Veterinary Physiology-III

RELEASE AND TRANSPORT OF THYROID HORMONES

• Two of the iodinated amino acids T₃ and T₄ are stored extracellularly in the colloid on the thyroglobulin and secreted into the blood stream when required.
• Thyroglobulin enters the thyroid cells by endocytosis.
• Thyroid hormones are excised from their thyroglobulin by protease digestion in lysosomes of thyroid epithelial cells and diffuse into blood.
• In contrast, the two iodotyrosines (MIT and DIT) are deiodinated within the gland by an enzyme 5' monodeiodi­nase. This cycle, which can be regarded as intrathyroidal, reclaims the iodine from tyrosine for use in the manufacture of the more active iodinated thyronines-T₃ and T₄.
• Thyroxine is predominant in all animals, i.e., T₄ is 3 to 4 times more than T₃.
• Most of the circulating T₃ are derived from peripheral deiodination of T₄ by the enzyme 5' monodeiodinase present in all the cells of the body.
• The T4/T3 ratio varies from 15:1 to 44:1 in most species of animals.
• In the blood, the thyroid hormone is transported bound with a serum globulin, the thyroid‑binding globulin (TBG - present in all animals except cat).
• In many species, thyroid hormones are also transported with an albumin, the thy­roid‑binding pre‑albumin (TBPA also termed as transthyrectin – specifically transports T4) and albumin itself is a thyroid hormone carrier;
• Collectively the iodine that is bound to all serum proteins is known as protein‑bound iodine or PBI.
• Transthyrectin transports thyroxine and vitamin A and thyroxine helps to convert carotene to vitamin A in liver cells.
• Less than 1% of T4 and T3 are transported as free hormones in the blood.
• T₄ has 3 to 4 times more affinity for the binding proteins than T₃, whereas T₃ in the free form is considerably more active as a thyroid hormone.
• T₄ may be a prohormone or a source of T₃ and T₃ is the active hormone – T₃ is five times more potent than T₄.
• T₄ has slower onset of action than T₃.
• The half life of T3 is about 1 day and for T4 it is about 6 to 7 days, which is very remarkable since other hormones have a half life in minutes only.