Deficient PTH leading to hypocalcemia and hyperphosphatemia; most commonly post-surgical.
Also known as: hypoparathyroidism, post-surgical hypoparathyroidism, DiGeorge syndrome, autoimmune polyglandular syndrome
Overview
Deficient PTH secretion (or end-organ resistance, in pseudohypoparathyroidism) producing hypocalcemia, hyperphosphatemia, and (when chronic) extra-skeletal calcification.
Epidemiology
Most cases are iatrogenic following thyroidectomy or parathyroidectomy (transient in 20-30%, permanent in 1-3% in experienced hands). Idiopathic/autoimmune forms are rare; congenital DiGeorge (22q11 deletion) and autoimmune polyglandular syndrome type 1 (APS-1/APECED, AIRE mutation) account for most non-surgical cases.
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Question 1EndocrineMedium
A 52-year-old woman with a history of total thyroidectomy presents with fatigue, paresthesias, and muscle cramps. The surgery was performed 3 months ago for multinodular goiter, and she takes levothyroxine daily. On examination she has a positive Chvostek sign. Which lab pattern best supports primary hypoparathyroidism rather than vitamin D deficiency or pseudohypoparathyroidism?
ALow calcium, low phosphorus, elevated PTH
BNormal calcium, high phosphorus, elevated PTH
CLow calcium, low phosphorus, low PTH
DLow calcium, high phosphorus, elevated PTH
ELow calcium, high phosphorus, low PTH
Reveal answer & full explanation
Correct answer: E — Low calcium, high phosphorus, low PTH
ALow calcium, low phosphorus, elevated PTH
BNormal calcium, high phosphorus, elevated PTH
CLow calcium, low phosphorus, low PTH
DLow calcium, high phosphorus, elevated PTH
ELow calcium, high phosphorus, low PTH✓
Why Low calcium, high phosphorus, low PTH is correct
In primary hypoparathyroidism (most commonly post-surgical), inadequate parathyroid hormone (PTH) leads to hypocalcemia and hyperphosphatemia (PTH normally promotes renal phosphate excretion), with an inappropriately low or low-normal PTH
Why the others are wrong
Normal calcium, high phosphorus, elevated PTH — pattern of secondary hyperparathyroidism from phosphate retention in chronic kidney disease; the rise in PTH holds calcium in the normal range, which cannot happen once parathyroid tissue has been removed
Low calcium, high phosphorus, elevated PTH — pattern of pseudohypoparathyroidism: shows low calcium and high phosphorus like hypoparathyroidism, but PTH is HIGH due to end-organ resistance
Low calcium, low phosphorus, elevated PTH — pattern of vitamin D deficiency: low calcium with LOW phosphorus and a compensatory ELEVATED PTH (secondary hyperparathyroidism)
Low calcium, low phosphorus, low PTH — tempting if parathyroid hormone is pictured as raising phosphorus along with calcium, but PTH's net renal effect is phosphaturic, so PTH deficiency causes phosphorus retention; a low phosphorus with a low PTH does not fit postsurgical hypoparathyroidism.
Additional high-yield points
Magnesium must be normal to interpret PTH — hypomagnesemia impairs PTH secretion and action
Question 2EndocrineMedium
A 44-year-old woman is evaluated on the first day after total thyroidectomy for multinodular goiter. She reports tingling around her mouth and in her fingertips and intermittent hand cramps. Tapping over the facial nerve produces an ipsilateral facial twitch. Laboratory studies show a corrected serum calcium of 6.9 mg/dL, phosphate of 5.8 mg/dL, magnesium of 2.0 mg/dL, and an inappropriately low intact PTH. Which of the following best explains the findings?
