The thyroid gland affects many body systems and physiological processes, including fertility and reproduction. Disorders of the thyroid gland, such as hypothyroidism and hyperthyroidism, can affect fertility, miscarriage and pregnancy outcomes.
Subfertility is a term used to describe a state of reduced fertility; where it takes a longer time than expected to conceive. Subfertility affects around 1-5% of couples globally. Associations have been drawn between thyroid disorders and subfertility. Hyperthyroidism has an incidence of 1.5% amongst women in the general population; however, this rate may be as high as 2.3% in women affected by subfertility. Hyperthyroidism can cause problems with the menstrual cycle, such as menstrual irregularity and heavy bleeding. Despite the effect of hyperthyroidism on fertility, there is little evidence to suggest that treatments for hyperthyroidism improve ovulation.
Hypothyroidism can also affect fertility. This is related to the hormonal control of ovulation. In hypothyroidism, the release of a hormone from the hypothalamus called gonadotrophin releasing hormone (GnRH). This hormone is usually released in bursts. When released in bursts, the GnRH stimulates the release of two hormones from the pituitary gland called follicle-stimulating hormone and luteinising hormone, which help to control ovulation. In hypothyroidism, GnRH is not released in bursts, which can affect the process of ovulation. Additionally, hypothyroid patients have high levels of thyroid stimulating hormone (TSH). High levels of TSH have been associated with disturbances to the menstrual cycle and an increase in the number of menstrual cycles in which ovulation does not occur (anovulatory cycles). There are higher rates of hypothyroidism amongst subfertile women, and also women who present with subfertility tend to have higher blood levels of TSH. Treatment with thyroxine can improve fertility in hypothyroid women.
Several autoimmune diseases have been associated with subfertility, including type 1 diabetes and lupus. Autoimmune thyroid disease and thyroid autoantibodies may also be associated with subfertility, as there is a higher prevalence of thyroid autoantibodies amongst subfertile patients than in the wider community.
Jefferys A, Vanderpump M, Yasmin E. Thyroid dysfunction and reproductive health. The Obstetrician & Gynaecologist, 2015, 17:39-45. http://onlinelibrary.wiley.com