Investigations
1st investigations 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 sub-clinical disease, levels are only mildly elevated; usually <20 mIU/L, but may also be <10 mIU/L.
Result
elevated
Investigations to consider
free serum thyroxine (T4)
Test
Usual normal free T4 range is 9.00 to 23.12 picomol/L (0.8-1.8 nanograms/dL) (may vary in different laboratories). Low free T4 with an elevated TSH is diagnostic of primary hypothyroidism.[50] Free T4 is normal in sub-clinical 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]
Result
low
antithyroid peroxidase antibodies (anti-TPOAbs)
FBC
Test
Recommended in assessment of patients with non-specific fatigue and weight gain. One study found patients with hypothyroidism had a higher risk of anaemia compared with euthyroid participants and suggested that a reduced thyroid function at baseline increased the risk of developing anaemia during the study follow-up; however, the underlying mechanisms of this link are unclear.[57]
Result
a mild, normocytic anaemia sometimes occurs
fasting blood glucose
Test
Recommended in assessment of patients with non-specific fatigue and weight gain. Primary hypothyroidism is associated with type 1 diabetes mellitus.[62]
Result
may be elevated
serum cholesterol
Test
Hypothyroidism increases total cholesterol and low-density lipoprotein concentrations.[2]
Result
often elevated
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