Primary Hyperparathyroidism: This condition is characterized by the excessive production of parathyroid hormone (PTH) due to one or more parathyroid glands being overactive. The high PTH levels can lead to hypercalcemia, which in turn can cause a range of symptoms. The presence of a parathyroid node (likely referring to a parathyroid adenoma or hyperplasia) and elevated PTH levels strongly support this diagnosis. Low iron levels could be secondary to increased calcium levels affecting iron absorption or metabolism.

Parathyroid Adenoma: A benign tumor on one of the parathyroid glands that causes it to become overactive, leading to an excessive production of PTH. This is a common cause of primary hyperparathyroidism and fits with the high PTH and parathyroid node findings.
Parathyroid Hyperplasia: A condition where all four parathyroid glands become overactive and produce excess PTH. This could also explain the high PTH levels and the presence of a parathyroid node, although typically, hyperplasia involves all glands rather than a single "node."
Familial Hypocalciuric Hypercalcemia (FHH): A rare genetic disorder that affects the calcium receptors in the parathyroid glands, leading to elevated PTH and calcium levels. However, it's less likely given the low iron levels and the specific mention of a parathyroid node.

Parathyroid Carcinoma: A rare but serious condition where a parathyroid gland develops cancer, leading to very high levels of PTH and severe hypercalcemia. Although rare, missing this diagnosis could be fatal, and any significant parathyroid abnormality warrants consideration of this possibility.
Multiple Endocrine Neoplasia (MEN) Syndromes: These are hereditary conditions that can lead to the development of tumors in multiple endocrine glands, including the parathyroids. Types 1 and 2A can involve primary hyperparathyroidism. While less common, these syndromes can have significant implications for the patient and their family.

Lithium-Induced Hyperparathyroidism: Long-term use of lithium can lead to increased PTH secretion, potentially causing hyperparathyroidism. This would be considered in patients with a history of lithium therapy.
Parathyroid Cysts: Rare, benign fluid-filled sacs in the parathyroid glands that can occasionally cause hyperparathyroidism if they become functional and secrete PTH.
Ectopic PTH-Producing Tumors: Very rare tumors located outside the parathyroid glands that produce PTH, leading to hyperparathyroidism. These could be considered in the differential diagnosis if other causes are ruled out and there's evidence of an ectopic tumor.