27.1 Introduction to the Thyroid and Parathyroid

Thyroid and Parathyroid Glands

The thyroid gland is a butterfly-shaped gland located at the front of the neck below the voice box (see Figure 27.2). This important gland regulates metabolism in the body through the production and secretion of thyroid hormones, which require iodine for synthesis. The thyroid gland produces the active thyroid hormone, triiodothyronine (T3), and the inactive thyroid hormone, thyroxine (T4). Thyroid follicular cells secrete T3 and T4, while parafollicular cells secrete calcitonin cells that aid calcium metabolism. Follicular cells line the follicles, which are small spherical structures in the thyroid gland. They absorb iodine from the blood and combine it with T3 and T4. Parafollicular cells are scattered between the follicles of the thyroid gland. Their main function is to secrete calcitonin for calcium regulation. Calcitonin lowers the levels of calcium in the blood by inhibiting the activity of osteoclasts—cells that break down bone tissue and release calcium into the blood. When calcium levels in the blood rise, the thyroid gland releases calcitonin, which binds to receptors on the surface of osteoclasts and inhibits their activity, reducing the amount of calcium released from the bone into the blood, thereby lowering calcium levels in the blood (McLaughlin & Jialal, 2022).

Figure 27.2 The thyroid and parathyroid play an important role in body functioning and metabolism. (credit: modification of work from Biology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Thyroid hormones affect multiple body systems:

• the cardiac system—they have a positive inotropic effect on cardiac output and stroke volume;
• metabolism—they play an important role through heat production and oxygen consumption;
• the respiratory system—they help by normalizing arterial oxygen;
• the nervous system—they stimulate the peripheral nervous system; and
• the reproductive system—they are necessary for growth and development.

The parathyroid glands are four tiny glands that are located on the upper and lower parts of the sides of the thyroid gland (see Figure 27.2). The parathyroid glands secrete parathyroid hormone (PTH), which along with dihydroxy-vitamin D₃ (vitamin D₃) and calcitonin are responsible for calcium homeostasis within the body. The major target organs for PTH are the kidneys, skeletal system, and intestines. The primary responses to PTH by the kidneys are to increase renal calcium reabsorption and phosphate excretion as well as to enhance the kidney’s ability to activate vitamin D so that more calcium is absorbed through the intestine. In addition, it increases the activity of osteoclasts, thereby increasing the resorption of bone (Lofrese et al., 2022; Khan et al., 2022).

When thyroid hormone levels are insufficient within the body, a series of responses is initiated. The process begins with the hypothalamus releasing thyrotropin-releasing hormone (TRH). This hormone acts as a communicator by interacting with the anterior pituitary gland, a small pea-sized gland located at the brain's base. In response to TRH, the anterior pituitary gland releases thyrotropin, also known as thyroid-stimulating hormone (TSH). TSH stimulates the thyroid gland, prompting it to either increase or decrease the production of both T4 and T3, essential thyroid hormones.

Conversely, in cases where an excess of thyroid hormones is present, a crucial negative feedback system comes into play. Excessive levels of T4 and T3 instruct the anterior pituitary and hypothalamus to restrain the release of their respective hormones. This control mechanism is of paramount importance in maintaining a balanced hormonal environment. The influence of thyroid hormones extends beyond their impact on metabolism and energy regulation. These hormones also interact with the parathyroid glands. The secretion of calcitonin, for instance, directly interacts with calcium ions and affects parathyroid hormones. Calcitonin, functioning as an antagonist to the parathyroid gland, contributes to the intricate balance of calcium in the body. Figure 27.3 illustrates thyroid hormones that control metabolism and parathyroid hormones that control calcium homeostasis.

Figure 27.3 Thyroid hormones control metabolism and impact parathyroid hormones that control calcium homeostasis within the body. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Hypothyroidism

Hypothyroidism arises from an inadequate secretion of T3 and T4 into the bloodstream, prompting the pituitary gland to release TSH. This dynamic is part of the hypothalamus-pituitary axis, in which the hypothalamus triggers the release of TRH to stimulate the anterior pituitary in the secretion of TSH. Hashimoto thyroiditis is the most common cause of hypothyroidism. This condition is caused by an autoimmune-mediated destruction of the thyroid gland, which leads to a dysfunctional thyroid gland and decreased secretion of thyroid hormones. If hypothyroidism is left untreated, the client may develop a myxedema coma, which is a life-threatening disorder that leads to hypothermia, cardiovascular collapse, hypoventilation, hyponatremia, hypoglycemia, lactic acidosis, and coma.

Symptoms of hypothyroidism include:

• Bradycardia
  • Thyroid hormones stimulate the heart, which increases the heart rate. However, in hypothyroidism, the reduced levels of thyroid hormone cause the heart rate to decrease.
• Cold intolerance
  • When thyroid hormones are low, the body’s ability to generate and maintain heat is reduced. This can result in feeling excessively cold, often with a decreased tolerance to cold environments.
• Constipation
  • Thyroid hormones play a role in promoting gastric motility. With hypothyroidism, the decreased levels of thyroid hormones cause slower movement in the digestive tract, resulting in constipation.
• Fatigue and decreased energy
  • Thyroid hormones are responsible for cellular metabolism and energy production. When the thyroid hormone levels are insufficient, it slows the metabolic process, resulting in decreased energy and fatigue.
• Weight gain
  • Thyroid hormones influence the body’s metabolic rate and the breakdown of fat. With reduced levels of thyroid hormones, the metabolic rate slows, resulting in a decreased ability to burn calories and fat.

