The blood chemistry screen and thyroid panel can
be used to differentiate the different types of hypothyroidism.
Primary hypothyroidism is the classic hypothyroid condition. The
locus of the problem lies in the thyroid itself. As such the output
of T4 from the thyroid is low and the conversion of T4 into T3 is
also decreased. On a blood test you will see high TSH, and low T4
and T3 levels.
Secondary hypothyroidism is a form of hypothyroidism is becoming
more common. The thyroid itself is working normally, but the anterior
pituitary is no longer able to produce TSH due to some kind of anterior
pituitary dysfunction. on a blood test you will see low TSH levels,
normal T4 and T3 levels.
Tertiary hypothyroidism is not so common and is caused by an inadequate
secretion of TRH from the hypothalamus causing a decreased TSH level.
How do you differentiate between these three
types of hypothyroidism?
A decreased TSH level may be due to an inadequate secretion of
Thyroid Releasing Hormone (TRH) from the hypothalamus. Clinically
an intravenous injection of TRH is administered to try to stimulate
the production of TSH from the pituitary.
- In primary
hypothyroidism there is an exaggerated TSH response after administration
- In secondary
hypothyroidism due to anterior pituitary hypofunction there is
no significant rise in TSH levels after administration of TRH.
- In tertiary
hypothyroidism due to a hypothalamic hypofunction there will be
a delayed rise in TSH of approximately 30 minutes after administration
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