Loss of sense of smell in adult, hypothyroid mice

Abstract

Adult hypothyroid humans can lose their sense of smell. The present study was designed to investigate whether anosmia follows hypothyroidism in mice. If so, this would provide an animal model in which to study the mechanism for this effect. Adult mice were made hypothyroid with propylthiouracil (PTU) in their drinking water. Their sense of smell was tested before and after treatment by measuring the amount of time they spent sniffing food vs water odours, after a 24-h fast. Thyroid function was assessed histologically or by radioimmunoassay of blood serum for free thyroxine or free triiodothyronine. In Expt. I mice treated for 50 days with PTU were hypothyroid and anosmie. Control, euthyroid mice maintained their sense of smell, as did PTU-treated mice which also received daily thyroxine injections. PTU-induced anosmia was reversible: 50 days after removal of treatment previously anosmic mice were euthyroid and had regained their sense of smell (Expt. 3). It was possible that hypothyroidism induced non-specific effects which indirectly affected the olfactory function tests. However, short-term treatment with PTU caused severe hypothyroidism but no anosmia (Expt. 2). Additionally, non-specific effects of hypothyroidism were examined in open-field activity tests after short- and long-term hypothyroidism: euthyroid and hypothyroid animals were similarly active (Expts. 1 and 2). We conclude that chronic hypothyroidism produces anosmia in mice, as it does in humans. This anosmia is prevented by daily injections of thyroxine, and the sense of smell can recover to normal once thyroid function is restored.