The Effect of Hypothyroid Function on Canine Behavior

In human medicine, a wide range of behavioral symptoms have been reported in hypothyroid patients. In the early stages of the disease, reduced cognitive function and concentration, together with impaired short-term memory, may be easily confused with attention deficit-hyperactivity disorder (AD/HD) (Hauser et al. 1993). Visual and auditory hallucinations can be mistaken for schizophrenia or psychosis. Fear—ranging from mild anxiety to frank paranoia; mood swings; and aggression have also been reported in hypothyroid patients (Denicoff et al. 1990). We have seen a comparable range of behavioral manifestation in dogs (Canis familiaris), particularly in those whose hypothyroidism has not progressed to the more traditional skin, coat and metabolic changes characteristic of the condition. Thyroid hormones modulate the activity of norepinephrine (Heal and Smith 1988), serotonin (Bauer et al. 2002) and their receptors (Sandrini et al. 1996). In hypothyroid animals, 5-HT turnover increases in the brainstem, while cortical 5-HT concentrations and 5-HT2A receptor density may decrease. Administration of thyroid hormone to hypothyroid animals increases cortical 5-HT concentration and desensitizes autoinhibitory 5-HT1A receptors in the raphe area, thereby disinhibiting cortical and hippocampal 5HT release. There is also evidence that thyroid hormones increase the sensitivity of 5-HT2 receptors (Bauer et al. 2002).