The hormone epinephrine binds to β adrenergic receptors on smooth muscle cells of the bronchioles, causing them to dilate (increase in diameter). thyroid hormone is essential for the synthesis of β adrenergic receptors; therefore, in the absence of thyroid hormone, epinephrine cannot trigger bronchodilation. this concept is known as

The answer is permissiveness.

Thyroid hormone exerts a significant effect on the ability of the smooth muscle cells in bronchioles to respond to other hormones, especially catecholamines (including epinephrine). Thyroid hormone binds to the receptors on the surface of the cells and allow more receptors to accept epinephrine, which in turn, allows bronchodilation.