Substrate-controlled hormone is
**Core Concept**
Substrate-controlled hormones are a class of hormones whose production is primarily regulated by the availability of their substrate. This type of regulation is essential for maintaining homeostasis and ensuring that the hormone is produced only when necessary.
**Why the Correct Answer is Right**
Substrate-controlled hormones rely on the availability of a specific substrate to be converted into the active hormone. For example, thyroxine (T4) is converted into triiodothyronine (T3) by the enzyme 5'-deiodinase, which is dependent on the presence of iodine. Similarly, testosterone is converted into estradiol by the enzyme aromatase, which is dependent on the presence of androgen precursors. This type of regulation allows for precise control over hormone production and ensures that the hormone is produced only when its substrate is available.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the type of hormone regulation being referred to. Substrate-controlled hormones are a specific type of hormone regulation, and this option does not provide enough information to determine its validity.
**Option B:** This option is incorrect because it is a type of hormone regulation that is not dependent on the availability of a substrate. Hormones that are regulated by feedback inhibition or negative feedback mechanisms do not rely on the presence of a substrate to be produced.
**Option C:** This option is incorrect because it is a type of hormone regulation that is not directly related to the availability of a substrate. Hormones that are regulated by changes in gene expression or transcription do not rely on the presence of a substrate to be produced.
**Clinical Pearl / High-Yield Fact**
One important clinical application of substrate-controlled hormones is in the treatment of hypothyroidism. In this condition, the thyroid gland is unable to produce enough thyroid hormone due to a lack of iodine. Administering iodine supplements can help to increase the production of thyroid hormone by providing the necessary substrate for its production.
**Correct Answer:** B. Growth hormone is a substrate-controlled hormone, meaning its production is primarily regulated by the availability of growth hormone-releasing hormone (GHRH) and somatostatin.