27.1 Introduction to the Thyroid and Parathyroid

Thyroid and Parathyroid Glands

The thyroid and parathyroid glands regulate two things you'll see constantly in clinical practice: metabolism and calcium balance. Thyroid hormones control how your body uses energy, while parathyroid hormone (PTH) manages calcium levels in the blood and bones. Nursing assessment and medication administration rely on recognizing when these systems malfunction and knowing which labs confirm it.

Thyroid and Parathyroid Hormone Roles

Thyroid hormones come in two forms: T3 (triiodothyronine) and T4 (thyroxine). T4 is produced in larger quantities but T3 is the more active hormone. Most T4 gets converted to T3 in peripheral tissues. Together, they:

• Increase basal metabolic rate (BMR), which is your body's baseline energy expenditure at rest
• Stimulate carbohydrate and fat metabolism
• Increase protein synthesis and breakdown
• Support normal growth and development (especially critical in pediatric patients)

Think of thyroid hormones as the thermostat for your metabolism. Too much and everything speeds up; too little and everything slows down.

Parathyroid hormone (PTH) is produced by the four small parathyroid glands sitting behind the thyroid. Its primary job is raising serum calcium levels, and it does this through three mechanisms:

1. Bone resorption: PTH stimulates osteoclasts to break down bone and release calcium into the blood
2. Renal reabsorption: PTH tells the kidneys to hold onto calcium instead of excreting it
3. Vitamin D activation: PTH promotes conversion of vitamin D to its active form (calcitriol), which increases calcium absorption from the intestines

PTH also decreases serum phosphate by inhibiting phosphate reabsorption in the kidneys. Calcium and phosphate have an inverse relationship, so when one goes up, the other tends to go down.

Hypothyroidism and Hyperthyroidism: Symptoms and Causes

The easiest way to remember these two conditions is that hypothyroidism slows everything down and hyperthyroidism speeds everything up.

Hypothyroidism (underactive thyroid) presents with:

• Fatigue, depression, memory impairment
• Weight gain, constipation
• Cold intolerance, dry skin, hair loss
• Bradycardia, hypothermia, delayed reflexes

Common causes include Hashimoto's thyroiditis (the most common cause in the U.S., an autoimmune condition where the body attacks thyroid tissue), iodine deficiency (more common globally), surgical removal or radiation of the thyroid, and certain medications like lithium and amiodarone.

Hyperthyroidism (overactive thyroid) presents with:

• Nervousness, anxiety, irritability, tremors
• Heat intolerance, increased sweating
• Tachycardia, palpitations
• Weight loss despite increased appetite, diarrhea
• Menstrual irregularities

The most common cause is Graves' disease, another autoimmune disorder where antibodies stimulate the thyroid to overproduce hormones. Other causes include thyroid nodules, excessive iodine intake, and thyroiditis (inflammation that causes stored hormone to leak into the bloodstream).

Diagnostic Tests for Thyroid and Parathyroid Function

Thyroid function tests:

TSH is the most sensitive screening test. Here's why: the pituitary gland releases TSH to tell the thyroid to produce more hormone. If the thyroid is underperforming, TSH rises because the pituitary is "yelling louder." If the thyroid is overproducing, TSH drops because the pituitary backs off. TSH moves in the opposite direction of the problem.

Parathyroid function tests:

Notice the inverse calcium-phosphate relationship in both conditions. In hyperparathyroidism, calcium is high and phosphate is low. In hypoparathyroidism, it flips. This pattern shows up frequently on exams.