Thyroid dysfunction presents differently depending on whether the gland is underactive or overactive, and symptoms can be subtle, especially in early stages. Hypothyroidism commonly causes fatigue that does not resolve with rest, unexplained weight gain, difficulty losing weight despite effort, cold intolerance, constipation, hair thinning, dry skin, brain fog, slow heart rate, depression, low libido, and menstrual irregularities. Hyperthyroidism more commonly produces anxiety, heart palpitations, heat intolerance, unexplained weight loss, loose stools, insomnia, shakiness, and increased appetite. Because these symptoms overlap with many other conditions, thyroid problems are often missed or misattributed. Functional medicine uses a detailed health history combined with comprehensive lab testing to identify patterns that standard evaluation overlooks.