The hormone that acts directly on the thyroid gland to promote the secretion of thyroid hormones T3 (triiodothyronine) and T4 (thyroxine) is Thyroid-Stimulating Hormone (TSH).

- TSH is produced by the anterior pituitary gland and plays a crucial role in regulating the function of the thyroid gland.
- When TSH binds to its receptors on the thyroid follicular cells, it stimulates the synthesis and release of the thyroid hormones T3 and T4.
- These hormones are essential for regulating metabolism, growth, and development in the body.

- Other hormones listed, such as Adrenocorticotropic Hormone (ACTH), primarily target the adrenal cortex to stimulate the release of cortisol.
- Growth Hormone (GH) influences growth and metabolism but does not act directly on the thyroid gland.
- Prolactin mainly targets the mammary glands for milk production and has no direct role in thyroid hormone secretion.

In summary, the key hormone responsible for controlling thyroid hormone secretion by acting directly on the thyroid gland is TSH.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th Edition, Chapter 74: The Thyroid Gland and Control of Thyroid Hormone Secretion