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[Skill Modules >> Thyroid >> Laboratory & Imaging ]

Laboratory & Imaging: Thyroid

Laboratory diagnosis of hypothyroidism: Interpretation of test results

The clinical diagnosis is supported by laboratory testing with the ultrasensitive TSH assay. If the TSH is elevated, repeat testing should be done as there may be transient elevations due to non-thyroidal illnesses. After confirming the TSH elevation, a free T4 level is measured, either directly by T4 RIA or estimated by obtaining a total T4 and a T3 resin uptake. Cost determines the choice of methods.

Note: In pregnancy, there may be increased T4 and decreased T3 resin uptake due to an increased production of binding protein, but free T4 and TSH are normal.

The interpretation of test results is presented below:

Clinical Status: NON-SPECIFIC SYMPTOMS (Fatigue, wt gain, constipation, etc.)
Initial TSH Test Results Repeat Results Implication Action
Normal Hypothyroidism unlikely Seek other causes. If high clinical suspicion of subclinical disease, consider retest in 6 months.
ELEVATED repeat NORMAL Transient non-thyroidal illness  
ELEVATED repeat ELEVATED < 10 mU/L Hypothyroidism unlikely
Possible subclinical disease
Consider repeat testing in 6 months
ELEVATED repeat ELEVATED 10-15 mU/L Possible subclinical disease Consider repeat testing in 6 mos.
Consider treating if T4
OR + antithyroid antibodies
ELEVATED repeat ELEVATED < 20 mU/L Subclinical or overt disease Check FT4:
if decreased, treat
if normal, consider
repeat testing in 6 mos or
Rx if + antithyroid antibodies

Initial TSH Test Results Repeat Results Implication Action
TSH low to normal ? Central hypothyroidism
? Apathetic hyperthyroidism in elderly
Check FT4:
if decreased, check TRH
+ endocrine referral
if increased,
evaluate for hyperthyroidism
TSH elevated repeat elevated Primary hypothyroidism Treat

Adapted from Pinsky LE

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