Which hormones influence muscle growth and breakdown, and how?

• A. Estrogen inhibits muscle growth; DHEA reduces hypertrophy; IGF-1 reduces protein synthesis; catecholamines degrade proteins.
• B. None of the above.
• C. Testosterone and growth hormone promote hypertrophy by increasing protein synthesis; cortisol promotes proteolysis; thyroid hormones increase metabolic rate; insulin promotes protein synthesis.
• D. Cortisol promotes hypertrophy; testosterone promotes proteolysis; thyroid hormones decrease metabolic rate; insulin inhibits protein synthesis.

Answer: (C)

Muscle mass is governed by the balance between anabolic signals that build protein and catabolic signals that break it down. Testosterone and growth hormone (via IGF-1) drive hypertrophy by increasing protein synthesis, which leads to larger and stronger muscle fibers. Growth hormone promotes IGF-1 production, and IGF-1 directly stimulates muscle protein synthesis and supports satellite cell activity, both of which contribute to muscle growth.

Insulin plays a key anabolic role as well, enhancing amino acid uptake and activating protein-building pathways like mTOR, while also helping to suppress protein breakdown. Cortisol, on the other hand, promotes proteolysis, freeing amino acids for gluconeogenesis during stress or energy deficit, which can oppose muscle growth.

Thyroid hormones increase metabolic rate, raising energy turnover and protein turnover overall; while not directly causing hypertrophy, they create a metabolic environment that can support growth when paired with anabolic signals and adequate nutrition.

This combination—testosterone and growth hormone promoting protein synthesis, cortisol promoting protein breakdown, thyroid hormones elevating metabolic rate, and insulin supporting protein synthesis—best reflects how these hormones influence muscle growth and breakdown. The other statements mischaracterize several hormones (for example, IGF-1 and estrogen’s effects, or the notion that cortisol promotes hypertrophy), so they don’t fit as well.