which the recurrent laryngeal or vagus nerves were at risk of injury, those with locally advanced cancer

and those with distorted anatomy due to large and/or substernal goiters.

The parathyroid glands are at risk during thyroid resection by virtue of their location which can be

firmly invested within the thyroid sheath or occasionally even within the thyroid capsule. Inferior

parathyroid glands are ultimately supplied by the ITA. Superior parathyroid glands are often served by

the ITA but may also have contributions from the superior thyroid artery. Ligation of the trunk of the

ITA proximal to the pedicle to the parathyroid glands must be avoided. Every attempt should be made

to mobilize the parathyroid glands away from the thyroid tissue while preserving the blood supply. If

the gland appears pale or dark and devascularized, the arterial supply is probably compromised. Such a

parathyroid gland is unlikely to survive after the operation and should be autotransplanted. This may be

done by mincing the gland into 1-mm pieces and inserting or injecting the pieces into well-vascularized

skeletal muscle (e.g., sternocleidomastoid, strap muscle, or pectoralis).84 With total thyroidectomy,

careful dissection is even more critical than with hemithyroidectomy. Temporary hypoparathyroidism is

relatively common and occurs in 20% to 40% of patients after total thyroidectomy and is due to mild

devascularization or venous congestion of parathyroid glands during mobilization. Symptoms of

hypocalcemia include paresthesias and numbness of the hands, feet, and lips which can be treated with

oral calcium with or without vitamin D supplements (e.g., calcitriol). Severe symptomatic hypocalcemia

can be treated with intravenous calcium gluconate. Temporary hypoparathyroidism complicating

thyroidectomy usually resolves over days to weeks, although occasionally it may take months to do so.

Hypoparathyroidism that persists longer than 6 months is usually destined to be permanent. The

transient hypocalcemia after total thyroidectomy may be worse in the patient with Graves disease

because of the increased bone turnover observed with hyperthyroidism; however, recovery is expected

and the incidence of permanent hypoparathyroidism should be no higher than in euthyroid patients.

Regardless of the specific extent or technique of thyroidectomy, these complications can largely be

avoided or ameliorated by delicate and deliberate surgical technique. A thorough understanding of the

function and anatomy, both normal and abnormal, of the thyroid gland, and of the rationale behind

various treatment options, is critical to ensure the best outcomes for our patients.

DISCLOSURES

The authors have nothing to disclose.

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Chapter 76

Parathyroid Glands

Gerard M. Doherty

Key Points

1 The normal parathyroid glands are flat, ovoid, and red-brown to yellow. Their dimensions are 5 to 7

mm × 3 to 4 mm × 0.5 to 2 mm, and they weigh between 30 and 50 mg each.

2 Parathyroid hormone (PTH) is the single most important hormonal regulator of calcium and

phosphate metabolism in humans with direct effects on the skeleton and kidney and indirect effects

on the intestine, mediated through vitamin D.

3 The demonstration of an elevated plasma PTH concentration alone does not establish the diagnosis

of hyperparathyroidism; with a simultaneous elevated serum calcium level, this finding is virtually

diagnostic.

4 A large proportion of patients with the diagnosis of hyperparathyroidism are minimally or

asymptomatic and the appropriate treatment for these patients remains controversial.

5 It is routinely possible to identify abnormal parathyroid glands prior to operation for most patients,

allowing the surgeon to know where to start the exploration; intraoperative PTH measurement can

be used to confirm removal of all hyperfunctioning parathyroid tissue, that is, when to stop the

operation.

ANATOMY

1 Typically, each person has four parathyroid glands – two superior and two inferior (Fig. 76-1).1 The

normal parathyroid glands are flat, ovoid, and red-brown to yellow. They measure 5 to 7 mm × 3 to 4

mm × 0.5 to 2 mm and weigh between 30 and 50 mg each. The lower glands are usually larger than

the upper glands. The superior glands are most often embedded in the fat on the upper posterior surface

of the thyroid lobe near the site where the recurrent laryngeal nerve enters the larynx. The inferior

glands are usually more ventral and lie close to or within the portion of the thymus gland that extends

from the inferior pole of the thyroid gland into the chest. Although this anatomy is fairly consistent,

substantial variations from the usual can occur, and it is essential that the surgeon have a thorough

understanding of these anatomic variations.

