge in anatomy, physiology, diseases, and diagnostic methods related to thyroid function in domestic ruminants. A structured research methodology was followed using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) extension for scoping reviews. Four databases were used: CAB Abstracts, Embase, PubMed and Scopus. Selection and screening process of the identified studies, as well as data extraction, were managed using Covidence software. Finally, 206 studies were included. Most studies involved cattle (<i>n</i> = 104), followed by sheep (<i>n</i> = 65) and goats (<i>n</i> = 28). The main study topic was on thyroid physiology (<i>n</i> = 127), followed by diseases (<i>n</i> = 48), diagnostic methods (<i>n</i> = 22) and histology (<i>n</i> = 9). Although many studies addressed the anatomy and physiology of the thyroid gland, few confirmed the euthyroid status (having a normally functioning thyroid gland) of these animals, warranting cautious interpretation of the results. Hypothyroidism is the most documented thyroid disease in ruminants, predominantly caused by iodine deficiency. The physiology of the thyroid gland has been extensively studied in relation to heat stress, reproduction, animal production and nutrition. However, there is much less literature available on diseases described in domestic ruminants and their diagnostic methods. Diagnostic tools for assessing thyroid metabolism in ruminants include assays for total thyroxine, total triiodothyronine, bovine thyroid stimulating hormone, total serum iodine, milk iodine, urine iodine, and plasmatic inorganic iodine.