AInadequate PTH lowering vitamin D activation and phosphate excretion
CEnd-organ resistance to PTH at renal and skeletal target receptors
DAutoimmune destruction of the parathyroid glands reducing PTH
EAvid skeletal remineralization drawing serum calcium into the bone
Why Inadequate PTH lowering vitamin D activation and phosphate excretion is correct
Inadequate PTH (here from inadvertent removal or devascularization of the parathyroids during thyroidectomy) lowers renal 1-alpha-hydroxylase activity, so less 1,25-OH vitamin D is made and intestinal calcium absorption falls.
PTH normally promotes renal phosphate excretion (phosphaturia); without it, phosphate is retained, producing the hyperphosphatemia seen here alongside hypocalcemia.
Loss of PTH also reduces osteoclast-mediated bone resorption and distal tubular calcium reabsorption, compounding the hypocalcemia. Low calcium with an inappropriately low PTH and high phosphate is the classic post-surgical hypoparathyroidism pattern.
Why the others are wrong
End-organ resistance to PTH at renal and skeletal target receptors — this is pseudohypoparathyroidism (GNAS mutation); it gives low calcium and high phosphate but with a HIGH PTH, not the low PTH measured here, and it is not a postoperative event.
Autoimmune destruction of the parathyroid glands reducing PTH — autoimmune hypoparathyroidism, isolated or as part of autoimmune polyglandular syndrome type 1, produces this same low-calcium, high-phosphate, low-PTH pattern, but it develops insidiously over months to years and does not declare itself on the first day after a thyroidectomy.
Calcium-sensing receptor activation suppressing parathyroid PTH release — an activating CaSR mutation (autosomal dominant hypocalcemia) or a calcimimetic such as cinacalcet suppresses PTH and lowers calcium, but neither is present here, and an abrupt presentation one day after neck surgery points to removal or devascularization of the glands.
Avid skeletal remineralization drawing serum calcium into the bone — hungry bone syndrome is the other classic cause of hypocalcemia right after neck surgery, but it follows correction of a high-turnover state, classically parathyroidectomy for hyperparathyroidism or thyroidectomy for long-standing hyperthyroidism, and it pulls phosphate and magnesium into bone along with calcium, so it lowers both rather than leaving the high phosphate and normal magnesium seen here.
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Inadequate PTH reduces osteoclast-mediated bone resorption, decreases renal calcium reabsorption (urinary calcium increases for a given filtered load), reduces 1-alpha-hydroxylation of vitamin D (less intestinal calcium absorption), and decreases renal phosphate excretion. Net effect: hypocalcemia, hyperphosphatemia, low 1,25-OH vitamin D, and (paradoxically) hypercalciuria during treatment.
Clinical presentation
Symptoms
Perioral and acral paresthesias
Muscle cramps, carpopedal spasm, tetany
Seizures, laryngospasm, bronchospasm
Fatigue, anxiety, depression, cognitive slowing
Cataracts and basal ganglia calcification with chronic disease (Fahr syndrome)
Signs / physical exam
Chvostek sign — facial twitch with tapping of CN VII anterior to ear (low sensitivity)
Trousseau sign — carpal spasm after 3-min BP cuff inflation above SBP (more specific)
Patient on POD 1 after total thyroidectomy with perioral tingling and a positive Chvostek sign.
Differential diagnosis
Pseudohypoparathyroidism — End-organ PTH resistance (GNAS mutation); high PTH with low calcium and high phosphate; Albright hereditary osteodystrophy phenotype in type 1a (short stature, round face, brachydactyly, obesity)
Acute pancreatitis — Calcium saponification in necrotic fat; acute setting
Massive transfusion / citrate — Acute symptomatic hypocalcemia; correct with IV calcium
Tumor lysis / rhabdomyolysis — Hyperphosphatemia binds calcium; acute setting with markers of cell turnover
Diagnostic workup
Diagnostic criteria
Hypocalcemia + LOW or inappropriately normal PTH + hyperphosphatemia + normal magnesium. If PTH elevated → pseudohypoparathyroidism or vitamin D deficiency.