Variations in parathyroid gland anatomy are primarily caused by differences in patterns of

embryogenesis. During the fourth and fifth weeks of fetal development, a series of four pharyngeal

pouches develop (Fig. 76-2). The superior parathyroid gland arises from the fourth pharyngeal pouch in

conjunction with the lateral thyroid, and the inferior gland arises from the third pouch along with the

thymus. The derivatives of each pouch then migrate together so that the superior parathyroid gland

usually remains in close association with the upper pole of the thyroid, although it may occasionally be

loosely attached by a long vascular pedicle, migrating caudad along the esophagus into the posterior

mediastinum. Occasionally, a gland may be totally embedded in the thyroid parenchyma. The inferior

parathyroid gland descends with the thymus, but this migration is extremely variable. Inferior glands

can be found anywhere from the pharynx to the mediastinum. Regardless of their location, they usually

adhere to the thymus or are within the thyrothymic ligament. Supernumerary glands can be identified

in up to 15% of patients, most often in association with the thymus. Autopsy studies suggest that four

parathyroid glands are virtually always present.

The arterial supply to both the superior and inferior parathyroid glands is usually from the inferior

thyroid artery, although it may arise from the superior thyroid or thyroidea ima arteries or from the

rich anastomosis of vessels supplying the larynx, trachea, and esophagus. A mediastinal parathyroid

gland that descended during embryonic development usually receives its blood supply from either the

internal mammary artery or small arteries within the thymus; however, an enlarged parathyroid gland

that grows into the mediastinum usually carries with it the corresponding branch of the inferior thyroid

2146

artery. The inferior, middle, and superior thyroid veins, which drain the parathyroid glands, empty into

the internal jugular vein or the innominate vein.

Histologically, the normal adult parathyroid gland is about half parenchyma and half stroma,

including fat cells (Fig. 76-3). In children, the gland is almost entirely composed of parenchymal chief

cells. Beginning at puberty, adipocytes appear and, with age, occupy an increasing proportion of the

gland. Also with increasing age, acidophilic, mitochondria-rich oxyphil cells are present in increasing

numbers and are intermixed with the glycogen-laden, polygonal, water-clear cells. The functional

significance of the various cell types remains unclear, although the water-clear cells and oxyphil cells

are probably derived from the chief cells and secrete parathyroid hormone (PTH).

PHYSIOLOGY

The primary physiologic role of the parathyroid gland is the endocrine regulation of calcium and

phosphate metabolism. Average daily exchanges of these ions from the gastrointestinal tract, bone, and

kidney are shown in Figure 76-4.

Calcium

Calcium ion plays a critical role in all biologic systems. It participates in enzymatic reactions and is a

mediator in hormone metabolism. Calcium is intimately involved in the physiology of

neurotransmission, muscle contraction, and blood coagulation. It is the major cation in bone and teeth.

It represents about 2% of the average body weight, and almost all calcium is contained in the skeleton.

The normal range of serum calcium is 9 to 10.5 mg/dL (4.5 to 5.2 mEq/L), and the daily variation in a

normal person is generally less than 10%. About half of the total serum calcium is in an ionized,

biologically active form; 40% is bound to serum protein, mainly albumin, and 10% forms compounds

with organic ions, such as citrate. The total serum calcium concentration is a function of the serum

protein content, and because hydrogen ion competes with calcium for the same binding sites on

albumin, the body fluid pH is important. In general, for every change of 1 g/dL in the serum albumin

level, a direct alteration of 0.8 mg/dL occurs in the serum calcium concentration. Almost all the

physiologically important activity of calcium is represented by the unbound, or free, fraction.

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