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
