Primary Hyperparathyroidism (PHPT) Causes
Primary hyperparathyroidism refers to a condition where the problem lies in the parathyroids themselves, resulting in the excessive secretion of parathyroid hormone and high blood calcium (hypercalcaemia). It is the most common type of hyperparathyroidism and the most amenable to surgery.
For the contributing factors to primary hyperparathyroidism, see webpage on Hyperparathyroidism. Detailed below are the pathological causes of the disease, that is, the underlying pathology found at surgery.
Pathological Causes
Primary HPT has three possible pathological causes:
1. Parathyroid adenoma (85-90%)
Fig.1: Parathyroid adenomaParathyroid adenomas (Fig. 1) are usually solitary, with the other glands showing evidence of suppressed activity. Double or multiple adenomas may occur occasionally, but more likely are misdiagnosed hyperplasia. Parathyroid tumours more often affect the lower glands than the upper glands, and intrathymic and intrathyroidal adenomas are not uncommon.
The adenomas generally are ovoid, soft, reddish brown tumours, usually a little darker than the normal glands. Microscopically, they are characterized by varying compositions of chief, clear and oxyphil cells, with chief cell adenoma by far the most common. Parathyroid adenomas generally exhibit a peripheral rim of condensed normal parathyroid tissue, separated by a slender capsule (Figs 2 & 3).
Fig.2: Histology of parathyroid adenoma with suppressed rim of normal parathyroid tissue (arrow)
Fig.3: High power view of histology with parathyroid adenoma on left and rim of normal parathyroid tissue on right
2. Parathyroid hyperplasia (5-10%)
Fig.4: Parathyroid hyperplasia from kidney failure patientThis is where there are two or more enlarged glands and affects less than 10% of patients as a cause of primary hyperparathyroidism.
The incidence of this condition appears to be decreasing since the advent of minimally invasive parathyroid surgery, where usually only a single gland is removed. This suggests either that hyperplasia was overdiagnosed in the era of open 4-gland exploration surgery or that it is not being picked up enough now – time will tell.
Of course, the most common cause of parathyroid hyperplasia is secondary hyperparathyroidism, typically from renal (kidney) failure (Fig. 4). Secondary HPT occurs because of a condition outside the parathyroids themselves.
When less than all four glands are involved, it is often referred to as double adenoma, but is more correctly referred to as asymmetric hyperplasia as it may involve two or three glands.
3. Parathyroid cancer (<1%)
Parathyroid cancers are often large and clinically palpable (30-70%), usually greater than 2 cm in size, and are nearly always functioning tumours. They account for less than 1% of cases of primary hyperparathyroidism. They tend to have a very florid presentation, are very symptomatic, and usually have a very high serum calcium and PTH.
Further details on this rare cancer can be found on the webpage Parathyroid Cancer.