MCAT Biology : Thyroid and Parathyroid

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #631 : Systems Biology And Tissue Types

The thyroid is responsible for making which of the following hormones?

Possible Answers:

Calcitonin

Parathyroid hormone (PTH)

Thyroid-stimulating hormone (TSH)

Aldosterone

Correct answer:

Calcitonin

Explanation:

The thyroid gland is responsible for helping to regulate basal metabolism and calcium levels. The thyroid gland secretes calcitonin, synthesized from C-cells, to reduce free calcium levels in the blood. Calcitonin antagonizes parathyroid hormone (PTH), which serves to increase the free calcium levels in the blood.

Parathyroid hormone is released from the parathyroid gland. Thyroid-stimulating hormone is released from the anterior pituitary. Aldosterone is released from the adrenal cortex.

Example Question #632 : Systems Biology And Tissue Types

What would be the result of increased levels of parathyroid hormone (PTH)?

Possible Answers:

Decreased blood potassium

Increased blood calcium

Increased blood potassium

Decreased blood calcium

Correct answer:

Increased blood calcium

Explanation:

Parathyroid hormone is secreted by the parathyroid glands, and acts in a few ways to directly and indirectly increase blood calcium levels. Parathyroid hormone increases calcium reabsorption in the collecting tubule of the kidney, increases calcium absorption in the gut, and increases bone resorption to free stored calcium.

Parathyroid hormone and calcitonin act in a negative feedback loop to maintain calcium levels. Calcitonin is released from the thyroid, and serves to decrease blood calcium.

Maintaining blood calcium levels is particularly important because heart attacks can become common in low calcium settings, as calcium is crucial to maintaining cardiac muscle contractions.

Example Question #1 : Thyroid And Parathyroid

A physician diagnoses a patient with iodine deficiency. Which of the following observations most likely helped the physician with this diagnosis?

Possible Answers:

An abundance of glucose in the blood

Decreased production of functional thyroid-stimulating hormone

Decreased production of functional thyroxine

Both a decrease in thyroid-stimulating hormone and thyroxine

Correct answer:

Decreased production of functional thyroxine

Explanation:

Iodine is an element that plays a key role in the function of thyroxine (T4), a thyroid hormone. There are two main types of thyroid hormones: T3 and T4 (or thyroxine). Both hormones contain iodine atoms (T3 contains three and T4 contains four) and both are essential in order to maintain basal metabolic rate. Since the doctor observed a decrease in iodine concentration, the patient must experience a decrease in the production of functional thyroxine. 

Glucose concentration does not depend on iodine; therefore, an increase or decrease in glucose concentration in blood is irrelevant to this question. Thyroid-stimulating hormone (TSH) is a peptide hormone released by the anterior pituitary gland. Its main function is to stimulate the thyroid gland to release T3 and T4. Iodine atoms are not necessary for the proper functioning and production of TSH. Iodine deficiency will cause TSH to stimulate the release of dysfunctional T3 and T4 hormones. 

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