Hypothyroidism is an endocrine condition characterized by the insufficient production of thyroid hormones by the thyroid gland, a small, butterfly-shaped gland located in the neck. These hormones, thyroxine (T4) and triiodothyronine (T3), play vital roles in regulating the body's metabolism, affecting how it uses energy. Without enough thyroid hormones, many of the body's functions slow down. Symptoms of hypothyroidism include fatigue, weight gain, constipation, dry skin, thinning hair, slowed heart rate, and feeling cold. It can also lead to more serious health issues if left untreated, such as heart problems and infertility. The most common cause worldwide is iodine deficiency, while in iodine-sufficient regions, autoimmune thyroiditis (Hashimoto's disease) prevails. Treatment involves lifelong thyroid hormone replacement, most commonly with synthetic thyroxine (levothyroxine). The interplay between thyroid function and testosterone is significant, as thyroid hormones influence the metabolism of sex hormones. Hypothyroidism can lead to alterations in sex hormone-binding globulin (SHBG) levels, affecting testosterone's availability. For men on testosterone replacement therapy (TRT), addressing any underlying thyroid issues is crucial to achieving hormonal balance and maximizing the efficacy of TRT.