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Thyroid hormones, primarily thyroxine (T4) and triiodothyronine (T3), play a crucial role in the regulation of various physiological processes in the body. Bone growth and protein synthesis are significantly influenced by these hormones.

Thyroid hormones promote the activity of osteoblasts, the cells responsible for bone formation, leading to increased bone turnover and growth. They are also vital for normal skeletal development during childhood and adolescence. In adults, adequate levels of thyroid hormones help maintain bone mass and density, preventing conditions like osteoporosis.

In terms of protein synthesis, thyroid hormones facilitate the transcription of certain genes and the translation of mRNA, which are essential processes for the production of proteins. This is crucial for growth, tissue repair, and overall metabolic regulation, as proteins are necessary building blocks for muscles, enzymes, and other vital components in the body.

The other processes mentioned, such as digestion and excretion, cognitive function and memory, and respiration and circulation, are influenced by thyroid hormones but are more indirect or secondary effects compared to the direct role they have on bone growth and protein synthesis. Therefore, selecting the option that includes bone growth and protein synthesis accurately reflects the significant and direct physiological roles of thyroid hormones.