Qualitative and semiquantitative assessment of thyroid hormone binding proteins in greyhounds and other dog breeds

Authors: SHIEL, ROBERT - University College - Ireland; NOLAN, CATHERINE - University College - Ireland; Nally, Jarlath; REFSAL, KENT - Michigan State University; MOONEY, CARMEL - University College - Ireland

Submitted to: Domestic Animal Endocrinology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/17/2021
Publication Date: 3/7/2021
Citation: Shiel, R.E., Nolan, C.M., Nally, J.E., Refsal, K.R., Mooney, C.T. 2021. Qualitative and semiquantitative assessment of thyroid hormone binding proteins in greyhounds and other dog breeds. Domestic Animal Endocrinology. 76:1-8. https://doi.org/10.1016/j.domaniend.2021.106623.
DOI: https://doi.org/10.1016/j.domaniend.2021.106623

Interpretive Summary: Interbreed variation in thyroid hormone concentrations is well-recognized in dogs. In particular, healthy greyhounds, considered as sighthounds, display markedly decreased total thyroxine (T4) concentrations in comparison to most other non-sighthound breeds. The cause of this interbreed variation is unknown, but it has clinical importance because it can lead to an erroneous diagnosis of hypothyroidism in greyhounds. In humans, over 99 percent of circulating T4 is bound to specific proteins: T4-binding globulin (TBG), transthyretin and albumin. In dogs, over 99 percent of T4 is also protein bound and it has been estimated that TBG, transthyretin and albumin bind 12-60 percent, 12-39 percent and 16-40.4 percent of T4, respectively. It has been hypothesized that variations in the sequence or concentration of thyroid hormone binding proteins could be at least partly responsible for the observed differences in total T4 concentration between greyhounds and other non-sighthound breeds. The aim of this study was to determine if differences in the sequence or quantity of the major thyroid hormone binding proteins was responsible for the decreased total T4 concentration observed in greyhounds compared to most other non-sighthound breeds. Results indicate that this is not the case.

Technical Abstract: Total thyroxine (T4) concentrations are lower in healthy greyhounds compared to most other non-sighthound breeds. In humans, variations in the structure or concentration of the major thyroid hormone binding proteins are responsible for most reported differences between total T4 concentrations in healthy individuals from different ethnic groups or other subpopulations. The aim of this study was to determine if such variations are also responsible for the lower total T4 concentrations in greyhounds. The predicted protein sequences of thyroxine binding globulin (TBG), transthyretin and albumin were determined in liver tissue from a euthyroid greyhound with decreased T4 concentration and a Jack Russell terrier using reverse-transcriptase PCR. Sequences were compared to each other and online reference sequences. Serum proteins from 21 greyhounds and 21 non-sighthound dogs were separated by denaturing electrophoresis and immunoblots probed with polyclonal antibodies to human TBG and transthyretin. Reactive bands were quantified by densitometry, expressed relative to the mean of reference samples included in each gel. Serum albumin concentrations were measured using a commercially-available assay. Several SNPs were identified but none was thought likely to explain the lower total T4 concentrations in greyhounds. There was no significant difference between the quantity of any of the binding proteins in serum from greyhounds and non-sighthound dogs. However, total T4 and transthyretin concentrations were highly correlated in the greyhound group (r equals 0.73, p equals 0.0002). Variation in the sequence of thyroid hormone binding proteins is not responsible for low greyhound total T4 concentrations. Further evaluation of the role of transthyretin is warranted.