Located in the front of the neck, the thyroid gland produces the hormones thyroxine (T₄) and triiodothyronine (T₃) that regulate the body's metabolic rate by helping to form protein ribonucleic acid (RNA) and increasing oxygen absorption in every cell. In turn, the production of these hormones are controlled by thyroid-stimulating hormone (TSH) that is produced by the pituitary gland. When production of the thyroid hormones increases despite the level of TSH being produced, hyperthyroidism occurs. The excessive amount of thyroid hormones in the blood increases the body's metabolism, creating both mental and physical symptoms.

Curiously, the thyroid gland is often enlarged whether it is making too much hormone, too little, or sometimes even when it is functioning normally. TSH increases the amount of thyroxin secreted by the thyroid and also causes the thyroid gland to grow.

• Hyperthyroid goiter--If the amount of stimulating hormone is excessive, the...