What is the primary function of the thyroid gland?

The primary function of the thyroid gland is to regulate metabolism through hormone production. The thyroid gland produces hormones, primarily thyroxine (T4) and triiodothyronine (T3), which play crucial roles in regulating the body's metabolic rate. These hormones influence how the body converts food into energy, affects heart rate, body temperature, and overall energy levels. An appropriate balance of these hormones is vital for maintaining normal metabolic processes.

The other options refer to different glands and functions within the body: insulin is produced by the pancreas and regulates blood glucose levels; adrenaline, which prepares the body for a 'fight or flight' response, is secreted by the adrenal glands; and the filtering of blood is a primary function of the kidneys, which remove waste products and excess substances.