In human endocrinology, which hormone secreted by the thyroid gland helps stimulate heart beat and increase metabolic rate?

Introduction / Context:
The cardiovascular and endocrine systems are closely linked because many hormones influence heart rate and cardiac output. Thyroid hormones are well known for their role in regulating basal metabolic rate, oxygen consumption, and heart function. Competitive exams often ask which hormone stimulates heart beat or increases heart rate. This question relates specifically to the thyroid hormone that has such an effect in humans.

Given Data / Assumptions:
• The question refers to a hormone that stimulates heart beat.• The options list thyroxine, gastrin, glycogen, dopamine, and oxytocin.• We assume standard textbook functions for each hormone or compound.• The focus is on endocrine control of heart rate and metabolism.

Concept / Approach:
Thyroxine, also called T4, is a major hormone secreted by the thyroid gland. It increases basal metabolic rate, enhances oxygen consumption by tissues, and has a positive chronotropic and inotropic effect on the heart, meaning that it can increase heart rate and force of contraction indirectly. Gastrin acts on the stomach to stimulate gastric juice secretion, glycogen is a storage carbohydrate rather than a hormone, dopamine is mainly a neurotransmitter, and oxytocin acts on the uterus and mammary glands. Therefore the correct choice is thyroxine.

Step-by-Step Solution:
Step 1: Identify that the question wants a hormone that stimulates heart beat and increases metabolism.Step 2: Recall that thyroid hormones, especially thyroxine, increase basal metabolic rate and oxygen consumption.Step 3: Remember that increased metabolic activity leads to higher demand for oxygen and therefore a faster heart rate.Step 4: Review the functions of the other options and see that they do not primarily stimulate heart beat.Step 5: Conclude that thyroxine is the hormone that best matches the description in the question.

Verification / Alternative check:
Clinical observations support this answer. Patients with hyperthyroidism, who have elevated levels of thyroxine, often present with symptoms such as palpitations, tachycardia, weight loss, and heat intolerance. These signs reflect increased metabolic rate and enhanced cardiac activity. Conversely, hypothyroidism can lead to bradycardia and reduced cardiac output. These real world patterns confirm that thyroxine has a stimulating effect on heart function, unlike gastrin or glycogen.

Why Other Options Are Wrong:
Gastrin is a gastrointestinal hormone that stimulates secretion of hydrochloric acid and pepsin in the stomach; it does not primarily regulate heart beat. Glycogen is not a hormone at all, but a polysaccharide storage form of glucose in liver and muscle. Dopamine functions mainly as a neurotransmitter in the brain and in some peripheral tissues; although it can affect blood vessels and heart in pharmacological doses, it is not the classic endocrine hormone asked here. Oxytocin stimulates uterine contraction during childbirth and milk ejection during lactation, and its main actions are related to reproduction, not to regulation of heart rate. Thus, these options do not fit the description in the question.

Common Pitfalls:
Students sometimes confuse the effects of adrenaline and thyroxine, since both can influence heart rate. However, adrenaline is secreted by the adrenal medulla, whereas this question clearly refers to a thyroid hormone. Another pitfall is to choose dopamine because it is known to affect the nervous system and circulation, but its primary textbook classification is as a neurotransmitter, not as the main hormone that stimulates heart beat in normal physiology. Keeping track of which gland secretes which hormone helps avoid these mistakes.

Final Answer:
The hormone that stimulates heart beat and increases metabolic rate is Thyroxine, a thyroid hormone that increases heart rate and metabolism.