Diagnosis and monitoring: blood tests

The most common tests ordered for the diagnosis of thyroid conditions have been listed below. Click on the links to find out more about these tests, including the possible meaning of test results.

• Thyroid stimulating hormone (TSH) (Pathology Tests Explained)

• TSH (Medline Plus)

• Free Thyroxine (FT4) (Pathology Tests Explained)

• Free T4 (Medline Plus)

• Free triiodothyonine (FT3) (Pathology Tests Explained)

• Thyroid Antibodies (Pathology Tests Explained)

• Antithyroglobulin Antibody (anti-Tg) (Medline Plus)

• Thyroid peroxidase antibody (Medline Plus)

• Thyroid Stimulating Immunoglobulin (TSI) (Medline Plus)

• Thyroid Stimulating Immunoglobulin (TSI) (Mayo Clinic)

• Thyroglobulin (Pathology Tests Explained)

For various reasons, doctors sometimes order more tests than the common thyroid function tests. We have included some of those other tests below:

• Antinuclear antibodies (ANA) (Pathology Tests Explained)

• Antinuclear Antibody (ANA) (Medline Plus)

• C-reactive protein (CRP) (Pathology Tests Explained)

• Cortisol (Pathology Tests Explained)

• Reverse T3 (rT3) (Mayo Clinic)

• Reverse T3 (rT3) (Australian Clinical Labs)

• Vitamin B12 and folate (Pathology Tests Explained)

• Vitamin D (Pathology Tests Explained)

• Iron Studies (Pathology Tests Explained)

Taking biotin supplements in doses that result in blood concentrations that are higher than those occurring naturally in the body may interfere with blood tests measuring thyroid hormones, TSH, thyroglobulin and thyroid receptor antibodies (among other things). It's recommended to stop taking biotin supplements for at least two days prior to having your blood collected for these thyroid function tests. This may prevent misdiagnosis and, therefore, mistreatment. (Note: biotin doesn't interfere with thyroid function itself, only with the tests.)

More info: FDA Warns that Biotin May Affect Some Lab Test Results. Testing.com, 2018.

Patients medicated with thyroid hormones (e.g., levothyroxine, T3/T4 combination therapy) are advised to have their blood collected before taking their daily dose of thyroid medication, when their serum Free T4 level has to be measured.

TSH levels vary during the day; levels are lowest between 4.00 and 8.00pm and peak between 2.00 and 4.00am. Therefore, ideally, patients have their blood drawn at the same time of the day, every time tests need to be done.

Patients being treated for hypothyroidism should have their TSH tested 4-8 weeks after:

• starting treatment;

• a change in dose;

• a change of brand;

• starting/discontinuing taking medicines that influence the absorption, plasma binding or metabolism of thyroid medication.

Once stabilised on an adequate dose, TSH tests can be reduced to once every 6 months and then 12 months, unless persisting or new symptoms require more frequent testing.

The thyroid function of patients with only mildly elevated TSH (subclinical hypothyroidism < 10 m/L), who are not treated with thyroid medication, should be re-checked within 8 to 12 weeks after the initial blood test. Thyroid autoantibody levels should then also be measured, if this has not been done previously. If TSH levels normalise, no further testing is necessary. If TSH remains mildly elevated, TSH tests should be repeated 6 monthly for at least the first two years, and then annually. If the patient's TSH increases above 10 mU/L or progresses to overt hypothyroidism, treatment with thyroid hormone replacement drugs should be re-considered.

Patients being treated with anti-thyroid drugs for hyperthyroidism should have their Free T4 and Free T3 tested about 2 - 6 weeks after starting treatment, depending on the severity of their condition. Once a patient's hormone levels have normalised, their dose of anti-thyroid drugs is usually adjusted and they should have new blood tests in 4-6 weeks. As soon as a patient has normal hormone levels on a minimal dose of their medicine, blood tests can be reduced to once every 2-3 months (or 6 months if on long term therapy).

Thyroid Stimulating Antibodies, a test used to diagnose Graves' disease, is often tested again prior to taking a decision of stopping treatment with anti-thyroid drugs. The level of antibodies helps estimate the chance of remission.

• LabPlus: mostly tests for hospitals in Auckland and Pacific Islands

• Awanui Labs - North (formerly Northland Pathology; Labtests): Northland, Auckland region: Tāmaki-Makaurau, Waitemata, Counties Manukau, Waiheke Island

• Health New Zealand | Te Whatu Ora Waikato Laboratories (formerly Waikato DHB Laboratory): Waikato Hospital (Hamilton, Te Kuiti, Taumarunui, Tokoroa, Thames)

• Pathlab: Bay of Plenty, Lakes District, Waikato

• Awanui Labs - Central (formerly Taranaki Pathology Services; Southern Community Laboratories Hamilton, Hawke's Bay or Wellington): Taranaki, Hamilton, Hawke's Bay, Wellington region, Kapiti Coast, Hutt Valley, Wairarapa 

• Awanui Labs - South (formerly Medlab South, Southern Community Laboratories): Marlborough, Nelson, Tasman, Canterbury, South Canterbury, Otago, Southland

• Canterbury Health Laboratories: Canterbury, West Coast?