95- What is perchlorate discharge test?
In newborns/children with primary hypothyroidism who have goiter and/or
normal or increased uptake on 99m Tc pertechnetate/^123 I scan, a diagnosis of
dyshormonogenesis (other than NIS defect) should be considered. This can be
confi rmed by perchlorate discharge test. Perchlorate is an anion which competes
with iodine for uptake by NIS. A baseline^123 I uptake study is performed after
2h of radioiodine ingestion. Thereafter, 200–1000 mg potassium perchlorate is
administered orally and repeat^123 I uptake study is performed after 30min to 1h.
Decrease in radioactivity by >10 % as compared to baseline uptake suggests
dyshormonogenesis. This is because radioiodine is taken up by the gland in
patients with dyshormonogenesis, but is not incorporated in tyrosine residues
of thyroglobulin and hence discharged from the thyroid gland due to compete-
tive inhibition by perchlorate at NIS.- A 10 - year - old boy presented with headache and visual impairment.
Neuroimaging revealed a sellar – suprasellar mass and the child was referred to
a neurosurgeon. Preoperative evaluation revealed T 4 2.4 μg / dl and TSH
150 μ IU / ml ; prolactin 104 ng / ml ; cortisol 400 nmol / L with prepubertal LH ,
FSH , and testosterone ; and low IGF1. Is surgery warranted?
In all patients with sellar–suprasellar mass, anterior pituitary function tests
including T 4 , TSH, 0800h cortisol, prolactin, LH/FSH, testosterone/estra-
diol, and IGF1 should be done. In the index child, hormonal profi le was sug-
gestive of primary hypothyroidism with hyperprolactinemia. Low IGF1 in
the index patient may be due to low T 4 or mass effect. In a patient with pri-
mary hypothyroidism, presence of a sellar–suprasellar mass should raise a
suspicion of thyro-lactotrope hyperplasia. In this scenario, optimal replace-
ment with L-thyroxine not only reverses features of hypothyroidism but also
results in visual improvement in 1–4 months followed by regression of thyro-
lactotrope hyperplasia. Hence, surgery should be avoided in these patients
(Fig. 3.16 ).
3 Thyroid Disorders in Children