Theory of Thyroid Panel
Thyroid hormones regulate the energy metabolism of the body, acting much as a governor on
a car or a thermostat. When the body is too hot or energy reserves are low,
thyroid hormone is reduced. Conversely, when the body is too cold and energy reserves are high,
thyroid hormone is increased.
Three main endocrine glands are involved in regulating thyroid hormone in the body:
The hypothalamus and pituitary gland (in the brain) and the thyroid gland (in the throat).
For a more in-depth treatment of thyroid hormone labs, see
Dickson B. Dunlap: Thyroid Function Tests
The main hormones involved in the thyroid system are:
- TRH (Thyroid Releasing Hormone) - produced by the hypothalamus.
- TSH (Thyroid Stimulating Hormone) - produced by the pituitary gland.
- T4 (Thyroxine, tetraiodothyronine) - produced by the thyroid gland.
Increases oxygen availability in all tissues [Friedman2005, pg 93].
- T3 (triiodothyronine) - made from T4, primarily in the liver and kidneys. Most active
thyroid hormone; increases oxygen availability in all tissues and interacts
with the mitochondria to increase energy production and thus regulate metabolic rate
[Friedman2005, pg 93]; [Starr2005].
The three endocrine glands work together in a system of negative feedback, constantly monitoring
and adjusting the status of the body:
- Low free-T4 levels in the blood prompts the hypothalamus to produce TRH.
- TRH prompts the anterior pituitary gland to produce TSH.
- TSH prompts the thyroid gland to release T4, which is produced by the thyroid gland.
- T4 is converted to T3 in the liver and kidneys.
- T3 is then disseminated in the blood throughout the body.
- If T4 or T3 levels rise too high, a negative feedback loop to the hypothalamus reduces TRH
production to maintain homeostasis.
One author suggests that the peripheral cells have some signaling mechanism back to the
hypothalamus to indicate a need for more circulating thyroid hormone, but does not
make clear the nature of this signaling mechanism [Starr2005, pg 67]. Most authors
consider free-T3 or free-T4 in the blood to be the primary signaling mechanism back to the
hypothalamus.
TSH (Thyroid Stimulating Hormone)
When TSH is within the normal range, conventional medicine considers that the upper control
centers in the brain hypothalamus and pituitary) have established homeostasis.
However, researchers have found that the symptom picture does not always correlate with TSH
levels [Zulewski1997].
Practitioners of functional medicine note that in cases of peripheral resistance to T4,
normal levels of TSH and T4 may be insufficient to eliminate the signs and symptoms of
hypothyroidism. In this case,
supplementation with desiccated thyroid or custom-formulated T4/T3 blend may be necessary to
attain remission of symptoms, even though TSH may be driven to subnormal levels (less than 0.01)
that most allopathic doctors consider indicative of
hyperthyroidism [Starr2005, pg 63]
Total-T4
Total-T3
T3 is the most biologically active thyroid hormone. Low levels of total-T3 when compared with
total-T4 indicate poor conversion of T4 into T3 by the peripheral tissues.
Reverse-T3
This is an inactive isomer of T3 that may be elevated when T3 is low. Elevated reverse-T3 may
indicate a selenium deficiency or caloric restriction.
Free-T4
This is the fraction of total-T4 that is not bound to blood proteins and hence represents
the amount of T4 actually available to the tissues. Free-T4 should be within
the upper limit of normal [Starr2005, pg 168]. There are several different methods
for measuring free-T4; The non-dialysis immunoassay) method is most commonly used.
However, according to
Mayo Clinic
"abnormal types or quantities of binding proteins found in some patients and most often related
to other illnesses or drug treatments, may interfere in the accurate measurement of free-T4
by analog immunoassays. These problems can be overcome by measuring free-T4 by equilibrium dialysis,
free from interfering proteins."
See also
Sonora Quest
Free-T3
This is the fraction of total-T3 that is not bound to blood proteins and hence represents
the amount of T3 actually available to the tissues. Free-T3 may be elevated if thyroid
medications are taken in the morning before the blood draw, otherwise should be within
the upper limit of normal [Starr2005, pg 168].
24 hour Urine T3
Another approach that has been discussed in the literature is an assay of T3 in 24-hour sample of
urine. While this test is reported to accurately correlate with symptoms of hypothyroidism,
the test is not widely used (yet) [Starr2005, pg 71]; [Hertoghe2001].
References regarding Thyroid Panel
[Friedman2005] Michael Friedman.
Fundamentals of Naturopathic Endocrinology
Toronto: Canadian College of Naturopathic Medicine (CCNM) Press (2005).
[Hertoghe2001] J. Hertoghe et al.
Thyroid insufficiency. Is thyroxine the only valuable drug?
Journal of Nutritional & Environmental Medicine 11:159-166 (2001).
Cited by [Starr2005, pg 176].
[Pagana2011] Kathleen Deska Pagana & Timothy J Pagana.
Mosby's Diagnostic and Laboratory Test Reference, Tenth Edition.
St Lewis: Elsevier/Mosby (2011).
[Starr2005] Mark Starr.
Hypothyroidism Type 2: The Epidemic
Columbia, MO: Mark Starr Trust (2005). 
Amazon
Updated 2011 added: Hashimoto's & Grave's diseases.
[Zulewski1997] Henryk Zulewski, Beat Müller, Pascale Exer, André R Miserez, Jean-Jacques Staub.
Estimation of tissue hypothyroidism by a new clinical score: evaluation of patients with
various grades of hypothyroidism and controls.
J Clin Endocrinol Metab. 1997 Mar;82(3):771-6. PMID: 9062480
Cited by [Starr2005, pg 70].
Unless specifically noted above, references used in the construction of this web
page include the following:
[FMU]
Lecture notes from Functional Medicine University.
[SCNM]
Lecture notes from Southwest College of Naturopathic Medicine.
[UT]
Lecture notes from the University of Tennessee graduate programs in
Chemistry, Microbiology, and Biochemistry.
