The thyroid gland is responsible for secreting thyroid hormones (T3 and T4), which are responsible for setting the basal metabolic rate (BMR). The concentrations of these hormones are what tell the cells which metabolic pathways to undergo. Since body heat is a byproduct of metabolism, thyroid hormone also determines body temperature.

The adrenal medulla secretes epinephrine (adrenaline), which is involved in the body's "fight of flight" response. Epinephrine is released in response to direct neural stimulation during periods of short-term stress and acts to stimulate the sympathetic nervous system.

The pituitary gland is known as the "master" endocrine gland because it secretes several hormones that control other endocrine glands. These are known as tropic hormones. Adrenocorticotropic hormone, thyroid-stimulating hormone, and follicle-stimulating hormone are some examples of pituitary tropic hormones.

Steroid hormones are the only ones derived from cholesterol and are always characterized by a four ring molecular structure. Some examples include sex hormones such as androgens and estrogens as well as some adrenal hormones like cortisol (a glucocorticoid) and aldosterone (a mineralocorticoid).  

Increased potassium levels will stimulate aldosterone to increase potassium secretion. The main regulators of aldosterone are potassium and the renin-angiotensin pathway. 

Hormones will be elevated in response to the conditions of the body. In other words, the body's hormones will be elevated in order to provide a negative feedback response. When blood glucose is high, insulin is released to lower these levels to the normal range. In contrast, when blood glucose is low, glucagon is released to raise these levels back to normal.

Thyroxine (T4) is released by the thyroid in order to increase the basal metabolic activity of the body, causing it to produce more ATP energy. Parathyroid hormone is released to stimulate osteoclast activity and raise blood calcium levels when they are too low.

TSH is secreted by the anterior pituitary in response to TRH (thyrotropin-releasing hormone), which is secreted by the hypothalamus. TSH then acts on the thyroid to stimulate the release of thyroid hormone into the blood. Adequate levels of thyroid hormone then cause negative feedback on the hypothalamus and pituitary to decrease the release of TRH and TSH. 

In primary hypothyroidism, the problem lies in the thyroid-- the thyroid is not releasing enough thyroid hormone and it can be due to a variety of causes. What's important here is that there is no negative feedback on the pituitary to decrease its synthesis of TSH and therefore, the TSH levels are higher. It's just not able to function to increase the level of thyroid hormone and bring it to normal. 

Thyrotropin-releasing hormone leads to the stimulation of prolactin at the same time as thyroid-stimulating hormone. Oxytocin and antidiuretic hormone are produced by the hypothalamus and stored and secreted by the posterior pituitary. Somatostatin and growth hormone are not influenced by thyrotropin-releasing hormone. 

Oxytocin and antidiuretic hormone are released from the posterior pituitary. All other choices are released from the anterior pituitary.

Sarcomeres are composed of thick and thin filaments. The thin filament is composed of polymerized actin, while the thick filament is composed of myosin. Titin is a protein that spans the full range of the sarcomere, and is involved in stability and elasticity in the muscle. Collagen is not a primary component of sarcomeres.