From the Guidelines
The best lab test for diagnosing and monitoring hypothyroidism is the thyroid stimulating hormone (TSH) test, typically combined with free thyroxine (free T4) measurement. These tests provide a comprehensive assessment of thyroid function. TSH is the most sensitive indicator of thyroid dysfunction, with elevated levels suggesting hypothyroidism as the pituitary gland produces more TSH to stimulate an underactive thyroid. Free T4 measures the actual thyroid hormone available in the bloodstream. Additional helpful tests include free triiodothyronine (free T3), thyroid antibodies (particularly thyroid peroxidase antibodies or TPOAb) to identify autoimmune causes like Hashimoto's thyroiditis, and reverse T3 in some cases. For initial diagnosis, doctors typically order TSH and free T4, then may add other tests based on results. Regular monitoring of TSH and free T4 is essential for patients on thyroid replacement therapy like levothyroxine to ensure proper dosing. These tests should be done fasting in the morning for the most accurate results, and medication should be taken after blood is drawn if on thyroid medication, as suggested by the U.S. Preventive Services Task Force recommendation statement 1.
Some key points to consider when interpreting these tests include:
- The serum TSH test is the primary screening test for thyroid dysfunction, with multiple tests done over a 3- to 6-month interval to confirm or rule out abnormal findings 1.
- Follow-up testing of serum T4 levels in persons with persistently abnormal TSH levels can differentiate between subclinical (normal T4 levels) and “overt” (abnormal T4 levels) thyroid dysfunction 1.
- The principal treatment for hypothyroidism is oral T4 monotherapy (levothyroxine sodium), and hyperthyroidism is treated with antithyroid medications or nonreversible thyroid ablation therapy 1.
- Risk factors for an elevated TSH level include female sex, advancing age, white race, type 1 diabetes, Down syndrome, family history of thyroid disease, goiter, previous hyperthyroidism, and external-beam radiation in the head and neck area 1.
Overall, the combination of TSH and free T4 tests provides a comprehensive assessment of thyroid function and is essential for diagnosing and monitoring hypothyroidism, as well as guiding treatment decisions.
From the FDA Drug Label
Assess the adequacy of therapy by periodic assessment of laboratory tests and clinical evaluation. In adult patients with primary hypothyroidism, monitor serum TSH levels after an interval of 6 to 8 weeks after any change in dosage. In patients on a stable and appropriate replacement dosage, evaluate clinical and biochemical response every 6 to 12 months and whenever there is a change in the patient’s clinical status In patients with hypothyroidism, assess the adequacy of replacement therapy by measuring both serum TSH and total or free-T4.
The best labs for hypothyroidism are:
- Serum TSH
- Total or free-T4 to assess the adequacy of replacement therapy and monitor clinical and biochemical response 2
From the Research
Best Labs for Hypothyroidism
The following lab tests are recommended for the diagnosis and monitoring of hypothyroidism:
- Thyroid-stimulating hormone (TSH) level: the preferred test for initial evaluation of suspected primary hypothyroidism 3
- Free thyroxine (T4) level: to further narrow the diagnosis if TSH is abnormal 3
- Triiodothyronine (T3) level: if TSH is undetectable and free T4 is normal, or to monitor central hypothyroidism 3
Lab Test Ordering Practices
Current guidelines recommend TSH alone as the best test to detect and monitor thyroid dysfunction, yet free thyroxine (FT4) and free triiodothyronine (FT3) are commonly ordered when not clinically indicated 4
- A study found that 36.0% of thyroid function tests (TFTs) ordered were free thyroid hormones, which could save up to $120,000 per year if ordered appropriately 4
- Inappropriate ordering of free thyroid hormone tests is common, and further evaluation is needed to determine strategies that can reduce excessive thyroid hormone testing 4
Monitoring Replacement Therapy
The TSH level is the most important parameter to monitor when assessing the adequacy of replacement therapy in primary hypothyroidism 3
- Levothyroxine doses should be optimized aiming for a TSH in the 0.3-2.0 mU/L range for 3 to 6 months before a therapeutic response can be assessed 5
- In some patients, it may be acceptable to have serum TSH below reference range (e.g. 0.1-0.3 mU/L), but not fully suppressed in the long term 5
Special Considerations
Some patients with confirmed overt hypothyroidism and persistent symptoms who have had adequate treatment with levothyroxine may benefit from a trial of liothyronine/levothyroxine combined therapy 5, 6
- The decision to start treatment with liothyronine should be a shared decision between patient and clinician, and individual clinicians should not feel obliged to start liothyronine or to continue liothyronine medication provided by other health care practitioners or accessed without medical advice, if they judge this not to be in the patient's best interest 5