This study focused on 215 patients possessing thyroid dysfunction and were distributed as follow: 100 patients with hypothyroidism, 100 patients with hyperthyroidism, 15 patients with thyroid cancer, and 20 healthy persons as control. Their ages ranged between (20-70) years. Blood samples of those subjects were collected at AL- Yarmok Teaching Hospital (Baghdad), ALKadhymia Teaching Hospital (Baghdad), St. Rafael Hospital (Baghdad), Marjan General Surgery Hospital (Babylon), AL-Zahra Hospital (Najaf), ALBasra General Hospital (Basra), AI- Ramadi General Hospital (Ramadi), ALShifaa General Hospital (Mousel), and Karkuk General Hospital (Karkuk). The first objective of this research was to study the relationship between gender and thyroid dysfunction, and age at which the illness appears. Results showed that hyperthyroidism is more frequent in male than female, whereas hypothyroidism is significantly higher (p<0.05) in female than in male. The disease symptoms were also age related since most patients with thyroidites were in age of 41 – 50 and 51 – 60 respectively in both genders. The second objective is to study the effect of thyroid dysfunction on fertility. Results showed that there was a significant effect of thyroid disorders on fertility in both genders. In male, seminal fluid analysis showed a significant decrease (P<0.05) in total count when compared with normal, whereas in women progesterone decreased significantly (P<0.05) when compared with normal value. The third objective is to determine the histopathic disorder in thyroid tissue. Histopathological examination of thyroid disorders showed the presence of papillary adenomas in patients with goiter and most of benign tumors appeared in female patients and were less in male. Cancer study showed that cancer cases studied were mostly of papillary carcinoma, whereas medullary carcinoma was less frequent. The fourth objective is the molecular analysis of thyroid dysfunction using RT – PCR performed on blood samples collected from subjects under study. Results showed no gene alteration in patients with hypothyroidism and hyperthyroidism, and illness may be due to physiological changes. In the case of cancer, molecular analysis of papillary carcinoma indicated the presence of point or deletion mutation that altered the normal function of thyroid gene(s), while in the case of medullary carcinoma, the mutation could be of amplification or gene earrangement in thyroid gland.