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Thyroid Blood Tests to Diagnose a Thyroid or Hormone Imbalance by Barbara Lougheed

Thyroid Blood Tests to Diagnose a Thyroid or Hormone Imbalance by Barbara Lougheed


[updated 6/9/12]

A healthy thyroid produces mostly T4 and just a little T3 [1], so the most important thyroid blood test is the total T4, because that tells whether there is even enough thyroid hormone to work with. Free T4 (FT4) can be artificially elevated from certain drugs like aspirin, diuretics, or heparin, so it is important to know the total T4 to get an accurate picture of thyroid levels. [2,3] In acute illness, the fall in in the totals is much more pronounced than the fall in the free levels, which is why its important to measure both total and free T4. [7]

Free T3 (FT3) is next most important thyroid blood test because if that lab result is on the low end while total T4 is mid-range or higher, it suggests improper conversion. If thats the case, one should correct factors that impede conversion like low selenium, low ferritin, etc. [4]

Free T4 would be the next thyroid blood test to look at, to see whether it correlates to the total T4 level. Sometimes there is an excess of thyroid hormone binding proteins, leading to a low Free T4, even though there is ample total T4. [5,6] This could be caused by high thyroid binding globulin which is caused by high estrogen levels, pregnancy, or non-thyroidal illness. Oftentimes the Free T4 can look close to mid-range, but the total T4 will show the true picture of a deficit, since it will be below mid-range.

Total T3 is not as good a barometer of healthy levels as Free T3, because it is the unbound or free part that exerts the biological effect. Free T3 levels correlate better with symptoms. My Free T3 has been over range (121%) while Total T3 was at 75% of range. Symptoms indicated I was on too much T3, not too little. Another patient also presented with a Free T3 over range (110%), but a Total T3 at only 41% of range. His SHBG and A1C were also over range, which suggests that T3 levels were too high, not low.

A study of 832 hypothyroid patients showed that 24 hour urine Free T3 had the highest inverse correlation with clinical symptoms. In other words, the higher the urine Free T3, the lower the symptoms and vice versa. Urine Free T3 is not influenced by binding globulins and correlated well with the severity of eight clinical hypothyroid symptoms: fatigue, depression, coldness, headache, muscle cramps, constipation, arthritis, and Achilles tendon reflex. Serum T4, Free T4, and TSH often had no correlation to these symptoms. [10]

Free T3 was the strongest (based on statistical analysis) and only independent predictor of mortality (lower the FT3, higher the chance of mortality) in a study in an intensive care unit (ICU). Individually, Total T3, Free T4, Total T4, TSH and reverse T3 could not predict the patients outcome. Interestingly, rT3 levels were similar in survivors and non-survivors. [8]

FT3 was the only thyroid lab result that was consistently out of range, in spite of the fact that these patients were seriously ill and in ICU. The percentages were calculated from 480 ICU patients in the study (59.79% male; mean age 71.7115.52 years). [8]

Total T3: 4.79% Low
Free T3: 54.38% Low
Total T4: 11.04% Low
Free T4: 10.00% Low
TSH: 3.54% High
rT3: 6.25% High

Another study of healthy men in Finland compared the same thyroid lab results over 14 months to see if there was any correlation with colder temperatures. Again, only Free T3 showed a statistically significant correlation with severe cold, being lowest in February and highest in August. Serum Total T3, Total T4, Free T4, rT3, and urinary T4 were basically unchanged throughout the year. Urinary T3 displayed an inverse relationship with serum T3, being significantly higher in winter than in summer, which indicates higher clearance or usage of the hormone by the body over the colder months. Interestingly, TSH did not show any correlation to the thyroid hormone levels, but did change throughout the year, with a peak in December, when sunlight is lowest. [9]

And yet, Free T3 is not routinely measured in someone being tested for hypothyroidism. It must be specifically requested and is not on most lab forms; TSH and T4 or FT4 are what are usually ordered, even though TSH levels often do not reflect thyroid levels, and T4 is not as biologically active as T3. Many have decent T4 levels with multiple hypothyroid symptoms, and the root problem only becomes apparent when Free T3 is tested.

