The thyroid gland is the largest endocrine gland in the human body, located in the anterior neck [1]. Its function is regulated by the anterior pituitary through the secretion of thyroid-stimulating hormone (TSH). The hypothalamic hormone that stimulates the secretion of TSH is thyrotrophin-releasing hormone (TRH). Three hormones are produced by the thyroid gland – that is, thyroxin or tetraiodothyronine (T4), triiodothyronine (T3) and calcitonin, which reduces the levels of calcium in blood. The production and secretion of T3 and T4, derived from the amino acid tyrosine, is controlled by TSH.

Infertility is defined as the inability of a couple to conceive after at least 1 year of regular unprotected sexual intercourse. According to the World Health Organization (WHO), infertility is a disease of the reproductive system [1]. In the literature other terms such as subfertility or sterility have been used at times. The latter, however, is no longer used as it implies almost permanent inability of conception, whereas today there are opportunities for treatment in almost all cases of infertility. Infertility is defined as primary when the woman has never been pregnant before and secondary, when the woman has had at least one clinical pregnancy regardless of whether it was full-term.

Up to now, there is no comprehensive pathophysiological explanation for PCOS. Its heterogeneity may reflect multiple mechanisms which in individual women are not necessarily involved simultaneously but help explain the close relation of metabolic–reproductive circuitry. Ovarian androgen excess and anovulation are key features of the disorder.

Folliculogenesis in humans is a long process. It has become clear that there are two main time points in follicular development, the “initial recruitment” that takes place at the stage of primordial follicles and the “cyclic recruitment” at the stage of small antral follicles [1]. In humans, the initial recruitment is gonadotropin independent, while cyclic recruitment is entirely dependent on the action of both follicle-stimulating hormone (FSH) and luteinizing hormone (LH) [2]. Locally produced substances affect the action of gonadotrophins on follicle maturation. The balanced effect of FSH and LH in the normal menstrual cycle ensures the selection of a single follicle.

Scientific research leads to the generation of new knowledge and is either basic or applied. A question is formulated first, followed by the creation of a hypothesis that needs to be tested. Research in obstetrics and gynaecology can explain the physiology of the reproductive system, the physiology of pregnancy and various clinical abnormalities. Clinical research is carried out through clinical trials. Randomized controlled trials are the gold standard and are performed in four phases with the involvement of patients. Systematic reviews and meta-analyses form the basis of evidence-based medicine. Performance of high-quality research should be controlled by the rules and regulations of national research governance policy to ensure patient safety and ethical issues are adhered to. Research misconduct is a serious failure that leads to incorrect results. Researchers must be aware of the moral rules governing research as well as the consequences for themselves and science.