Background: Although hypothyroidism is easily treated with levothyroxine (LT4) replacement, results from several studies indicate that people receiving LT4 replacement often have under- or over-replaced thyroid function. Poor medication adherence and factors interfering with thyroxine absorption and bioavailability may result in such under-replacement. Using the COM-B (Capability, Motivation, and Opportunity - Behavior) model, a health education intervention using patient decision aid (PDA), was developed for primary care physicians to educate individuals about thyroxine replacement. Besides imparting health literacy, PDAs provide greater comfort of the patients to participate in decision making. Intervention with the PDA aims to improve the medication adherence by increasing patients' knowledge about thyroxine replacement. Specific aims: The primary aim is to determine effectiveness of PDA in improving the knowledge, medication adherence and quality of life of the under-replaced hypothyroid adults in a primary care setting. Secondary aims are to explore their understanding and acceptance on the PDA and to assess their perceived usability of this PDA. Methodology: A randomised controlled trial will be conducted at SingHealth Polyclinics (SHP) using mixed-method study design to determine effectiveness of PDA. Patient participants will be randomly allocated in a 1:1 ratio to either the intervention or control group. A total of 236 (118 in each group) patient participants will be enrolled by simple randomization. Eligible patient must be of age ≥21 years years and have LT4 under-replacement with Thyroid stimulating hormone (TSH) \>3.7 mIU/L within the last 6 months. Relevance/significance of the study: Findings from the study may add evidence to the scientific knowledge of using PDA to improve medication adherence and recommend development of similar interventions for other chronic medical conditions.