What are thyroid hormones?

Glands such as the thyroid secrete chemical messengers (hormones) into the bloodstream. These messengers circulate to specific target organs and induce a response. Such messengers produced and secreted by the thyroid gland are called as thyroid hormones. 

Levothyroxine or L-thyroxine is one such hormone and this hormone contains four iodine molecules leading to the name T4. The thyroid also produces triiodothyronine, T3, which is identical to thyroxine except that instead of four iodine molecules T3 contains only three iodine molecules and thus leading to the name T3. 

Thyroid hormones are the same regardless of the species; T4 is the same in frog, dog, pig, sheep, or cow as in humans. In man, eighty to ninety percent of the thyroid hormone produced and released is T4; the reminder is T3. 

The thyroid also secretes another hormone, calcitonin, in very low amounts, this is produced from the para-follicular (C-Cells) of the thyroid gland. The main function of "Calcitonin" is that it acts to reduce blood calcium (Ca²⁺), opposing the effects of parathyroid hormone (PTH) which is secreted from the four para-thyroid glands behind the thyroid gland.

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What is Thyroid Binding Protein (TBP)?

Thyroid hormones circulate bound to binding proteins (TBP) which lead act as a reservoir and lead to stable levels of thyroxine (T4). 

Facts about thyroid hormone values: 

• 99.98% of T4 is bound to protein in circulation.
• 99.8% of T3 is bound to protein in circulation.
• The half life of T4 is 7-8 days, it means it takes this length of time for the levels of T4 to fall from its original value to one-half of its basal value.
• The half life of T3 is about 1 day. 
• Circulating levels of T4 are 10 times higher than that of T3.
• Unbound T4 is called Free T4 (FT4) and is about 0.02% of T4.
• Unbound T3 is called Free T3 (FT3) and is about 0.2% of T3.
• Only the free hormones are metabolically active and free to bind to target tissue.
• The TBP acts as a equilibrium control system. 
• Normal levels of Serum T4 (Bound T4) range from 5ug/dL to 12ug/dL.
• The reference levels depends on the type of assay but in general can be estimates as - Free T4 (unbound T4) is about 1000 times less than the Bound (T4).

• Since it would take this long for the levels to fall, a change in dose of oral thyroxine dosage in patients ( thyronorm or eltroxine) would minimum require 4-6 weeks to reach a stable dose and therefore is required only after that. This is the main reason why - thyroid levels are tested only after 6 weeks of initiating a dose for evaluation of the doses effects in the patient.    

(note: this article should not be read in isolation and if more understanding about the thyroid function and the hormone production is required please go through the other articles in this blog. Dr Ajith Joy k)