When thyroid hormone levels are below the optimal range, the body's core metabolic processes slow down, which can result in widespread physiological dysfunction and a higher risk of death.

When there is too much thyroid hormone in the body, the rate at which the body uses energy increases, resulting in more frequent bowel movements, loss of body weight, and a higher body temperature.

When thyroid hormone levels are low, the body's energy use slows down, leading to less fat burning, increased weight, constipation, and tiredness.

In adults taking thyroid hormone medication, consistently having too much hormone in the blood—measured by very low TSH or high free thyroxine—is linked to a higher chance of developing heart failure.

Adults on thyroid hormone replacement therapy who consistently have very high TSH levels (above 20 mIU/L) are at significantly higher risk of developing heart failure over time compared to those with normal TSH levels, with the risk increasing substantially after five years.