NCLEX: Pituitary and Thyroid

The neuroendocrine system, which is controlled by the pituitary and hypothalamus, coordinates body functions by transmitting messages between individual cells and tissues. This contrasts with the nervous system, which communicates locally through electrical impulses and neurotransmitters directed through neurons to other neurons or to specific target organs, such as muscle or glands. Nerve impulses generally act within milliseconds. The endocrine system releases hormones into the bloodstream, which carries chemical messengers to target cells throughout the body.

Hormones have a much broader range of response time than do nerve impulses, requiring from seconds to days, or longer, to cause a response that may last for weeks or months. The two regulatory systems are closely interrelated. For example, in several instances, the release of hormones is stimulated or inhibited by the nervous system, and some hormones can stimulate or inhibit nerve impulses. Chapters 25 to 27 focus on drugs that affect the synthesis and/or secretion of specific hormones and their actions. In this chapter, the central role of the hypothalamic and pituitary hormones in regulating body functions is briefly presented.

Pituitary and Thyroid

 

Pituitary and Thyroid: HYPOTHALAMIC AND ANTERIOR PITUITARY HORMONES

Focus topic: Pituitary and Thyroid

The hormones secreted by the hypothalamus and the pituitary are all peptides or low molecular weight proteins that act by binding to specific receptor sites on their target tissues. The hormones of the anterior pituitary are regulated by neuropeptides that are called either “releasing” or “inhibiting” factors or hormones. These are produced in the hypothalamus, and they reach the pituitary by the hypophyseal portal system.

The interaction of the releasing hormones with their receptors results in the activation of genes that promote the synthesis of protein precursors. The protein precursors then undergo post-translational modification to produce hormones, which are released into the circulation. Each hypothalamic regulatory hormone controls the release of a specific hormone from the anterior pituitary.

Although a number of pituitary hormone preparations are currently used therapeutically for specific hormonal deficiencies, most of these agents have limited therapeutic applications. Hormones of the anterior and posterior pituitary are administered intramuscularly (IM), subcutaneously, or intranasally because their peptidyl nature makes them susceptible to destruction by the proteolytic enzymes of the digestive tract.

 

Pituitary and Thyroid

 

A. Adrenocorticotropic hormone (corticotropin)

Focus topic: Pituitary and Thyroid

Corticotropin-releasing hormone (CRH) is responsible for the synthesis and release of the peptide pro-opiomelanocortin by the pituitary. Adrenocorticotropic hormone (ACTH) or corticotropin [kor-ti-koe-TROE-pin] is a product of the post-translational processing of this precursor polypeptide. [Note: CRH is used diagnostically to differentiate between Cushing syndrome and ectopic ACTH-producing cells.] Normally, ACTH is released from the pituitary in pulses with an overriding diurnal rhythm, with the highest concentration occurring in the early morning and the lowest in the late evening. Stress stimulates its secretion, whereas cortisol acting via negative feedback suppresses its release.

  • Mechanism of action: ACTH binds to receptors on the surface of the adrenal cortex, thereby activating G protein–coupled processes that ultimately stimulate the rate-limiting step in the adrenocorticosteroid synthetic pathway (cholesterol to pregnenolone;.This pathway ends with the synthesis and release of the adrenocorticosteroids and the adrenal androgens.
  • Therapeutic uses: The availability of synthetic adrenocorticosteroids with specific properties has limited the use of corticotropin mainly to serving as a diagnostic tool for differentiating between primary adrenal insufficiency (Addison disease, associated with adrenal atrophy) and secondary adrenal insufficiency (caused by the inadequate secretion of ACTH by the pituitary). Therapeutic corticotropin preparations are extracts from the anterior pituitaries of domestic animals or synthetic human ACTH. The latter, cosyntropin [ko-sin-TROEpin], is preferred for the diagnosis of adrenal insufficiency. ACTH is also used in the treatment of infantile spasm (West syndrome).
  • Adverse effects: Short-term use of ACTH for diagnostic purposes is usually well tolerated. With longer use, toxicities are similar to those of glucocorticoids and include hypertension, peripheral edema, hypokalemia, emotional disturbances, and increased risk of infection.

 

Pituitary and Thyroid

 

B. Growth hormone (somatotropin)

Focus topic: Pituitary and Thyroid

Somatotropin is a large polypeptide that is released by the anterior pituitary in response to growth hormone (GH)-releasing hormone produced by the hypothalamus. Secretion of GH is inhibited by another hypothalamic hormone, somatostatin (see below). GH is released in a pulsatile manner, with the highest levels occurring during sleep. With increasing age, GH secretion decreases, accompanied by a decrease in lean muscle mass. Somatotropin influences a wide variety of biochemical processes (for example, cell proliferation and bone growth are promoted). Synthetic human GH (somatropin [soe-mah-TROE pin]) is produced using recombinant DNA technology.

