Differences Between T3 And T4

Thyroid gland of human control most of the metabolism of body. It is an endocrine gland and has follicular and C cells. It is the follicular cells of thyroid glands that produce thyroid hormons i.e thyroxine and tri iodo thyronine. I.e T4 and T3. Both of these hormones are made iodine and amino acid tyrosine. The step of synthesis of these hormones are listed below:

Formation Of Thyroid Hormones:

To have some idea of thyroxine and triiodothyronine, you must know the process of formation of both of these hormones.

1. First of all iodine enter thyroid. This occurs by active transport through Sodium iodide symporter. This is facilitated by TSH (thyroid stimulation hormone) which is released by hypothalamus.
2. After entrapping, iodide is converted into organic iodide by the action of peroxidase.
3. Then tyrosine (which is present in thyroglobulin) is converted to mono and di iodo tyrsoine i.e MIT and DIT. It is called iodination of thyrosine. This reaction is also catalyzed by thyroid peroxidase enzyme.
4. After this two DIT combine to form T4 and one MIT and one DIT combine to form T3.
5. When required, T3 and T4 are broken from thyroglobulin and released into blood by the action of protease enzyme.

So, this was a short introduction about step involved in the formation of T3 and T4. Other difference between both of these are listed below:

Difference Between Triiodothyronine And Thyroxine:

Formation:

T3 is formed by DIT Plus MIT. Whereas T4 (tyroxine) is formed by combination of two DIT.

Onset of Action:

T3 has relatively rapid onset of action while T4 has slow onset of action.

Duration of action:

T3 has short duration of action as compared to t4.

Half life:

T3 has less half life as compared to t4. i.e the half life of t3 is about 1 day and that of t4 is about 7 days.

Potency:

Triiodothyronine is more potent than thyroxine.

Uses:

T3 is used to treat myxoedema coma. While t4 is used to treat both myxoedema coma and regular treatment of myxoedema.

Transformation in target cells:

T4 is converted to T3 in target cells while T3 is not converted.