Treatment for hypothyroidism involves replacing the body’s natural thyroid hormone when the levels are low or absent within the body and will be discussed later in this chapter.

Hyperthyroidism

Hyperthyroidism occurs when the body secretes too much T3 and T4 into the blood. The autoimmune disorder Graves’ disease is the most common cause of hyperthyroidism. In Graves’ disease, the production of TSH receptor antibodies stimulate the thyroid gland to grow and secrete additional thyroid hormone. Thyroid storm (thyrotoxic crisis) is a rare but severe complication of hyperthyroidism, which leads to severe tachycardia, fever, dehydration, heart failure, and coma. It occurs in clients who are inadequately treated for hyperthyroidism or initially after the removal of the thyroid.

Symptoms of hyperthyroidism include:

• Weight loss
  • Increased levels of thyroid hormones can accelerate the body’s metabolic rate, causing an increase in calorie burning. This can lead to unintended weight loss, even with a normal or increased appetite.
• Heat intolerance
  • Thyroid hormones influence thermoregulation and can affect how the body generates and dissipates heat. In hyperthyroidism, the elevated levels of thyroid hormones can lead to an increased sensitivity to heat, resulting in an intolerance to hot temperatures.
• Diarrhea
  • Excessive thyroid hormones speed up gastric motility, leading to frequent bowel movements and diarrhea.
• Fine tremors
  • Increased thyroid hormones cause an increased excitability within the nervous system, leading to tremors, usually in the hands and fingers.
• Tachycardia
  • Thyroid hormones have a stimulatory effect on the heart, thereby increasing the heart rate and the force of contractions.
• Frequent mood changes
  • Thyroid hormones can influence neurotransmitters in the brain, affecting mood regulation. In hyperthyroidism, fluctuations in thyroid hormone levels contribute to mood swings, irritability, and anxiety.
• Muscle weakness
  • Elevated thyroid hormone levels can cause increased breakdown of muscle protein and impair muscle function, which leads to muscle wasting and weakness.

Treatment for hyperthyroidism includes surgery to remove all or part of the thyroid gland and antithyroid drugs, which will be discussed later in this chapter.

Hypoparathyroidism

Hypoparathyroidism occurs when the body does not secrete enough PTH in the blood. This leads to dangerously low levels of calcium in the blood. Because calcium and phosphate have an inverse reaction, the phosphate levels in the body increase. Hypoparathyroidism is typically caused by an autoimmune disorder, can be idiopathic, or is a result of thyroid/parathyroid surgery.

Symptoms of hypoparathyroidism are tetany, contraction of facial muscles after tapping the facial nerve (Chvostek sign), induction of carpal pedal spasm (Trousseau sign), paresthesia, and a prolonged QT wave (total time from ventricular depolarization to complete repolarization) on an electrocardiogram. Treatment for hypoparathyroidism includes intravenous calcium gluconate, oral calcium, and vitamin D supplements, which will be discussed later in this chapter.

Hyperparathyroidism

Hyperparathyroidism occurs when the body secretes too much PTH in the blood, leading to dangerously elevated levels of calcium in the blood. The inverse reaction of calcium and phosphate causes the phosphate levels in the body to decrease. Hyperparathyroidism is typically caused by a lack of response to the normal feedback mechanism of the calcium homeostasis cycle.

Symptoms of hyperparathyroidism include:

• Nausea and vomiting
  • Elevated levels of PTH can disrupt the normal balance of calcium in the body, leading to gastrointestinal symptoms.
• Constipation
  • Excessive PTH slows gastric motility, resulting in constipation.
• Kidney stones
  • Increased levels of calcium in the blood due to hyperparathyroidism can lead to the deposition of calcium crystals in the kidneys, leading to the development of kidney stones.
• Polyuria
  • Elevated PTH levels increase the excretion of water in the urine, leading to increased urination.
• Impaired sodium-water reabsorption
  • Elevated PTH levels can interfere with the normal resorption of sodium and water in the kidneys, leading to fluid imbalance.
• Muscle weakness
  • Excessive PTH promotes the loss of calcium from muscles, impairing muscle function and causing muscle weakness.
• Bone pain and osteoporosis
  • Increased PTH causes increased calcium release from bones, resulting in bone pain, osteoporosis, and an increased risk of fractures.
• Depression, psychosis, or an altered mental status
  • High levels of PTH can affect neurotransmitter function, contributing to mood changes and cognitive function impairment and affecting overall mental well-being.

Treatment for hyperparathyroidism includes increasing fluid intake and supplementing with calcimimetics, vitamin D supplements, phosphate binders, and calcitriol, which will be discussed later in this chapter.

Diagnostic Testing

Diagnostic testing can help determine if the thyroid and parathyroid glands are functioning appropriately. Table 27.1 provides a list of tests used to diagnose common thyroid and parathyroid conditions.

Other diagnostic tests that assist in diagnosing thyroid and parathyroid conditions include:

• Ionized serum calcium: This test is performed for conditions that affect the body’s ability to balance the amounts of ionized calcium and bound calcium in the blood. The normal range is 1.12–1.23 mmol/L, although ranges may vary slightly depending on the laboratory (American Board of Internal Medicine, 2023).
• Vitamin D (calciferol): This test is performed to determine the amount of vitamin D in the blood, which helps the body to absorb calcium. The normal range is 30–60 ng/mL, although ranges may vary slightly depending on the laboratory (American Board of Internal Medicine, 2023).