  • Mechanism of action: Although many physiologic effects of GH are exerted directly at its targets, others are mediated through the somatomedins—insulin-like growth factors 1 and 2 (IGF-1 and IGF-2). [Note: In acromegaly (a syndrome of excess GH due to hormone-secreting tumors), IGF-1 levels are consistently high, reflecting elevated GH.]
  • Therapeutic uses: Somatropin is used in the treatment of GH deficiency or growth failure in children. Somatropin is also indicated for growth failure due to Prader-Willi syndrome, management of AIDS wasting syndrome, and GH replacement in adults with confirmed GH deficiency. [Note: GH administered to adults increases lean body mass, bone density, and skin thickness, whereas adipose tissue is decreased. Many consider GH an “antiaging” hormone. This has led to off-label use of GH by older individuals and by athletes seeking to enhance performance.] Somatropin is administered by subcutaneous or IM injection. Although the half-life of GH is short (approximately 25 minutes), it induces the release of IGF-1 from the liver, which is responsible for subsequent GH-like actions.
  • Adverse effects: Adverse effects of somatropin include pain at the injection site, edema, arthralgias, myalgias, flu-like symptoms, and an increased risk of diabetes. Somatropin should not be used in pediatric patients with closed epiphyses, patients with diabetic retinopathy, or obese patients with Prader-Willi syndrome.

 

Pituitary and Thyroid

 

C. Somatostatin (Growth hormone-inhibiting hormone)

Focus topic: Pituitary and Thyroid

In the pituitary, somatostatin binds to receptors that suppress GH and thyroid-stimulating hormone release. Originally isolated from the hypothalamus, somatostatin is a small polypeptide that is also found in neurons throughout the body as well as in the intestine, stomach, and pancreas. Somatostatin not only inhibits the release of GH but also that of insulin, glucagon, and gastrin. Octreotide [ok-TREE-ohtide] and lanreotide [lan-REE-oh-tide] are synthetic analogs of somatostatin. Their half-lives are longer than that of the natural compound, and depot formulations are available, allowing for administration once every 4 weeks.

They have found use in the treatment of acromegaly and in diarrhea and flushing associated with carcinoid tumors. An intravenous infusion of octreotide is also used for the treatment of bleeding esophageal varices. Adverse effects of octreotide include diarrhea, abdominal pain, flatulence, nausea, and steatorrhea. Gallbladder emptying is delayed, and asymptomatic cholesterol gallstones can occur with long-term treatment. [Note: Acromegaly that is refractory to other modes of therapy may be treated with pegvisomant (peg-VIH-soe-mant), a GH receptor antagonist.]

D. Gonadotropin-releasing hormone

Focus topic: Pituitary and Thyroid

Pulsatile secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus is essential for the release of the gonadotropins folliclestimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary. However, continuous administration of GnRH inhibits gonadotropin release through down-regulation of the GnRH receptors on the pituitary. Continuous administration of synthetic GnRH analogs, such as leuprolide [loo-PROE-lide], goserelin [GOE-se-rel-in], nafarelin [NAFF-a-rel-in], and histrelin [his-TREL-in], is effective in suppressing production of the gonadotropins.

[Note: Several of these agents are available as implantable formulations that provide convenient continuous delivery of the drug.] Suppression of gonadotropins, in turn, leads to reduced production of gonadal steroid hormones (androgens and estrogens). Thus, these agents are effective in the treatment of prostate cancer, endometriosis, and precocious puberty. In women, the GnRH analogs may cause hot flushes and sweating, as well as diminished libido, depression, and ovarian cysts. They are contraindicated in pregnancy and breast-feeding. In men, they initially cause a rise in testosterone that can result in bone pain. Hot flushes, edema, gynecomastia, and diminished libido may also occur.

 

Pituitary and Thyroid

 

E. Gonadotropins

Focus topic: Pituitary and Thyroid

The gonadotropins (FSH and LH) are glycoproteins that are produced in the anterior pituitary. The regulation of gonadal steroid hormones depends on these agents. They find use in the treatment of infertility. Menotropins [men-oh-TROE-pinz] (also known as human menopausal gonadotropins or hMG) are obtained from the urine of postmenopausal women and contain both FSH and LH. Urofollitropin [yoor-ohfol-li-TROE-pin] is FSH obtained from postmenopausal women and is devoid of LH. Follitropin [fol-ih-TROE-pin] alfa and follitropin beta are human FSH products manufactured using recombinant DNA technology.

