Anxiety or Stress: Mild tachycardia can often be attributed to anxiety or stress, which are common and can cause an increase in heart rate without any underlying severe medical condition. The normal CBC and TSH help to rule out other potential causes such as anemia or thyroid disorders.

Dehydration: Even mild dehydration can lead to tachycardia as the body tries to maintain blood pressure and perfusion of vital organs. This is a common and easily treatable condition.
Caffeine or Nicotine Use: Consumption of caffeine or nicotine can lead to increased heart rate. Given their widespread use, these substances are a common cause of mild tachycardia.
Physical Conditioning or Recent Exercise: Athletes or individuals who have recently exercised may exhibit a resting tachycardia due to their body's adaptation to increased physical demands.

Pregnancy: In pregnant women, mild tachycardia can be a normal finding due to increased blood volume and cardiac output. However, it's crucial to consider pregnancy as it has significant implications for management and treatment.
Cardiac Conditions (e.g., Mitral Valve Prolapse, Cardiomyopathy): Although less common, certain cardiac conditions can present with asymptomatic mild tachycardia. Early detection is critical for appropriate management and prevention of complications.
Electrolyte Imbalance (e.g., Hypokalemia, Hypomagnesemia): Abnormal levels of potassium or magnesium can affect heart rhythm, leading to tachycardia. These conditions require prompt identification and treatment to prevent serious cardiac arrhythmias.

Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodic or sustained hypertension and tachycardia due to excessive catecholamine production.
Thyroid Storm (with Normal TSH due to Laboratory Error or Pituitary Issue): Although TSH is normal, a thyroid storm, which is a life-threatening condition, could be considered if there were other suggestive symptoms or signs, and the TSH result was questionable.
Autonomic Dysfunction: Certain conditions affecting the autonomic nervous system can lead to abnormal heart rate regulation, resulting in tachycardia. These conditions are less common and might require specialized testing for diagnosis.