Laboratory Studies
See the list below:
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Total and ionized serum calcium: In hypoparathyroidism and pseudohypoparathyroidism (PHP), total and ionized calcium levels are low. Ionized serum calcium level should be evaluated when patient has low protein or albumin in the blood.
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Serum phosphate: Serum phosphate levels are elevated in hypoparathyroidism and PHP, although they can be within the reference range, especially in the infant without enteral feeding and low phosphate/protein intake.
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Serum magnesium: Serum magnesium levels are obtained to rule out hypomagnesemia as a cause of hypoparathyroidism. In this condition, hypocalcemia could be corrected very rapidly with magnesium therapy. In general, magnesium levels are within the reference range in hypoparathyroidism and PHP.
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Intact parathyroid hormone (iPTH): Obtain iPTH at the time of hypocalcemia. Nomograms have been developed for the interpretation of serum iPTH concentration with respect serum calcium. In hypoparathyroidism, iPTH is low. An iPTH that falls within the reference range must be interpreted with caution. The value might be considered low in the face of hypocalcemia. In PHP, iPTH is usually elevated.
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BUN and creatinine levels: BUN and creatinine concentrations are obtained to assess renal function. These test results are normal in hypoparathyroidism and PHP.
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25-hydroxyvitamin D and 1,25-dihydroxyvitamin D: 25-Hydroxyvitamin D and 1,25-dihydroxyvitamin D concentrations are obtained to rule out a vitamin D–deficient or vitamin D–resistant state as the etiology of hypocalcemia. 25-Hydroxyvitamin D levels are within the reference range in hypoparathyroidism and PHP. 1,25-Dihydroxyvitamin D levels are expected to be low in hypoparathyroid states because of lack of PTH-stimulated 1-alpha-hydroxylase activity. Elevated 1,25-dihydroxyvitamin D concentrations have been documented in PHP, but the mechanism remains unclear.
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Urine calcium and creatinine ratio: Urine calcium is elevated in PTH-resistant and PTH-deficient states and particularly elevated in calcium-sensing receptor mutations.
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Thyroid studies (thyroid-stimulating hormone [TSH], thyroxine, and thyroid antibodies [antiperoxidase and antithyroglobulin antibodies]. If an autoimmune process is suspected as the etiology of hypoparathyroidism, thyroid studies may uncover a concomitant hypothyroid state. TSH resistance could be associated with PHP Ia.
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Adrenocorticotropic hormone (ACTH) and adrenal antibodies: If an autoimmune process is suspected, concomitant primary adrenal insufficiency can be revealed by an elevated ACTH level, and adrenal antibodies may be present.
Imaging Studies
See the list below:
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Chest radiography: Thymic aplasia is associated with the 22q11 deletion syndrome and can be assessed with chest radiography.
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Echocardiography: An infant with a murmur and in whom hypoparathyroidism is suggested should have echocardiography performed to assess for conotruncal lesions that are associated with the 22q11 deletion syndrome.
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Renal ultrasonography: Treatment of hypoparathyroidism can lead to nephrocalcinosis as a result of hypercalciuria. Baseline renal ultrasonography with initial treatment should be performed.
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Left hand and wrist radiography (bone age): Brachymetacarpals (shortening of fourth and fifth metacarpals) are a feature of Albright hereditary osteodystrophy (AHO) phenotype and can aid in the diagnosis of PHP Ia. There may be some degrees of advanced bone age as well in those patients with PHP.
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Brain MRI: Basal ganglia calcifications are quite common in both hypoparathyroidism and PHP. These suggest a long-standing presence of calcium disorder and are more common with PHP. These calcifications occur even before the treatment of hypocalcemia was initiated.
Other Tests
See the list below:
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Electrocardiography: A prolonged QTc interval is found with hypocalcemia and resolves with correction of serum calcium.
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ACTH stimulation testing: Adrenal insufficiency can be life threatening. If APS I is suggested, an ACTH stimulation study should be performed to assess adrenal function (if basal ACTH levels are elevated).
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Thyrotropin-releasing hormone stimulation testing: PHP Ia is associated with generalized hormone resistance. Hypothyroidism may be subtle and may only be detected with a thyrotropin-releasing hormone (TRH) stimulation study. Unfortunately, TRH is not commercially available at this time.
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Genetic studies, when applicable
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In neonates, a fluorescence in situ hybridization (FISH) for the chromosome band 22q11 deletion
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Electrocardiogram (ECG) findings in severe hypocalcemia.
