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The primary hormones responsible for muscle growth include testosterone, growth hormone, and insulin-like growth factor (IGF). Testosterone is known for its anabolic effects, promoting protein synthesis and muscle hypertrophy. It increases the availability of amino acids, thereby facilitating muscle repair and growth. Growth hormone also plays a significant role in muscle development; it stimulates the synthesis of proteins and the growth of muscle cells. Additionally, insulin-like growth factor, which is stimulated by growth hormone, encourages the growth of muscle tissues and improves recovery following exercise.

When we consider the roles of other hormones mentioned in different choices, cortisol is a catabolic hormone that can actually hinder muscle growth if levels are elevated over prolonged periods. While insulin is important for overall muscle metabolism, its role is distinct and works in conjunction with other growth hormones. Estrogen and progesterone are primarily involved in reproductive functions rather than directly in muscle growth. Thyroxine and adrenaline have metabolic roles but do not directly stimulate muscle hypertrophy in the same way as testosterone, growth hormone, and IGF. Therefore, the combination highlighted is critical for effectively promoting muscle growth.