lab studies----

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Lab Studies:

The sensitive TSH assay is the most useful test in screening for

hypothyroidism and for confirming of the diagnosis. Additional tests

of free T4, total T4, T3 resin uptake, thyroid autoantibodies

(antimicrosomal or antithyroid peroxidase [anti-TPO]), and

antithyroglobulin (anti-Tg) may be helpful to determine the etiology.

Hypothyroidism is virtually the only disease that raises TSH levels

in a sustained fashion. As the TSH level increases, an increase in T4

conversion to T3 occurs; thus, T3 levels are maintained. In early

hypothyroidism, TSH is increased, T4 is normal to low, and T3 is

normal.

Newer and more sensitive generations of TSH assays can detect levels

as low as 0.01 mU/L. (The normal range of TSH value is 0.4-4 mU/L.)

In most healthy patients, TSH values are 0.5-1.5 mU/L. TSH levels

peak in the evening and are lowest in the afternoon, with marked

variations due to physiologic conditions such as illness, psychiatric

disorders, and low energy intake. A threshold for TSH inhibition may

exist, as evidenced by the finding that some patients have a normal

TSH level and low free T4 levels.

A free T4 test is recommended over a total T4 test or other

measurement because it is not affected by thyroid hormone binding

proteins.

A free T3 test, as with a T3 test, may be indicative; however,

results are frequently within reference ranges early in the disease.

In some situations, the level of thyroid hormones (eg, free T4) may

be a better indicator of thyroid status.

In severely ill patients with non–thyroid disease, TSH secretion is

decreased, T4 is decreased, and T3 is markedly decreased. In these

patients, the primary abnormality is the peripheral production of

thyroid hormones (mostly T3).

The TRH stimulation test is rarely needed now because of improved TSH

assays. It may be helpful to confirm hypothyroidism by an exaggerated

TSH response in patients who have a low free T4 level with a normal

TSH level.

TSH assays are not useful in patients with pituitary or hypothalamic

failure and secondary hypothyroidism. In these patients, TSH levels

are low in the face of low free T4 and low free T3 levels. Since

isolated pituitary loss of TSH is very rare, other signs and symptoms

to support the diagnosis of pituitary failure and secondary

hypothyroidism are usually present.