The hypothalamus mainly inhibits the production and secretion of PRL. Disruption of the pituitary stalk and hypothalamohypophyseal portal vessels can lead to increased PRL levels but decreased levels of ACTH, TSH, FSH, LH, and GH. PRL circulates freely in the blood and has a short half-life of about 20 minutes. Both men and women have similar basal serum concentrations of PRL. The hypothalamus normally inhibits the release of PRL through dopaminergic tracts that secrete dopamine in the median eminence. There is also a prolactin-releasing factor (PRF), although its exact nature is unknown. TRH and hormones in the glucagon family (secretin, glucagon, VIP, and GIP) can stimulate the release of PRL.

PRL is one of the hormones released in response to stress, such as surgery, fear, arousal, and exercise.

Upon completion of this chapter, the student should be able to answer the following questions:

1. Describe the structure of the thyroid gland and its follicles.
2. Explain the synthesis of thyroid hormones within the thyroid gland, including iodine uptake, iodination of tyrosine residues in thyroglobulin by thyroid peroxidase/dual oxidase, and the formation of T4 and T3.
3. Describe the process of endocytosis, which retrieves thyroglobulin from the follicle lumen and produces T3 and T4 for secretion into the blood.
4. Diagram the hypothalamic-pituitary-thyroid axis, showing how TSH regulates thyroid function and how thyroid hormones provide feedback regulation. Provide examples of how central input can alter the axis's set point.
5. Discuss the role of thyroid-binding proteins in the transport and stability of thyroid hormones and the function of peripheral deiodinases in activating T4 to T3 or inactivating it to reverse T3. Contrast the cellular location and function of the D1 and D2 deiodinases.
6. Describe the mechanisms of thyroid hormone action, including the location and nature of the thyroid hormone receptor and its ability to activate or repress target gene transcription.
7. Discuss the actions of thyroid hormone during development, particularly its effects on the central nervous system (CNS) and skeleton, as well as the consequences of severe hypothyroidism.
8. Describe the effects of thyroid hormone on basal metabolic rate and thermogenesis, the cardiovascular system (heart rate, cardiac output, systemic vascular resistance), and other organ systems (skin, skeletal muscle, digestive tract).

The thyroid gland produces the prohormone tetraiodothyronine (T4 or thyroxine) and the active hormone triiodothyronine (T3). Iodine is necessary for the synthesis of T4 and T3 and can be limited in some regions. Peripheral conversion of T4 to T3 also contributes to T3 production. Thyroid hormone primarily acts through a nuclear receptor that regulates gene transcription. T3 is essential for normal brain and bone development and has wide-ranging effects on metabolism and cardiovascular function in adults.

The thyroid gland consists of two lobes located on the anterolateral sides of the trachea.
Production and secretion of PRL are predominantly under inhibitory control by the hypothalamus. Thus disruption of the pituitary stalk and the hypothalamohypophyseal portal vessels (e.g., secondary to surgery or physical trauma) results in an increase in PRL levels but a decrease in ACTH, TSH, FSH, LH, and GH. PRL circulates unbound to serum proteins and thus has a relatively short half-life of about 20 minutes. Normal basal serum concentrations are similar in men and women. Release of PRL is normally under tonic inhibition by the hypothalamus. This is exerted by dopaminergic tracts that secrete dopamine in the median eminence. There is also evidence for existence of a prolactin-releasing factor (PRF). The exact nature of this compound is not known, although many factors, including TRH and hormones in the glucagon family (secretin, glucagon, VIP, and gastric inhibitory polypeptide [GIP]) can stimulate release of PRL.
PRL is one of the many hormones released in response to stress. Surgery, fear, stimuli causing arousal, and exercise are
Upon completion of this chapter the student should be able to answer the following questions: 1.
Describe the anatomy and histology of the thyroid gland, including the structure of the thyroid follicle.
2.
Explain how thyroid hormones are synthesized within the thyroid gland, including the processes of iodine uptake, iodination of tyrosine residues in thyroglobulin by thyroid peroxidase/dual oxidase, and coupling to form T4 and T3.
3.
Describe the process of endocytosis by which thyroglobulin is retrieved from the follicle lumen and processed to yield T3 and T4, which are secreted into the circulation.
4.
Diagram the hypothalamic-pituitary-thyroid axis to show how TSH regulates thyroid function and how thyroid hormones feed back to regulate the axis. List examples of how central input can alter the set point of the axis.
5.
Discuss the role of thyroid-binding proteins in the transport and stability of thyroid hormones, and the role of peripheral deiodinases in the activation of T4 to T3 or inactivation to reverse T3. Contrast the cellular location and function of the D1 and D2 deiodinases.
6.
Describe the mechanisms of thyroid hormone action, including the nature and location of the thyroid hormone receptor and its ability to either repress or activate target gene transcription.
7.
Discuss the actions of thyroid hormone during development, especially on the central nervous system (CNS) and skeleton, including the consequences of severe hypothyroidism.
8.
Describe the effects of thyroid hormone on basal metabolic rate and thermogenesis, on the cardiovascular system (heart rate, cardiac output, systemic vascular resistance), and on other organ systems (skin, skeletal muscle, digestive tract).
he thyroid gland produces the prohormone tetraiodothyronine (T4, also called thyroxine) and the active hormone triiodothyronine (T3). Synthesis of T4 and T3 requires iodine, which can be a limiting factor in some parts of the world. Much of T3 is also made by peripheral conversion of T4 to T3. Thyroid hormone acts primarily through a nuclear receptor that regulates gene transcription. T3 is critical for normal brain and bone development and has broad effects on metabolism and cardiovascular function in adults.
Anatomy and Histology of the Thyroid Gland
The thyroid gland is composed of right and left lobes that sit anterolateral to the trachea (