Human chorionic gonadotropin (hCG) is a placental hormone that is excreted in the urine of pregnant women. The effects of hCG and choriogonadotropin [kore-ee-oh-goe-NAD-oh-troe-pin] alfa (made using recombinant DNA technology) are essentially identical to those of LH. All of these hormones are injected via the IM or subcutaneous route. Injection of hMG or FSH products over a period of 5 to 12 days causes ovarian follicular growth and maturation, and with subsequent injection of hCG, ovulation occurs. Adverse effects include ovarian enlargement and possible ovarian hyperstimulation syndrome, which may be life threatening. Multiple births are not uncommon.

F. Prolactin

Focus topic: Pituitary and Thyroid

Prolactin is a peptide hormone that is also secreted by the anterior pituitary. Its primary function is to stimulate and maintain lactation. In addition, it decreases sexual drive and reproductive function. Its secretion is inhibited by dopamine acting at D2 receptors. [Note: Drugs that act as dopamine antagonists (for example, metoclopramide and antipsychotics such as risperidone) can increase the secretion of prolactin.]

Hyperprolactinemia, which is associated with galactorrhea and hypogonadism, is treated with D2 receptor agonists, such as bromocriptine and cabergoline. Both of these agents also find use in the treatment of pituitary microadenomas. Bromocriptine is also indicated for the treatment of type 2 diabetes. Among their adverse effects are nausea, headache and, sometimes, psychiatric problems.

Pituitary and Thyroid: HORMONES OF THE POSTERIOR PITUITARY

Focus topic: Pituitary and Thyroid

In contrast to the hormones of the anterior lobe of the pituitary, those of the posterior lobe, vasopressin and oxytocin, are not regulated by releasing hormones. Instead, they are synthesized in the hypothalamus, transported to the posterior pituitary, and released in response to specific physiologic signals, such as high plasma osmolarity or parturition. Both hormones are administered intravenously and have very short half-lives.

A. Oxytocin

Focus topic: Pituitary and Thyroid

Oxytocin [ok-se-TOE-sin] is used in obstetrics to stimulate uterine contraction and induce labor. Oxytocin also causes milk ejection by contracting the myoepithelial cells around the mammary alveoli. Although toxicities are uncommon when the drug is used properly, hypertension, uterine rupture, water retention, and fetal death have been reported. Its antidiuretic and pressor activities are much less pronounced than those of vasopressin.

B. Vasopressin

Focus topic: Pituitary and Thyroid

Vasopressin [vas-oh-PRESS-in] (antidiuretic hormone) is structurally related to oxytocin. Vasopressin has both antidiuretic and vasopressor effects. In the kidney, it binds to the V2 receptor to increase water permeability and reabsorption in the collecting tubules. Thus, the major use of vasopressin is to treat diabetes insipidus. It also finds use in the management of cardiac arrest and in controlling bleeding due to esophageal varices.

Other effects of vasopressin are mediated by the V1 receptor, which is found in liver, vascular smooth muscle (where it causes constriction), and other tissues. The major toxicities of vasopressin are water intoxication and hyponatremia. Abdominal pain, tremor, and vertigo can also occur. Desmopressin [des-moe-PRESS-in], an analog of vasopressin, has minimal activity at the V1 receptor, making it largely free of pressor effects.

This analog is longer acting than vasopressin and is preferred for the treatment of diabetes insipidus and nocturnal enuresis. For these indications, desmopressin may be administered intranasally or orally. [Note: The nasal spray should not be used for enuresis due to reports of seizures in children using this formulation.] Local irritation may occur with the nasal spray.

 

Pituitary and Thyroid

 

Pituitary and Thyroid: THYROID HORMONES

Focus topic: Pituitary and Thyroid

The thyroid gland facilitates normal growth and maturation by maintaining a level of metabolism in the tissues that is optimal for their normal function. The two major thyroid hormones are triiodothyronine (T3; the most active form) and thyroxine (T4). Inadequate secretion of thyroid hormone (hypothyroidism) results in bradycardia, poor resistance to cold, and mental and physical slowing. In children, this can cause mental retardation and dwarfism. In contrast, excess secretion of thyroid hormones (hyperthyroidism) can cause tachycardia and cardiac arrhythmias, body wasting, nervousness, tremor, and heat intolerance.

A. Thyroid hormone synthesis and secretion

Focus topic: Pituitary and Thyroid

The thyroid gland is made up of multiple follicles that consist of a single layer of epithelial cells surrounding a lumen filled with thyroglobulin, which is the storage form of thyroid hormone. Thyroid function is controlled by thyroid-stimulating hormone (TSH; thyrotropin), which is synthesized by the anterior pituitary. [Note: TSH generation is governed by the hypothalamic thyrotropin-releasing hormone (TRH).]

TSH action is mediated by cAMP and leads to stimulation of iodide (I−) uptake by the thyroid gland. Oxidation to iodine (I2) by a peroxidase is followed by iodination of tyrosines on thyroglobulin. [Note: Antibodies to thyroid peroxidase are diagnostic for Hashimoto thyroiditis, a common cause of hypothyroidism.] Condensation of two diiodotyrosine residues gives rise to T4, whereas condensation of a monoiodotyrosine residue with a diiodotyrosine residue generates T3. The hormones are released following proteolytic cleavage of the thyroglobulin.

