What laboratory tests are indicated for thyroid-related symptoms?

From the Guidelines

For thyroid-related symptoms, the most important initial laboratory test to order is the serum TSH test, as it is the primary screening test for thyroid dysfunction, according to the US Preventive Services Task Force recommendation statement 1. This test should be done in multiple instances over a 3- to 6-month interval to confirm or rule out abnormal findings. Some key points to consider when evaluating thyroid function include:

• The serum TSH test is the most sensitive initial screening test for thyroid dysfunction.
• Follow-up testing of serum T4 levels in persons with persistently abnormal TSH levels can differentiate between subclinical and overt thyroid dysfunction.
• Additional tests such as free thyroxine (free T4) and free triiodothyronine (free T3) may be helpful in certain situations.
• Thyroid antibodies, such as thyroid peroxidase antibodies (TPOAb) and thyroglobulin antibodies (TgAb), can help diagnose autoimmune thyroid conditions like Hashimoto's thyroiditis.
• A complete blood count (CBC) and comprehensive metabolic panel (CMP) can provide valuable background information on overall health status.

It's essential to interpret these tests together rather than in isolation, as thyroid function exists on a spectrum, and symptoms alone are not reliable indicators of thyroid status 1. The optimal screening interval for thyroid dysfunction is unknown, and there is a need for long-term randomized, blinded, controlled trials to determine the benefits and harms of screening for thyroid dysfunction 1. In clinical practice, treatment is generally recommended for patients with a TSH level that is undetectable or less than 0.1 mIU/L, particularly those with overt Graves disease or nodular thyroid disease, while treatment is typically not recommended for patients with TSH levels between 0.1 and 0.45 mIU/L or when thyroiditis is the cause 1.

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. Monitor TSH and total or free-T4 in pediatric patients as follows: 2 and 4 weeks after the initiation of treatment, 2 weeks after any change in dosage, and then every 3 to 12 months thereafter following dosage stabilization until growth is completed For pregnant patients with pre-existing hypothyroidism, measure serum TSH and free-T4 as soon as pregnancy is confirmed and, at minimum, during each trimester of pregnancy.

The laboratory tests indicated for thyroid-related symptoms are:

• Serum TSH levels: to assess the adequacy of therapy and monitor for primary hypothyroidism
• Total or free-T4: to assess the adequacy of replacement therapy in patients with hypothyroidism
• Serum free-T4 levels: to maintain in the upper half of the normal range in patients with secondary and tertiary hypothyroidism These tests should be monitored at specific intervals, including:
• Every 6 to 8 weeks after a change in dosage
• Every 6 to 12 months in patients on a stable replacement dosage
• 2 and 4 weeks after initiation of treatment in pediatric patients
• Every 3 to 12 months thereafter in pediatric patients until growth is completed
• During each trimester of pregnancy in pregnant patients with pre-existing hypothyroidism 2

From the Research

Laboratory Tests for Thyroid-Related Symptoms

The following laboratory tests are indicated for thyroid-related symptoms:

• Thyroid-stimulating hormone (TSH) level: the preferred test for initial evaluation of suspected primary hypothyroidism 3
• Free thyroxine (T4) level: used to further narrow the diagnosis if TSH is abnormal 3
• Triiodothyronine (T3) level: obtained if TSH is undetectable and free T4 is normal 3
• Thyroglobulin (Tg) and thyroglobulin antibodies (Tg-Ab): used to diagnose Hashimoto's thyroiditis 4
• Thyroid peroxidase antibodies (TPO-Ab): used to diagnose Hashimoto's thyroiditis 4, 5
• TSH receptor antibodies (TRAb): used to diagnose Graves' disease 4
• Calcitonin: an important tumor marker used in the management of medullary thyroid carcinoma (MTC) 4

Special Considerations

• TSH and free thyroid hormone tests are frequently used to assess the functional status of the thyroid 4
• Liquid chromatography-tandem mass spectrometry (LC-MS/MS) may be used to evaluate thyroid hormone levels in patients who continue to experience hypothyroid symptoms on levothyroxine (L-T4) therapy 6
• Thyroid function tests must be interpreted carefully because acute illness, diet, and drugs may alter values 7
• The clinical context should be considered when interpreting results, as thyroid function may appear abnormal in the absence of actual thyroid dysfunction during pregnancy and in critical illness 4