Labs
Serum calcium (correct for albumin) and ionized calcium
PTH (low or inappropriately normal for the level of calcium)
Phosphate (elevated)
Magnesium (correct first if low — PTH won't respond)
25-OH vitamin D, 1,25-OH vitamin D
BMP, creatinine
24-hour urinary calcium (to titrate treatment and avoid hypercalciuria)
Imaging
ECG (prolonged QTc)
Head CT or MRI if cognitive symptoms (basal ganglia calcification)
DEXA — bone density is paradoxically increased in hypoparathyroidism
Slit-lamp exam for cataracts in chronic disease
Diagnostic algorithm
flowchart TD
A[Hypocalcemia + paresthesias<br/>Chvostek/Trousseau positive] --> B[Check ionized Ca, PTH,<br/>Mg, phosphate, vitamin D]
B --> C{Magnesium low?}
C -->|Yes| D[Replete Mg first<br/>recheck PTH and Ca]
C -->|No| E{PTH level}
E -->|Low / inappropriately normal| F[Hypoparathyroidism]
E -->|Elevated| G{Phosphate level}
G -->|High| H["CKD (check creatinine/eGFR)<br/>or pseudohypoparathyroidism<br/>(end-organ PTH resistance)"]
G -->|Low| I["Vitamin D deficiency<br/>or 2° hyperparathyroidism"]
F --> J["Treat: oral Ca + calcitriol<br/>± thiazide; goal Ca 8.0-8.5"]
F --> K["Acute symptomatic: IV Ca gluconate<br/>continuous monitoring"]
Hypocalcemia workup — PTH and magnesium-driven decision tree.
Treatment
Acute symptomatic or severe hypocalcemia
Severe symptoms (tetany, carpopedal spasm, seizures, laryngospasm), prolonged QTc, or an acute fall in corrected total Ca to ≤7.5 mg/dL: IV calcium gluconate 1-2 g (10-20 mL of 10% solution) in 100 mL D5W over 10-20 min; follow with calcium gluconate infusion (10 g in 1 L D5W at 50 mL/h, titrate)
Correct magnesium concurrently if low (2 g IV MgSO4)
Monitor ionized Ca q4-6 h
Continuous ECG monitoring
Chronic
Oral calcium — calcium carbonate 500-1000 mg elemental 3 times daily with meals (or citrate if PPI/achlorhydria)
Activated vitamin D — calcitriol 0.25-1 mcg BID (does not require renal/hepatic activation — PTH-independent)
Thiazide diuretic to reduce urinary calcium losses if hypercalciuria limits therapy
PTH replacement (palopegteriparatide; rhPTH 1-84 was discontinued in 2024) for patients inadequately controlled on or intolerant of conventional calcium plus calcitriol
Goal serum calcium: low-normal (8.0-8.5 mg/dL) to minimize hypercalciuria and nephrocalcinosis
ALWAYS check and correct magnesium first — hypomagnesemia produces a functional hypoparathyroidism that does not respond to calcium.
Chvostek sign is insensitive and nonspecific (absent in many hypocalcemic patients, positive in up to about 25% of normal people); Trousseau sign is more specific.
Treatment target is low-normal calcium (~8.0-8.5) — aggressive correction to mid-normal causes hypercalciuria, stones, and nephrocalcinosis.
Thiazides reduce urinary calcium; loops increase it — use thiazides as adjuncts in hypoparathyroidism.
DiGeorge syndrome = 22q11.2 deletion — think hypocalcemia + cardiac anomaly + immunodeficiency in a child.
References
ESE 2015 — European Society of Endocrinology Clinical Guideline: Treatment of Chronic Hypoparathyroidism in Adults (Bollerslev et al., Eur J Endocrinol 2015)
Workshop 2016 — Management of Hypoparathyroidism: Summary Statement and Guidelines (Brandi et al., J Clin Endocrinol Metab 2016)
Second International Workshop 2022 — Evaluation and Management of Hypoparathyroidism Summary Statement and Guidelines from the Second International Workshop (Khan AA et al., J Bone Miner Res 2022;37(12):2568-2585)
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