T3 is more active than T4

Explanation:
The thyroid gland produces two main hormones: thyroxine (T4) and triiodothyronine (T3). Both T4 and T3 are iodine-containing hormones that play a crucial role in regulating metabolism, growth, and development in the body.

Differences between T3 and T4:
1. Structure: T4 contains four iodine atoms, while T3 contains three iodine atoms.
2. Conversion: T4 is the major hormone secreted by the thyroid gland, and it is converted into T3 in various tissues, including the liver, kidney, and skeletal muscle. This conversion is catalyzed by enzymes called deiodinases.
3. Activity: Despite being less in quantity, T3 is more active than T4. T3 has a faster onset and a shorter duration of action compared to T4.
4. Receptor binding: T3 has a higher affinity for thyroid hormone receptors (TRs) compared to T4. This means that T3 binds more tightly to TRs and has a stronger effect on target cells.
5. Biological effects: T3 is responsible for the majority of the biological effects of thyroid hormones, including increasing basal metabolic rate, regulating body temperature, promoting growth and development, and maintaining cardiovascular function.

Importance of T4:
Although T3 is more active, T4 also plays a crucial role. T4 serves as a reservoir for T3, as it is converted into T3 as needed. This conversion allows for a more precise and regulated control of thyroid hormone levels in the body. T4 also has some direct effects on target tissues, although its potency is lower compared to T3.

Clinical significance:
In some medical conditions, there may be an imbalance in T3 and T4 levels. For example, hyperthyroidism is a condition characterized by excessive production of thyroid hormones, leading to high levels of T3 and T4. On the other hand, hypothyroidism is a condition characterized by low levels of thyroid hormones, resulting in decreased T3 and T4. These imbalances can lead to various symptoms and require appropriate medical management.

In conclusion, T3 is the more active hormone compared to T4. T3 has a higher affinity for thyroid hormone receptors and exerts stronger biological effects on target cells. However, T4 serves as a precursor for T3 and plays an important role in maintaining thyroid hormone balance in the body.