Today we’re gonna talk
about thyroid disorders.
Let’s start by revising some
of the thyroid physiology.
Hypothalamic thyroid releasing hormone triggers the
pulsatile release of TSH on the anterior pituitary.
TSH being thyroid stimulating hormone.
This activates thyroid cell growth, it has
a role in effecting iodide metabolism,
and then finally also causes the synthesis of
thyroid hormone within the thyroid gland
which mainly manifest as T4 and T3.
T4 and T3 once produced, then feedback
negatively on the hypothalamus to reduce TSH.
The ratio of T4 to T3 secretion is nearly 20:1
Most T3, in fact 80% of it requires deiodination
on the level of kidney to become T3
Only a tiny amount of free T4 and T3 is biologically
active, the rest being bound to circulating proteins.
Thyrotropin or TSH is the best single test for
screening and function of the thyroid.
If the TSH is abnormal, additional evaluation
of the thyroid function should be considered
to determine the extent of the dysfunction.
Measure T4 when thyroid
stimulating hormone is elevated
and measure both T4 and T3 when thyroid
stimulating hormone is suppressed.
TSH is very sensitive for
detecting thyroid dysfunction.
Check the T4 level with the TSH level
to evaluate for central hypothyroidism.
The TSH level may reflect
hypofunction when TSH is high.
Hyperfunction when TSH is low
or be within the normal range
which is usually in the
0.5-5 milliunit per liter range.
There are three factors that alter
the range of normal TSH levels.
Pregnancy, aging and pituitary dysfunction.
Checking a serum T3 is necessary if the
patient has a suppressed TSH level
because some patients with thyrotoxicosis,
only T3 may be preferentially secreted over T4
and this is a condition
known as T3 toxicosis.
This is also very very rare.
In patients with an elevated TSH level
which indicates hypothyroidism,
T3 will be within the normal range
even with significant disease.