Thyroid hormone

'Thyroid Hormone' is an umbrella term referring to Triiodothyronine, T₃, and Thyroxine, T₄, two Tyrosine-based hormones produced by the Thyroid gland. Their synthesis involves the iodination of two precursors, monoiodothyronine, T_1, and Diiodothyronine, T_2. Although thyroxine is produced in larger quantities, it must be noted that triiodothyronine is three to eight times more active. Thus, thyroxine is metabolised into triiodothyronine for active use. The thyroid hormones are involved in growth, working synergistically with Growth hormone, in cellular metabolism and in bodily Thermogenisis.

The level of thyroid hormones carried in the blood plasma must be regulated; over-active thyroid tissue leads to an increase in circulating thyroid hormones, termed Hyperthyroidism, the symptoms of which include Goitre. The converse, a deficiency of thyroid hormones, or Hypothyroidism can have serious acute effects as well as chronic effects, the latter most serious when presented congenitally.

Synthesis

Thyroxine and Triiodothyronine are based on the Amino acid Tyrosine and which has been iodinated to varying degrees.

Thyroid tissue is composed of distinct functional units called Follicles. These consist of a central Colloid-containing Lumen surrounded by a thin Epithelium of follicular cells .