In veterinary literature, data about the interaction between thyroid gland and female reproductive status are lacking. Furthermore, previous studies have often referred to male dogs and few data are available about thyroid influence on different phases of the bitch oestrous cycle.

The aim of this study was to determine the influences of the different phases of oestrous cycle on thyroid hormones in German Shepherd bitches.

Seventeen clinically healthy German Shepherd bitches (9 months to 6 years old) were monitored during a complete oestrus cicle (proestrus, oestrus, diestrus, anestrus); the different phases were determined by vaginal cytology. Two blood samples were collected by cephalic vein in each phase, between 11 am and 2 pm, after 12h–24h fasting. Thyroid hormones (TT4, fT4, TT3, fT3), TSH, progesterone, 17-ß-estradiol, serum triglycerides and cholesterol were assessed in each blood samples. At the beginning and at the end of the study a complete haematological and biochemical evaluation was also performed. Seven bitches were bred during the study, and hormone concentrations were also evaluated during pregnancy.

Data were statistically analysed: correlation test was performed between thyroid and pituitary hormones with age, between the different hormones assessed (thyroid, pituitary and sexual hormones) and between estradiol and cholesterol. One-way ANOVA was used to compare the averages of each parameter (thyroid/pituitary) in different phases of the cycle and in diestrus gravidarum and not gravidarum.

Effect size (Cohen's d) or root mean square standardized effect were calculated to measure the magnitude and strength of the statistically significant research findings.

A significant negative correlation between age and TT4 (r=-0,367; p<0,05), fT4 (r=-0,266; p<0,05), fT3 (r=-0,335; p<0,05) was found.

One way ANOVA showed that TT4 concentration during estrus and diestrus phase, was significantly higher than in proestrus and anestrus phase (p=0,0332).

TSH concentration during diestrus and pregnancy was not significantly different by two tails T-test (p=0.0507), even though a higher TSH concentration during pregnancy was evident.

TT4 (r=0,40289; p<0,001) and fT4 (r=0,260; p=0,0067) were positively and significantly associated with progesterone.

TSH was positively and significantly correlated with 17-ß-estradiol (r=0,2; p=0,0388), while TT4 was negatively and significantly correlated with 17-ß-estradiol (r=-0,3179; p=0,0008).

Cholesterol was negatively and significantly correlated with 17-ß-estradiol (r=-0,3355; p=0,0001) and was significantly higher during diestrus (r=-0,3355; p=0,0001). There was no significant correlation between total triglycerides and 17-ß-estradiol.

Results showed an influence of reproductive status on thyroid function, especially during progesterone-prevalent phase (diestrus) and especially relating to TT4 concentration.

Disclosures

No disclosures to report.