A good baseline of tests to request:

FREE T3 (not part of a thyroid panel and must be written in on a lab request)




TPO (Thyroid Peroxidase) antibodies

Tgab (Thyroglobulin) antibodies

TSI (Thyroid Stimulating Immunoglobulin) if Graves hyperthyroidism is suspected

TRab (TSH Receptor) or TBII (Thyrotropin Binding Inhibitory Immunoglobulin) antibodies if Graves hyperthyroidism is suspected; either test is fine, they are the same test called different names by different labs

Full iron panel (includes serum iron, iron binding capacity, and % saturation) [iron is essential to thyroid metabolism]

Ferritin (not included in the full iron panel and must be requested separately)

CBC or Complete Blood Count (includes RBC, hemoglobin, hematocrit, MCH)

Vit D

Vit B12

Lipid panel (includes cholesterol, triglycerides)
[high cholesterol is a symptom of low thyroid]

Comprehensive Metabolic Panel (includes fasting glucose, sodium, potassium)

Morning cortisol (24-hr saliva labs preferred, but at least this gives one reading) [cortisol is essential to thyroid metabolism]

Estradiol (men and women; women should test on day 19-21 of their cycle, or 5-7 days after ovulation)

Progesterone (men and women; women should test on day 19-21 of their cycle, or 5-7 days after ovulation)

Total testosterone (men and women)

SHBG (Sex Hormone Binding Globulin) men and women [high SHBG correlates with osteoporosis or bone loss]

[reference links inactivated for search engines; copy and paste the url at the end of the reference into your browser if you want to view the reference]

1. Sapin R, Schlienger JL. [Thyroxine (T4) and tri-iodothyronine (T3) determinations: techniques and value in the assessment of thyroid function]. Ann Biol Clin (Paris). 2003 Jul-Aug;61(4):411-20.

2. Lim CF, Bai Y, Topliss DJ, Barlow JW, Stockigt JR. Drug and fatty acid effects on serum thyroid hormone binding. J Clin Endocrinol Metab. 1988 Oct;67(4):682-8.

3. Stockigt JR. Free thyroid hormone measurement. A critical appraisal. Endocrinol Metab Clin North Am. 2001 Jun;30(2):265-89.

4. Michael B. Zimmermann, Josef Khrle. The Impact of Iron and Selenium Deficiencies on Iodine and Thyroid Metabolism: Biochemistry and Relevance to Public Health. Thyroid. October 2002, 12(10): 867-878.

5. Bartalena L, Bogazzi F, Brogioni S, Burelli A, Scarcello G, Martino E. Measurement of serum free thyroid hormone concentrations: an essential tool for the diagnosis of thyroid dysfunction. Horm Res. 1996;45(3-5):142-7.

6. Ferrari C, Paracchi A, Parisio E, Codecasa F, Mucci M, Boghen M, Gerevini G, Rampini P. Serum free thyroid hormones in different degrees of hypothyroidism and in euthyroid autoimmune thyroiditis. Acta Endocrinol (Copenh). 1987 Apr;114(4):559-64.

7. Warner MH, Beckett GJ. Mechanisms behind the non-thyroidal illness syndrome: an update. J Endocrinol. 2010 Apr;205(1):1-13. Epub 2009 Dec 16.

8. Feilong Wang, Wenzhi Pan, Hairong Wang, Shuyun Wang, Shuming Pan and Junbo Ge. Relationship between thyroid function and ICU mortality: a prospective observation study. Critical Care 2012, 16:R11 doi:10.1186/cc11151. Published: 19 January 2012.

9. J Hassi, K Sikkil, A Ruokonen and J Leppluoto. The pituitarythyroid axis in healthy men living under subarctic climatological conditions. Journal of Endocrinology (2001) 169, 195203.

10. W. V. Baisier, J. Hertoghe and W. Eeckhaut. Thyroid Insufficiency. Is TSH Measurement the Only Diagnostic Tool? Journal of Nutritional & Environmental Medicine (2000) 10, 105113.


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