A 60-year-old woman presented with 2 months of aching limbs and 2 weeks of nausea and a left sided neck mass. She was clinically euthyroid with a 5 cm non-tender left thyroid nodule. There was no cervical lymphadenopathy and the remainder of the examination was unremarkable. There was no family history of any endocrine disorder.
Investigations showed serum TSH concentration 5.94 μ/l and serum free T4 concentration 12 pmol/l. Thyroid peroxidase antibody titres were not raised. Her corrected serum calcium concentration was elevated at 3.03 mmol/l with phosphate concentration 1.21 mmol/l. Plasma intact parathyroid hormone concentration was inappropriately raised at 166 ng/l. The 24 h urine calcium excretion was 13.6 mmol/24 h excluding benign familial hypercalcaemia. She had vitamin D insufficiency (28 nmol/l) with a normal alkaline phosphatase. Bone densitometry showed spinal osteoporosis and hip osteopenia. No renal calculi were seen on ultrasound.
Neck ultrasound revealed an apparent 4 cm left inferolateral thyroid cyst, thought to be a degenerative cystic thyroid nodule. There was no parathyroid gland enlargement. 99mTc-sestamibi scan showed no parathyroid adenoma and the pertechnetate study matched the MIBI study. Thyroid uptake was normal (2.2%). FNAC of the thyroid nodule showed only cyst fluid.
Given her hyperparathyroidism, osteoporosis and non-diagnostic FNAC she underwent neck exploration of the parathyroid glands and removal of the presumed thyroid mass. The cystic lesion was the only abnormality found, but following its removal the perioperative PTH concentration fell by 50%.
Histological examination showed an intrathyroidal parathyroid adenoma with marked cystic degeneration, haemorrhage and fibrosis.
The postoperative serum calcium concentration fell to 2.38 mmol/l with phosphate 1.2 mmol/l. The PTH normalized at 29.2 ng/l.
Learning point: Cystic parathyroid adenomas may present as an apparent thyroid mass. Furthermore, in these large cystic lesions 99mTc-sestamibi parathyroid scan may be negative.