General

Synonym/Acronym:
Parathyroid scintiscan.

Rationale
To preoperatively locate the parathyroid glands, assess the parathyroid gland toward diagnosing cancer, and perform postoperative evaluation of the parathyroid gland.

Patient Preparation
There are no food, fluid, activity, or medication restrictions unless by medical direction.

Normal Findings

• No areas of increased perfusion or uptake in the thyroid or parathyroid.

Critical Findings and Potential Interventions
N/A

Overview

(Study type: Nuclear scan; related body system: Endocrine system.)

There are four parathyroid glands located near or embedded in tissue at the back of the thyroid gland. The parathyroid glands produce parathyroid hormone (PTH), which regulates calcium levels in a feedback cycle of interactions also involving phosphorus and vitamin D. For additional information regarding PTH, refer to the study titled “Parathyroid Hormone.” Increased blood calcium levels initiate a negative feedback loop by signaling the parathyroid glands to stop release of PTH. Dysfunction of one or more parathyroid glands results in unregulated overproduction of PTH that leads to hypercalcemia and hypophosphatemia. The three types of hyperparathyroidism include

• Primary hyperparathyroidism
• Secondary hyperparathyroidism
• Tertiary hyperparathyroidism

For additional information relating to the types of hyperparathyroidism, refer to the study titled “Parathyroid Hormone.”

Preoperative Identification and Localization
Parathyroid scanning is performed to assist in the preoperative identification and localization of the parathyroid glands. Parathyroid imaging is usually requested in the presence of elevated PTH and hypercalcemia. Hyperplasia of the parathyroids generally enlarges all four glands; parathyroid adenoma usually causes enlargement of a single gland while suppressing growth of the normal glands. The scan is also performed after surgery to verify the presence of the parathyroid glands after thyroidectomy. Technetium-99m sestamibi is the most commonly used radionuclide for parathyroid scanning and is considered the most sensitive and specific type of nuclear scan, especially when combined with single-photon emission computed tomography (SPECT) imaging, creating a three-dimensional image. The radionuclide is absorbed by the overactive parathyroid but not by the healthy glands. An initial scan is taken 10 to 15 min after injection of the radionuclide as the Tc-99m sestamibi is quickly absorbed by the thyroid gland and by one or more of the abnormal, overactive parathyroid glands. A delayed scan is taken 90 min to 3 hr following injection because the radionuclide leaves the thyroid quickly and leaves the parathyroid more slowly. The two scans are compared to identify the abnormal parathyroid(s).

Minimally Invasive Radioguided Parathyroidectomy (MIRP)
MIRP is a surgical method that combines the use of

• Technetium-99 sestamibi scan to identify the abnormal parathyroid glands.
• An intraoperative gamma probe to assist in decisions regarding the need to continue neck exploration if multiple or ectopic glands are suspected or to avoid waiting for laboratory studies to confirm a successful surgical cure. Immediately following the initial incision, the gamma probe measures the baseline background counts of radioactivity over the bridge between the butterfly nodes or isthmus of the thyroid. After the target hyperactive parathyroid gland is removed, a count of the gland’s radioactivity is measured. Counts greater than 20% of background indicate that the gland removed is a hyperactive parathyroid.
• Methylene blue dye is sometimes used as an adjunctive visual indicator. It is given at a low dose and in the absence of concurrent use of selective serotonin reuptake inhibitors (e.g., antidepressants) to avoid an adverse reaction to the dye.
• Measurement of intact PTH levels.

The measurements that indicate an effective cure vary by facility. For example, some facilities state that a cure is confirmed by a decrease of 50% between the intraoperative or postoperative serum PTH from the preoperative PTH measurement. Cure may also be indicated by intraoperative gamma probe readings that show the ex vivo or removed gland emits radioactive counts greater than 20% of the baseline background count.
Other Imaging Methods
Ultrasound, Tc-99m sestamibi, and SPECT-CT are the most commonly ordered imaging studies used for preoperative localization of diseased parathyroid glands. CT, magnetic resonance imaging, and positron emission tomography imaging are also useful as adjuncts, for example, to provide anatomic correlation in suspected ectopic glands or in cases of suspected cancer for surgical planning. Fine-needle aspiration biopsy guided by ultrasound is occasionally necessary to differentiate thyroid pathology, as well as pathology of other tissues, from parathyroid neoplasia.

Indications

• Aid in the diagnosis of hyperparathyroidism.
• Differentiate between extrinsic and intrinsic parathyroid adenoma but not between benign and malignant conditions.
• Evaluate the parathyroid in patients with severe hypercalcemia or in patients before parathyroidectomy.

Interfering Factors

Contraindications
Pregnancy is a general contraindication to procedures involving radiation.

Factors that may alter the results of the study

• Ingestion of foods containing iodine (e.g., iodized salt) and medications containing iodine (e.g., cough syrup, potassium iodide, vitamins, Lugol solution, thyroid replacement medications), which can decrease uptake of the radionuclide.
• Metallic objects (e.g., jewelry, body rings) within the examination field, other nuclear scans or radiographic procedures using iodinated contrast medium done within the previous 24 to 48 hr, or retained barium from a previous radiological procedure, which may inhibit organ visualization and cause unclear images.
• Improper injection of the radionuclide that allows the tracer to seep deep into the muscle tissue can produce erroneous hot spots.
• Inability of the patient to cooperate or remain still during the procedure, because movement can produce blurred or otherwise unclear images.

Potential Medical Diagnosis: Clinical Significance of Results

Abnormal Findings In:

• Intrinsic and extrinsic parathyroid adenomas

Nursing Implications, Nursing Process, Clinical Judgement