Prolonged hypothyroidism severely reduces ovarian follicular reserve in adult rats

Abstract

Background: There is substantial evidence both in humans and in animals that a prolonged reduction in plasma thyroid hormone concentration leads to reproductive problems, including disturbed folliculogenesis, impaired ovulation and fertilization rates, miscarriage and pregnancy complications. The objective of the present study is to examine the consequences of chronic hypothyroidism, induced in adulthood, for the size of the ovarian follicle pool. In order to investigate this, adult female rats were provided either a control or an iodide deficient diet in combination with perchlorate supplementation to inhibit iodide uptake by the thyroid. Sixteen weeks later animals were sacrificed. Blood was collected for hormone analyses and ovaries were evaluated histologically.

Results: At the time of sacrifice, plasma thyroid-stimulating hormone concentrations were 20- to 40-fold increased, thyroxine concentrations were negligible while tri-iothyronin concentrations were decreased by 40% in the hypothyroid group, confirming that the animals were hypothyroid. Primordial, primary and preantral follicle numbers were significantly lower in the hypothyroid ovaries compared to the euthyroid controls, while a downward trend in antral follicle and corpora lutea numbers was observed. Surprisingly the percentage of atretic follicles was not significantly different between the two groups, suggesting that the reduced preantral and antral follicle numbers were presumably not the consequence of increased degeneration of these follicle types in the hypothyroid group. Plasma anti-Müllerian hormone (AMH) levels showed a significant correlation with the growing follicle population represented by the total ovarian number of primary, preantral and antral follicles, suggesting that also under hypothyroid conditions AMH can serve as a surrogate marker to assess the growing ovarian follicle population.

Conclusions: The induction of a chronic hypothyroid condition in adult female rats negatively affects the ovarian follicular reserve and the size of the growing follicle population, which may impact fertility.

Keywords: AMH; Follicular atresia; Growing follicle population; Hypothyroidism; Ovary; Primordial follicle reserve.

Fig. 1

Plasma concentrations of thyroid-stimulating hormone (TSH) (a), thyroxine (T₄) (b), and tri-iodothyronine (T₃) (c) were measured in euthyroid control rats (open bars, n = 11) and hypothyroid rats (filled bars, n = 9). Values represent mean ± SEM. ****p < 0.0001; *p < 0.05

Fig. 2

Plasma follicle-stimulating hormone (FSH) (a), luteinizing hormone (LH) (b) and anti-Müllerian hormone (AMH) concentrations (c) in euthyroid control rats (open bars, n = 11) and hypothyroid rats (filled bars, n = 9). Values represent mean ± SEM. *p < 0.05

Fig. 3

Effects of mild hypothyroidism on ovarian follicle numbers in euthyroid control rats (open bars) and hypothyroid rats (filled bars). Total number of ovarian primordial (a), primary (b), preantral (c), antral follicles (d), total ovarian number of corpora lutea (CLs) (e) and percentage of atretic follicles (f). Values represent mean ± SEM; n = 4. *p < 0.05; **p < 0.01

Fig. 4

Correlations between the ovarian follicle numbers and thyroid status: primordial follicle number and TSH concentration (a), T₄ concentration (b) and T₃ concentration (c) respectively; primary + preantral + antral follicle number and TSH concentration (d), T₄ concentration (e) and T₃ concentration (f) respectively

Fig. 5

Effects of mild hypothyroidism on the size of the growing ovarian follicle population (consisting of primary plus preantral and antral follicle numbers) in euthyroid control (open bar) and hypothyroid rats (filled bar) (a), and the correlation between the growing ovarian follicle population and plasma AMH concentration in hypothyroid rats (b). Values represent mean ± SEM; n = 4