 

Pituitary and Thyroid

 

B. Mechanism of action

Focus topic: Pituitary and Thyroid

Most of the hormone (T3 and T4) is bound to thyroxine-binding globulin in the plasma. The hormones must dissociate from thyroxine-binding globulin prior to entry into cells. In the cell, T4 is enzymatically deiodinated to T3, which enters the nucleus and attaches to specific receptors. The activation of these receptors promotes the formation of RNAM and subsequent protein synthesis, which is responsible for the effects of T4.

C. Pharmacokinetics

Focus topic: Pituitary and Thyroid

Both T4 and T3 are absorbed after oral administration. Food, calcium preparations, and aluminum-containing antacids can decrease the absorption of T4. Deiodination is the major route of metabolism of T4. T3 also undergoes sequential deiodination. The hormones are also metabolized via conjugation with glucuronides and sulfates and excreted into the bile.

D. Treatment of hypothyroidism

Focus topic: Pituitary and Thyroid

Hypothyroidism usually results from autoimmune destruction of the gland or the peroxidase and is diagnosed by elevated TSH. Levothyroxine (T4) [leh-vo-thye-ROK-sin] is preferred over T3 (liothyronine [lye-oh-THYE-roe-neen]) or T3/T4 combination products (liotrix [LYE-oh-trix]) for the treatment of hypothyroidism. It is better tolerated than T3 preparations and has a longer half-life. Levothyroxine is dosed once daily, and steady state is achieved in 6 to 8 weeks.

Toxicity is directly related to T4 levels and manifests as nervousness, palpitations and tachycardia, heat intolerance, and unexplained weight loss. Drugs that induce the cytochrome P450 enzymes, such as phenytoin, rifampin, and phenobarbital, accelerate metabolism of the thyroid hormones and may decrease the effectiveness.

 

Pituitary and Thyroid

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E. Treatment of hyperthyroidism (thyrotoxicosis)

Focus topic: Pituitary and Thyroid

Graves disease, an autoimmune disease that affects the thyroid, is the most common cause of hyperthyroidism. In these situations, TSH levels are reduced due to negative feedback. [Note: Feedback inhibition of TRH occurs with high levels of circulating thyroid hormone, which, in turn, decreases secretion of TSH.] The goal of therapy is to decrease synthesis and/or release of additional hormone. This can be accomplished by removing part or all of the thyroid gland, by inhibiting synthesis of the hormones, or by blocking release of the hormones from the follicle.

  • Removal of part or all of the thyroid: This can be accomplished either surgically or by destruction of the gland with radioactive iodine (131I), which is selectively taken up by the thyroid follicular cells. Most patients become hypothyroid as a result of this drug and require treatment with levothyroxine.
  • Inhibition of thyroid hormone synthesis: The thioamides, propylthiouracil [proe-pil-thye-oh-YOOR-ah-sil] (PTU) and methimazole [me-THIM-ah-zole], are concentrated in the thyroid, where they inhibit both the oxidative processes required for iodination of tyrosyl groups and the condensation (coupling) of iodotyrosines to form T3 and T4. PTU also blocks the peripheral conversion of T4 to T3. [Note: These drugs have no effect on thyroglobulin already stored in the gland. Therefore, clinical effects of these drugs may be delayed until thyroglobulin stores are depleted.] Methimazole is preferred over PTU because it has a longer half-life, allowing for once-daily dosing, and a lower incidence of adverse effects. However, PTU is recommended during the first trimester of pregnancy due to a greater risk of teratogenic effects with methimazole. PTU has been associated with hepatotoxicity and, rarely, agranulocytosis.
  • Blockade of hormone release: A pharmacologic dose of iodide inhibits the iodination of tyrosines (“Wolff-Chaikoff effect”), but this effect lasts only a few days. More importantly, iodide inhibits the release of thyroid hormones from thyroglobulin by mechanisms not yet understood. Iodide is employed to treat thyroid storm or prior to surgery, because it decreases the vascularity of the thyroid gland. Iodide is not useful for long-term therapy, because the thyroid ceases to respond to the drug after a few weeks. Iodide is administered orally. Adverse effects include sore mouth and throat, swelling of the tongue or larynx, rashes, ulcerations of mucous membranes, and a metallic taste in the mouth.
  • Thyroid storm: Thyroid storm presents with extreme symptoms of hyperthyroidism. The treatment of thyroid storm is the same as that for hyperthyroidism, except that the drugs are given in higher doses and more frequently. β-blockers, such as metoprolol or propranolol, are effective in blunting the widespread sympathetic stimulation that occurs in hyperthyroidism.
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