Of the following thyroid hormones, which is considered the most biologically active?

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Multiple Choice

Of the following thyroid hormones, which is considered the most biologically active?

Explanation:
The most biologically active thyroid hormone is the free triiodothyronine. Activity depends on the hormone being available to enter cells and bind its receptors. When hormones are bound to transport proteins like thyroxine-binding globulin (TBG), they cannot access tissues, so they don’t activate receptors. Among the active species, T3 is more potent than T4 because it binds thyroid hormone receptors with higher affinity and produces stronger gene regulation. Free T3 is the form that can diffuse into cells and exert these effects, making it the most biologically active. Free T4, while active, is less potent and mainly serves as a reservoir that can be converted to T3 in tissues.

The most biologically active thyroid hormone is the free triiodothyronine. Activity depends on the hormone being available to enter cells and bind its receptors. When hormones are bound to transport proteins like thyroxine-binding globulin (TBG), they cannot access tissues, so they don’t activate receptors. Among the active species, T3 is more potent than T4 because it binds thyroid hormone receptors with higher affinity and produces stronger gene regulation. Free T3 is the form that can diffuse into cells and exert these effects, making it the most biologically active. Free T4, while active, is less potent and mainly serves as a reservoir that can be converted to T3 in tissues.

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