Click here for Frequently Asked Questions on Blood Tests.
Thyroid blood tests generally include determination of the levels of circulating thyroid hormones (Free T4 and Free T3 and thyroid stimulating hormone TSH). These tests, especially the TSH, are highly sensitive and reliable, and the levels of thyroid hormones or TSH do not fluctuate widely during the day, or from day to day. Hence it is highly unlikely that a significant disturbance of thyroid function (hypo or hyperthyroidism) is present if the TSH is normal, even if only a single TSH determination is carried out. The results of thyroid blood tests can be affected by other medications a patient may be taking, so be sure that this information is provided to your physician. Patients who are being treated for hyperthyroidism usually require more than just a TSH determination to assess their thyroid status, since the TSH level can remain low for a prolonged period of time, even as the hyperthyroidism is getting better and the levels of Free T4 and T3 are dropping.
TSH is made by specialized cells in the pituitary called thyrotrophs. These cells are highly sensitive to the levels of circulating thyroid hormone in our body, and function like a sensitive thermostat. If the levels of thyroid hormone drop, the pituitary gland makes more TSH to stimulate the thyroid gland to work harder and make more thyroid hormone. Accordingly, an increased level of TSH strongly suggests the presence of hypothyroidism. Conversely, if thyroid hormone levels are too high, the pituitary gland shuts off production of TSH and the level of TSH will become low, or even undetectable, indicating a hyperthyroid state. The reliance on TSH as a key indicator of thyroid status depends on normal function of the hypothalamus and pituitary. Patients with a history of pituitary disease may not always be able to produce TSH normally, rendering the TSH less than 100% reliable in some patients with known pituitary problems.
Although the TSH test is currently considered the gold standard for the initial assessment of thyroid function, one must remember that the TSH tells us only what the pituitary thinks of the circulating levels of thyroid hormone. Although the TSH is the best indicator we have for knowing what our body thinks of the levels of thyroid hormones, it remains possible that there may be subtle differences in how our peripheral tissues (liver, muscles, heart etc) sense optimal levels of thyroid hormone. At present, it is not possible to sensitively or specifically assess how peripheral tissues perceive levels of thyroid hormone, so the TSH test remains our best surrogate marker of thyroid function. Although other parameters have been advocated as sensitive indicators of thyroid function, such as basal body temperature, these parameters are not specific indicators of thyroid status and may be influenced by other variables independent of thyroid function.
The optimal level of TSH to aim for may differ in various patients, depending on the precise diagnosis of thyroid disease present and what the clinical endpoints are. Moreover, there is some evidence that "normal" TSH levels may vary with age, as older individuals may have somewhat higher levels of TSH, yet still have no evidence of thyroid dysfunction, as outlined in Age-Specific Distribution of Serum TSH and Antithyroid Antibodies in the United States Population; Implications for the Prevalence of Subclinical Hypothyroidism. J Clin Endocrinol Metab. 2007 Oct 2; [Epub ahead of print] and Age-Related Changes in Thyroid Function: A Longitudinal Study of a Community-Based Cohort J Clin Endocrinol Metab. 2012 Feb 16
Although newer TSH assays may indicate that many individuals have TSH levels that appear somewhat lower with the current generation of assays, there is no strong consensus among experts about whether the lower limit of normal for TSH determinations should be redefined, as outlined in The thyrotropin reference range should remain unchanged. J Clin Endocrinol Metab. 2005 Sep;90(9):5489-96 and in The evidence for a narrower thyrotropin reference range is compelling. J Clin Endocrinol Metab. 2005 Sep;90(9):5483-8. A brief discussion of this issue is also found in the section on osteoporosis.
Thyroid antibodies may be ordered to search for evidence of autoimmune thyroid diseases, such as Hashimoto's or Grave's disease. Thyroid antibodies may remain positive for years, and do not provide an indication of whether the person has normal or abnormal thyroid function. Furthermore, some patients with Hashimoto's disease may have negative levels of circulating antibodies, and conversely, patients with positive levels of thyroid antibodies may never develop thyroid disease during their lifetime. It is generally not useful to repeatedly measure levels of thyroid antibodies in the blood. Although the presence of antibodies predicts a slightly higher rate of progression to hypothyroidism, some studies suggest that a slightly higher TSH in the mid upper normal range might convey the same prognostic information. See Serum thyrotropin is a better predictor of future thyroid dysfunction than thyroid autoantibody status in biochemically euthyroid patients with diabetes: implications for screening. Thyroid. 2004 Oct;14(10):853-7.
