A wide range of reproductive disorders such as irregular menstruation and frank infertility is found in women with hypothyroidism. Most research done on these patients has focused on steroid and gonadotropin hormone profiles, whilst there has been little work on uterine morphology. Studies on hypothyroid animals have also demonstrated increases in fetal wastage, but there have been few studies of uterine structure in the hypothyroid rat. The present study has used hypothyroid Wistar rats as a model for investigating the effects of hypothyroidism on uterine structure. Three groups of Wistar rats were studied. One was made hypothyroid with methimazole (MMI), the 2nd was also made hypothyroid with methimazole but in addition the rats were simultaneously given daily thyroxine intraperitoneally (MMI + T4), and the 3rd was an untreated euthyroid group (control). Daily vaginal smears were obtained from rats in all 3 groups. All rats were aged 6 wk at the start of treatment and were killed after a further 6 wk. Uterine horns were removed and studied. Systematic random transverse sections were obtained from the proximal, middle, and distal regions of the horn and subjected to morphometric analysis. Difference between regions was assessed using 2-way analysis of variance. Absolute volume of endometrium in the uteri of hypothyroid rats was reduced by 45.1% (P < 0.05), whilst that of the muscle layer was decreased by 33.6% (P < 0.05). The cross-sectional area and absolute volume of the uterine horns were also reduced in hypothyroid animals (P < 0.05). In hypothyroid rats given thyroxine (MMI + T4) all variables increased significantly above those of hypothyroid rats. These changes suggest that hypothyroidism has an effect on uterine structure, which demonstrably improves under exogenous thyroxine administration. The observed structural changes might well play a significant role in the reproductive difficulties observed during hypothyroidism.

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