Tests
1st tests to order
serum thyroid-stimulating hormone (TSH)
Test
Normal TSH range is 0.4 to 4.0 mIU/L (may vary by laboratory standard) and levels are elevated in primary hypothyroidism.
In subclinical disease, levels are only mildly elevated; usually <20 mIU/L, but may also be <10 mIU/L.
Do not order more tests until the results of the TSH test are available as a TSH value within the reference range excludes the majority of primary thyroid diseases.[29]
Result
elevated
Investigations to avoid
thyroid ultrasound
Recommendations
Do not order thyroid ultrasound if there is no palpable abnormality of the thyroid gland.[32]
Rationale
Thyroid ultrasound is not part of the routine evaluation of hypothyroidism unless the patient also has a large goiter or a lumpy thyroid. Incidentally discovered thyroid nodules are common. Overzealous use of ultrasound will frequently identify nodules that are unrelated to the abnormal thyroid function.[32] Imaging for thyroid morphology does not help to identify a specific cause of hypothyroidism. Therefore, the result of a thyroid ultrasound would have no effect on treatment options.[33]
Tests to consider
free serum thyroxine (T4)
Test
Usual normal free T4 range is 0.8 to 1.8 nanograms/dL (may vary in different laboratories). Low free T4 with an elevated TSH is diagnostic of primary hypothyroidism.[26] Free T4 is normal in subclinical hypothyroidism despite a mildly elevated TSH.
If clinical hypothyroidism is suspected and TSH is low, a free T4 should also be obtained. A low free T4 in this situation is diagnostic of secondary or central hypothyroidism.[1]
If clinical hypothyroidism is suspected and TSH is low, a free T4 should also be obtained. A low free T4 in this situation is diagnostic of secondary or central hypothyroidism.[1] Do not order free T4 if TSH levels are within the normal range.[29][34]
Result
low
antithyroid peroxidase antibodies
CBC
Test
Recommended in assessment of patients with nonspecific fatigue and weight gain. One study found patients with hypothyroidism had a higher risk of anemia compared with euthyroid participants and suggested that a reduced thyroid function at baseline increased the risk of developing anemia during the study follow-up; however, the underlying mechanisms of this link is unclear.[31]
Result
a mild, normocytic anemia sometimes occurs
fasting blood glucose
Test
Recommended in assessment of patients with nonspecific fatigue and weight gain. Primary hypothyroidism is associated with type 1 diabetes mellitus.[8]
Result
may be elevated
serum cholesterol
Test
Hypothyroidism increases total cholesterol and low-density lipoprotein concentrations.[1]
Result
often elevated
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