Regulation of Diurnal Cycle is not a function of thyroid hormones.

Thyroid hormones play a crucial role in the regulation of various physiological processes in the body. They are produced by the thyroid gland, which is located in the neck. Thyroid hormones primarily include thyroxine (T4) and triiodothyronine (T3). Here is an explanation of the given options and why option D is the correct answer:

a) Regulation of Basal Metabolic Rate:
Thyroid hormones are known to regulate the basal metabolic rate (BMR) of the body. BMR refers to the minimum amount of energy required to maintain essential bodily functions at rest. Thyroid hormones increase the metabolic rate, enhancing energy expenditure and heat production in the body. This function helps in maintaining body temperature and overall energy balance.

b) Erythropoiesis:
Thyroid hormones also influence the production of red blood cells through a process called erythropoiesis. They stimulate the bone marrow to produce more red blood cells, which are responsible for carrying oxygen throughout the body. This function ensures an adequate oxygen supply to various tissues and organs.

c) Maintenance of Electrolyte Balance:
Thyroid hormones play a role in maintaining electrolyte balance in the body. They regulate the levels of sodium, potassium, calcium, and other electrolytes by influencing their absorption, excretion, and distribution. This function is essential for proper nerve conduction, muscle contraction, and overall cellular function.

d) Regulation of Diurnal Cycle:
The diurnal cycle, also known as the circadian rhythm, is primarily regulated by the suprachiasmatic nucleus (SCN) in the brain. The SCN is responsible for coordinating various physiological and behavioral processes that occur in a 24-hour cycle. While the circadian rhythm can influence the secretion of certain hormones, including cortisol, melatonin, and growth hormone, thyroid hormones do not directly regulate the diurnal cycle. Therefore, option D is the correct answer as it does not represent a function of thyroid hormones.

In conclusion, thyroid hormones regulate the basal metabolic rate, erythropoiesis, and maintenance of electrolyte balance. However, they do not directly regulate the diurnal cycle.