The cTSH test gives relatively poor predictability for primary hypothyroidism in dogs [~ 70%] vs people [95%], because the dog has another pathway to regulate the pituitary-thyroid-hypothalamic axis via growth hormone. False negatives and false positives (i.e. discordant results) occur in ~ 30% of cases. New research just published has shown that unlike humans where growth hormone has minimal influence on thyroid regulatory control, the dog uses this additional important regulatory pathway along with endogenous TSH.