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4 Principles of Endocrinology 33

While some diagnostic laboratories still use RIA, most be used to measure the hormone in samples from other
VetBooks.ir hormone determinations today are made using reporters animal species. Considerations involved in determining
other than radioactivity. The basic methodology is simi­
the usefulness of such assay methods include the fact
lar to RIA (competitive binding to antibodies) but the tag
significantly across species (e.g., T4 levels are approxi­
employed is nonradioactive, using methods such as that circulating levels of a particular hormone can vary
chemiluminescense or an enzyme label (as in enzyme‐ mately four times higher in humans compared with dogs
linked immunosorbent assay [ELISA]). Another method and cats), protein binding differs across species and
that is particularly well suited to the measurement of other materials in serum or plasma (e.g., lipid content)
protein hormones employs two antibodies, directed can impact results obtained in a particular assay. Protein
against different regions of the molecule. The presence hormones can vary structurally across species and this
of the hormone allows for formation of a “sandwich” may have a profound effect on the usefulness of an assay
complex, which is reported by a tag present on one of the to validate across species. For example, TSH levels
antibodies. cannot be determined in samples from dogs or cats using
Measurement of hormones in companion and domes­ human TSH assay methods. Especially in the case of
tic animals presents some special challenges. Many protein hormones, users should contact the reference
assays are commercially available to measure hormones laboratory to determine if the assay has been validated
in samples from humans. Some, but not all, of these can for use in that species.

Further Reading

Chakravarti B, Chattopadhyay N, Brown EM. Signaling Peterson ME. More than just T₄: diagnostic testing for
through the extracellular calcium‐sensing receptor hyperthyroidism in cats. J Feline Med Surg 2013; 15(9):
(CaSR). Adv Exp Med Biol 2012; 740: 103–42. 765–77.
Dooley R, Harvey BJ, Thomas W. Non‐genomic actions of Rutter GA, Pullen TJ, Hodson DJ, Martinez‐Sanchez A.
aldosterone: from receptors and signals to membrane Pancreatic β‐cell identity, glucose sensing and the
targets. Mol Cell Endocrinol 2012; 350(2): 223–34. control of insulin secretion. Biochem J 2015; 466(2):
Finch NC, Syme HM, Elliott J. Parathyroid hormone concen­ 203–18.
tration in geriatric cats with various degrees of renal
function. J Am Vet Med Assoc 2012; 241(10): 1326–35.