The thyroid gland is a butterfly-shaped organ located in the neck anterior to the trachea. Its hormones regulate basal metabolism, oxygen use, nutrient metabolism, the production of ATP, and calcium homeostasis. They also contribute to protein synthesis and the normal growth and development of body tissues, including maturation of the nervous system, and they increase the body’s sensitivity to catecholamines. The thyroid hormones triiodothyronine (T3) and thyroxine (T4) are produced and secreted by the thyroid gland in response to thyroid-stimulating hormone (TSH) from the anterior pituitary. Synthesis of the amino acid–derived T3 and T4 hormones requires iodine. Insufficient amounts of iodine in the diet can lead to goiter, cretinism, and many other disorders.
calcitonin
peptide hormone produced and secreted by the parafollicular cells (C cells) of the thyroid gland that functions to decrease blood calcium levels
colloid
viscous fluid in the central cavity of thyroid follicles, containing the glycoprotein thyroglobulin
goiter
enlargement of the thyroid gland either as a result of iodine deficiency or hyperthyroidism
hyperthyroidism
clinically abnormal, elevated level of thyroid hormone in the blood; characterized by an increased metabolic rate, excess body heat, sweating, diarrhea, weight loss, and increased heart rate
hypothyroidism
clinically abnormal, low level of thyroid hormone in the blood; characterized by low metabolic rate, weight gain, cold extremities, constipation, and reduced mental activity
neonatal hypothyroidism
condition characterized by cognitive deficits, short stature, and other signs and symptoms in people born to women who were iodine-deficient during pregnancy
thyroid gland
large endocrine gland responsible for the synthesis of thyroid hormones
thyroxine
(also, tetraiodothyronine, T4) amino acid–derived thyroid hormone that is more abundant but less potent than T3 and often converted to T3 by target cells
triiodothyronine
(also, T3) amino acid–derived thyroid hormone that is less abundant but more potent than T4
1. Which of the following statements about the thyroid gland is true?
A) It is located anterior to the trachea and inferior to the larynx.
B) The parathyroid glands are embedded within it.
C) It manufactures three hormones.
D) all of the above
D
2. The secretion of thyroid hormones is controlled by ________.
A) TSH from the hypothalamus
B) TSH from the anterior pituitary
C) thyroxine from the anterior pituitary
D) thyroglobulin from the thyroid’s parafollicular cells
B
3. The development of a goiter indicates that ________.
A) the anterior pituitary is abnormally enlarged
B) there is hypertrophy of the thyroid’s follicle cells
C) there is an excessive accumulation of colloid in the thyroid follicles
D) the anterior pituitary is secreting excessive growth hormone
C
4. Iodide ions cross from the bloodstream into follicle cells via ________.
A) simple diffusion
B) facilitated diffusion
C) active transport
D) osmosis
C
1. Explain why maternal iodine deficiency might lead to neurological impairment in the fetus.
Iodine deficiency in a pregnant woman would also deprive the fetus. Iodine is required for the synthesis of thyroid hormones, which contribute to fetal growth and development, including maturation of the nervous system. Insufficient amounts would impair these functions.
2. Define hyperthyroidism and explain why one of its symptoms is weight loss.
Hyperthyroidism is an abnormally elevated blood level of thyroid hormones due to an overproduction of T3 and T4. An individual with hyperthyroidism is likely to lose weight because one of the primary roles of thyroid hormones is to increase the body’s basal metabolic rate, increasing the breakdown of nutrients and the production of ATP.