Background: Functional Thyroid Disorders (FTDs) including hyperthyroidism and hypothyroidism conditions of the thyroid gland can be fatal and pose life-threatening health risks to older adults. The geriatric population tends to be affected with FTDs more than younger populations and manifestations in older persons are subtle due to the ageing process and thus are often ignored making it a challenge to diagnose and treat. In this context, implementing a training program for nurses who care for geriatric patients with FTDs to develop and or enhance their competency is a valuable priority. Objective: To evaluate the effect of implementing a training program on nurses' competency in the care of geriatric patients with FTDs at the outpatient clinic. Setting: The study was conducted at the outpatient clinics at the one-day services Centre which is an extension of the Main University Hospital (El Amiri Hospital). Design and Sample: A quasi-experimental research design was used for the study (one study group and one control group). A convenient sample of sixty nurses was selected and divided equally into the control and the study group. Tools: Two main tools were developed and used to evaluate nurses' competency on geriatric FTDs. Tool one: Nurses' socio-demographic and work-related characteristics questionnaire and tool two: Nurses' competency in caring for geriatric patients with functional thyroid disorders questionnaire which has three parts. Results: There was a statistically significant increase in total mean scores of nurses' competency levels regarding FTDs in the study group Compared to the control group, immediately post-implementation of the intervention (94±19.348, 62±19.66294). Conclusion: The current study reveals the effectiveness of applying a training program on nurses' FTDs competency. Nurses' competency levels in caring for geriatric patients with FTDs improved significantly for the study group after implementation of the proposed training intervention program as compared to before implementing the program and then those of the control group. Recommendations: In-service training programs on FTDs should be offered on a regular basis to the nurses at the outpatient clinic.