Anxiety or Stress-related Condition: Given the patient's age and the presence of sinus tachycardia without other significant findings on physical examination, chest X-ray, or TSH levels, anxiety or stress could be a common cause of her symptoms, including dyspnea and tachycardia.

Deconditioning or Poor Physical Fitness: A year of noted dyspnea could suggest a lack of physical activity leading to decreased cardiovascular fitness, resulting in symptoms like dyspnea and tachycardia during exertion.
Asthma or Other Obstructive Airway Diseases: Although the chest X-ray is normal, asthma or other obstructive airway diseases could cause dyspnea and potentially lead to sinus tachycardia, especially if the patient is experiencing an exacerbation.
Anemia: Anemia can cause dyspnea and tachycardia due to decreased oxygen delivery to tissues. It's a common condition that could be missed if not specifically tested for.

Pulmonary Embolism: Although less common in teenagers, pulmonary embolism is a life-threatening condition that can present with dyspnea and tachycardia. It's crucial to consider, especially if there's a history of immobilization, surgery, or family history of clotting disorders.
Cardiac Conditions (e.g., Cardiomyopathy, Arrhythmias): Certain cardiac conditions can present with dyspnea and sinus tachycardia. While less common, these are critical diagnoses not to miss due to their potential impact on the patient's life.
Thyroid Storm: Although the TSH is reported as normal, thyroid function tests can sometimes be misleading. Thyroid storm is a life-threatening condition that requires immediate attention.

Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodic hypertension, tachycardia, and dyspnea due to excessive catecholamine release.
Mitral Valve Prolapse: A condition where the two valve flaps of the mitral valve do not close smoothly or evenly, which can lead to symptoms like dyspnea and palpitations, including sinus tachycardia in some cases.
Chronic Fatigue Syndrome: A condition characterized by extreme fatigue that cannot be explained by any underlying medical condition. It might present with dyspnea and tachycardia among other symptoms, although it's more of a diagnosis of exclusion.