T3 and T4 are two hormones which are created by the thyroid gland in response to the release of thyroid stimulating hormone, commonly known as TSH, from the pituitary. T3/T4 and TSH levels normally control each other in a negative feedback loop; when T3 and T4 are high, production of TSH drops, and when they get low, TSH production increases.
T3 is the active form of the hormone, while T4 is a precursor that is converted to T3 in the liver. Consistently high levels of T3 in the bloodstream create a condition known as hyperthyroidism. Symptoms include excessive sweating, weakness, weight loss, hair loss, nervousness, tremors, heart palpitations, and difficulty in concentration and mental tasks. In advanced cases, muscle weakness and protrusion of the eyeballs, caused by fluid buildup in back of the eyes, may occur. There are also many anecdotal reports of symptoms such as stuttering and various skin ailments.
The commonest cause of elevated T3 levels is Graves disease, in which the thyroid becomes overactive due to an autoimmune condition. Other possible causes are excessive intake of iodine, and thyroid or pituitary tumors.