What is the principle of the uptake study?

A 30-year-old woman presents with complaints of palpitations, tremulousness and weight loss of around 4 kg (despite a good appetite) over the last 2 months. On examination, she is anxious, has a pulse rate of 100/min, warm and moist peripheries, fine tremors of the outstretched hands and a visible goiter. No obvious abnormalities in the eyes. Her last menstrual period was 15 days ago. Her primary care physician orders for laboratory examination which reveals normal hemogram with an erythrocyte sedimentation rate of 25 mm in the first hour. Thyroid stimulating hormone (TSH) <0.001 mIU/L and total T4 28 mg/dL. A diagnosis of thyrotoxicosis is made. She is started on beta-blockers and she is asked to get a technetium uptake scan and review.

What is the principle of the uptake study?