What is the connection between thyroid functionand fertility problems? Once again, there is no simple answer to this frequently asked question. In principle, there is little, if any, direct connection between female infertility and hypothyroidism(usually Hashimoto’s thyroiditis), reflected in abnormally high TSH levels, or hyperthyroidism(Grave’s disease), characterized by abnormally low TSH levels. Yet the thyroid gland must be carefully “watched” in infertile women anyhow.
There are quite a number of reasons for this conclusion. First of all, thyroid disease represents the most frequent autoimmune disease in women of reproductive age. Overt hypothyroidismhas been reported to occur in this age group in ca. 0.5% of women. Subclinical hypothyroidism(TSH > 4.5 mU/L), however, is much more prevalent at 2-4% in the general population but at ca. 25% in women with infertility. Under currently widely accepted standards of care, it must be treated in women trying to conceive or already pregnant in attempts to maintain TSH levels < 2.5 mU/L. In such patients, treatment is recommended not in order to improve fertility but to prevent adverse effects on IQ in offspring. The prevalence of hyperthyroidism in women with infertility is only ca. 10% of that of hypothyroidism (ca. 2.3%). Treatment is always recommended for general medical reasons, not to improve fertility chances.
One, therefore, can in principle state that how well or poorly the thyroid gland functions does not appear to affect female fertility, per se. Yet, the thyroid canaffect female fertility indirectly: For example, hypothyroidism can lead to hyperprolactinemia(excess production of prolactin by the pituitary gland) which, in turn, can inhibit ovulation.
At least one study in the literature reported decreased fertilization rates in IVF in women with elevated TSH levels. Presence of thyroid hormone receptors on granulosa and cumulus cells also support a potential physiological function of TSH on follicles, though no such function has yet been defined. What further complicates the picture is that thyroid disease, at least theoretically, can affect female fertility not only via the TSH function. As an autoimmune disease, thyroid disease is characterized by presence of three distinct autoantibodies, anti-thyroid peroxidase antibody (TPO Ab),anti-thyroglobulin antibody (TG Ab), both associated with hypothyroidism, and anti-thyroid receptor antibody (TR Ab), associated with hyperthyroidism.
Though detection of these autoantibodies has not been directly associated with female infertility, especially TPO Abs have been statistically associated with increased miscarriage risks. How much autoimmunity and how much thyroid function, individually, contribute to this risk has remained quite controversial. One study that at the time received considerable attention claimed that this miscarriage risk could be, simply, reversed by reducing TSH levels to below 2.5 mU/L. Other investigators have, however, not been able to confirm these results.