A Thyroglobulin level may be useful for monitoring patients with thyroid cancer, particularly after surgery and treatment with radioactive iodine. Many benign thyroid diseases can be associated with increased levels of thyroglobulin, hence an elevated thyroglobulin alone is not a specific indicators for the presence or absence of malignancy. At the Toronto General Hospital, normal values are:
0 - 34 ug/L for patients who still have their thyroid gland and
0 - 3 ug/L for
Thyroid stimulating immunoglobulins may be measured in patients with Grave's disease, but are not required to make a clinical diagnosis. The presence or absence of these antibodies in a mother may also provide predictive information about whether a fetus or infant has an increased risk of developing transient hyperthyroidism.
A calcitonin blood test may be ordered if a diagnosis of medullary thyroid carcinoma (MTC) is suspected. Occasionally, a stimulation test may be carried out to examine the increase in levels of calcitonin in response to secretory agents such as calcium or pentagastrin.
A serum calcium may be done to assess parathyroid function, particularly in patients following thyroid surgery. For more information on regulation of blood calcium, see the parathyroid gland section.
Do the levels of thyroid antibodies vary depending on the assay used?
Yes, the various test kits can give different results, which may influence the interpretation of thyroglobulin levels. See Comparison of some different methods for analysis of thyroid autoantibodies: importance of thyroglobulin autoantibodies. Thyroid. 2001 Mar;11(3):265-9.
My levels of Free T4 and T3 are normal, yet my TSH is abnormal, how can this be?
The levels of circulating thyroid hormones that are "normal" for any given individual may vary tremendously. For example, one subject may have a normal level of Free T4 at the upper end of the normal range, and a slight decrease in this Free T4 may cause an increase in TSH, despite still "high normal" levels of Free T4. Conversely, another individual may have a "normal" Free T4 at the lower end of the range, and even a slight increase in this persons level of circulating thyroid hormones may be associated with mild hyperthyroidism and a suppressed TSH. Hence, one needs to individualize each persons range of normal circulating thyroid hormones, and look at the laboratory limits of what is normal for a large population in this context. See Narrow individual variations in serum t(4) and t(3) in normal subjects: a clue to the understanding of subclinical thyroid disease. J Clin Endocrinol Metab. 2002 Mar;87(3):1068-72.
Why do my levels of thyroglobulin (Tg) fluctuate?
There are several possibilities, including the presence or absence of antibodies against thyroglobulin, and the use of different assays to measure thyroglobulin, as outlined in Discordant serum thyroglobulin results generated by two classes of assay in patients with thyroid carcinoma: correlation with clinical outcome after 3 years of follow-up. Cancer. 2003 Jul 1;98(1):41-7. Indeed, the newer more sensitive thyroglobulin assays which do not use radioactive tracers are more widely used, but are more prone to interference from circulating antithyroglobulin antibodies which can greatly compromise the accuracy of the thyroglobulin test. Hence, measurement of thyroid antibodies should always be done at the same time the thyroglobulin is measured. Some experts have advocated that the use of the newer highly sensitive Tg assays may one day supplant the need for a TSH-stimulated thyroglobulin in many patients. For an excellent overview of this area, see Measuring thyroglobulin and thyroglobulin autoantibody in patients with differentiated thyroid cancer Nat Clin Pract Endocrinol Metab. 2008 Apr;4(4):223-33
I have heard that measurement of the levels of thyroglobulin (Tg) RNA in a blood test may be a more sensitive way to look for recurrent tumors-is this test useful?
Although theoretically measurement of very low levels of Tg RNA in the circulation might provide a more sensitive test than the currently utilized blood test which measures the level of the circulating protein, ongoing evaluation of the clinical utility of such a test has shown inappropriately low utility, with suboptimal specificity in follow-up of patients with thyroid cancer. See Low specificity of blood thyroglobulin messenger ribonucleic acid assay prevents its use in the follow-up of differentiated thyroid cancer patients. J Clin Endocrinol Metab. 2004 Jan; 89(1): 33-9.
Does age affect levels of thyroid hormones?
Although levels of T4 and T3 do not change much in older individuals, the TSH response to hypothyroidism is age-dependent, with older subjects demonstrating a less robust TSH elevation in response to declining thyroid hormone levels Age and the Thyrotropin Response to Hypothyroxinemia J Clin Endocrinol Metab. 2010 May 19. [Epub ahead of print]