Investigations
1st investigations to order
serum free thyroxine (FT4)
Test
Serum thyroid-stimulating hormone (TSH) and FT4 should be ordered as an initial diagnostic step in any patient suspected of central hypothyroidism. Low FT4 is observed in both primary and central hypothyroidism.
Result
low
serum thyroid-stimulating hormone (TSH)
Test
Serum TSH and FT4 should be ordered as an initial diagnostic step in any patient suspected of central hypothyroidism. The TSH may be low, normal, or elevated and suggests central hypothyroidism if it is discordant with the FT4 level.
Result
may be inappropriately low, normal, or elevated in the setting of low serum FT4
Investigations to consider
pituitary MRI
Test
MRI should be obtained in any patient with biochemical evidence of hypothalamic or pituitary dysfunction, or in any patient with physical examination findings strongly suggestive of a pituitary lesion (e.g., a bitemporal hemianopia).
The use of MRI in detecting pituitary microadenomas has a sensitivity of 82% to 94%.[34][35]
If MRI is unavailable, a contrast-enhanced CT scan with coronal views through the pituitary is a reasonable alternative.[39]
Result
may reveal a variety of lesions, most commonly pituitary adenomas
head CT
serum prolactin (PRL)
Test
If levels are mildly elevated, differential diagnoses include pituitary stalk pathologies and drugs with dopamine antagonistic effects.
A serum PRL >8696 picomol/L (>200 micrograms/L) is highly suggestive of a PRL-secreting pituitary adenoma (prolactinoma). If levels are elevated to a lesser degree, the differential diagnosis includes several aetiologies including drug-induced hyperprolactinaemia.[40]
Untreated primary hypothyroidism may itself be associated with modest hyperprolactinaemia.
Result
may be elevated due to stalk effect of a pituitary mass
fasting morning serum cortisol
Test
Adrenocorticotrophic hormone (ACTH) deficiency causes adrenal atrophy. A standard 250 microgram or a 1 microgram corticotrophin test can be used to establish adrenal insufficiency.[41]
Most normal individuals have cortisol levels above 497 nanomols/L (18 micrograms/dL) at 30 and/or 60 minutes after 250 micrograms intravenous or intramuscular ACTH.
Result
low in the presence of ACTH deficiency due to compression from a pituitary mass
serum testosterone
Test
A serum total testosterone level of <5.2 nanomol/L (<150 nanograms/dL) in men is suggestive of a pituitary tumour, especially when associated with clinical signs of headache and diplopia, or laboratory evidence of other hormone abnormalities (most commonly, PRL elevation).
Result
low in the presence of gonadotrophin deficiency due to a pituitary mass
serum gonadotrophins
Test
Low/normal serum gonadotrophins in post-menopausal women and low serum gonadotrophins in combination with a low serum total testosterone in men indicate hypogonadotrophic hypogonadism, for which a pituitary mass should be ruled out.
Result
may be low in the presence of a pituitary mass
genetic analyses
Test
Recommended in congenital cases of central hypothyroidism, and in cases of onset during childhood or at any age when central hypothyroidism remains unexplained.[3] Testing is by direct sequencing following a phenotype-driven approach or next-generation sequencing using a panel of candidate genes.[3]
Result
mutation detected
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