3. Objectives of this lecture:
1. Hypothalamus- pituitary- thyroid axis
2. Common thyroid health problems
3. Tests performed for diagnosis of various
thyroid problems in labs
4. Normal values of different parameters
5. How to interpret the tests.
6. Differential diagnosis different thyroid
disorders.
4.
5.
6. T4
T4- Alb
T4- TTR T3
T4-TBG (70-75%)
T3-TBG (70-75%)
T3- TTR
T3- Alb
RT3
Thyroid Hormone Transport
TTR= transthyretin, TBG = thyroid binding globulin; RT3 = reverse T3
7. Given the protein levels, how does T4/T3
distribute across TBG,TTR, & Alb?
9. Thyroid function Tests performed in labs
• TRH
• Plasma FT4
• Total T4
• Plasma FT3
• Total T3
• RT3
• Free T4 index (FTI)
• TSH
• TBG
• TPO Ab (thyroid peroxidase
antibodies)
• TBG Ab (thyroglobulin antibodies)
• TSI (thyroid stimulating
immunoglobulins)
• TRAb (TSH receptor antibodies)
• Scintigraphy with radioactive iodine
Not all tests are useful in all situations
10. Approximate reference values
FT4 (0.9 − 2.5 ng/dL)
TT4 (5.0- 10.5 μg/dl)
FT3 (20- 45 pg/dL)
TT3 (70- 200 ng/dL)
TSH (0.4- 4.0 mIU/L)
FTI ( 1.1- 4.3 μg/dl)
TBG (1.5 to 3.4 mg/dl)
Urine Iodine (100-200mg/L)
Blood iodine (45-100 μg/l)
1. There are different reference values for children and pregnant
women
2. Reference range vary from lab to lab please check your local
guidelines.
12. The serum TSH is the best initial test of thyroid
function.
13. TSH; A high TSH level indicates that the
thyroid gland is not making enough thyroid
hormone (primary hypothyroidism).
The opposite situation, in which the TSH level
is low, usually indicates that the thyroid is
producing too much thyroid hormone
(hyperthyroidism).
Occasionally, a low TSH may result from an
abnormality in the pituitary gland adenoma,
which prevents it from making enough TSH to
stimulate the thyroid (secondary
hypothyroidism)
Then further tests are required
19. 1. A normal TSH and normal T4 show a Euthyroid.
2. A low TSH and high T4 generally means an Hyperthyroid.
3. A high TSH and low T4 means an Hypothyroid.
4. A high TSH and normal T4 means an subclinical Hypothyroid
5.The T3 usually ordered if T4 tests show elevation. confirmatory test.
6. TBG Lab Tesst: TBG binds T3 and T4/ hormone are inactive when
bound to TBG increased in hypothyroid, (false result increased in
Pregnancy, Oral Contraceptive Pills, hepatitis, Phenothiazine, opiates)
7. Thyroglobulin (Tg) Tg is not a primary measure of thyroid hormone
function. Levels can by elevated in thyroiditis, Graves’disease or
thyroid cancer. Used to monitor effectiveness of treatment
6. Iodine deficiency: The iodine in blood and urine is checked to
assess the iodine levels
Screening test or patch test for iodine deficiency is done on mass
level. PI2 on forearm disappears instantly in deficiency.
20. 7. FTI: Thyroxine (T4)/thyroid binding capacity.
Hyperthyroidism causes increased FTI, and hypothyroidism causes
decreased values.
8. Thyroid Peroxidase (TPO) antibodies, or Antithyroid
Peroxidase Antibodies (TPOAb).
Peroxidase enzyme that plays a part in the T4-to-T3
conversion and synthesis process.
TPO antibodies can be evidence of tissue destruction,
such as Hashimoto's thyroiditis and other thyroiditis such
as post-partum thyroiditis.
9,Testing for thyroglobulin antibodies (also called
antithyroglobulin antibodies) is common. Graves' disease, having
high levels of thyroglobulin antibodies means that you are more
likely to eventually become hypothyroid.
21. 10. Stimulatory TSH receptor antibody (TSI). This antibody
causes the thyroid to be overactive in Graves’ Disease.
TSH receptor antibody test (TSHR or TRAb), which detects
both stimulating and blocking antibodies.
levels in Graves’ patients may help to assess response to
treatment of hyperthyroidism, to determine when it is
appropriate to discontinue antithyroid medication, and to assess
the risk of passing antibodies to the fetus during pregnancy.
11.. Radioactive iodine uptake test (RAIU): Individual
swallow iodine pill or liquid, which is radioactive. Iodine is
taken up by thyroid. Radioactive I131 emits rays which is
recorded by gamma counter probe. High uptake in
thyrotoxicosis
22. Indirect Links to Other Systems
Give false results
Glucocorticoid
Excess ↓ TSH, TBG, TTR, T3, T4, ↑rT3
Deficiency ↑ TSH
Estrogens
TBG sialylation & serum t1/2
T4 requirement in hypothyroidism
↑ TSH in postmenopausal women
Androgens
TBG
↓ T4 turnover in women
T4 requirement in hypothyroidism
23. Differential Diagnosis of different thyroid disease
• Primary hypothyroidism: Low T4 and a raised TSH.
Autoimmune thyroiditis: TPO Ab and TBG Ab
Iodine deficiency Hypothyroid: Iodine deficiency
• Subclinical hypothyroidism: normal T4 and a raised TSH.
• Secondary hypothyroidism: Low T4 and T3 and low TSH
Decreased production or secretion of TRH and TSH results in
decreased stimulation of the thyroid gland cause decreased
thyroid hormones. Secondary to Pituitary or hypothalamic
disorders. Pituitary or hypothalamic tumors.
24. • Primary hyperthyroidism: Raised T3 and T4 and low TSH
• Causes of primary hyperthyroidism include:
• Graves’ disease (75% of all cases)
• Toxic multinodular goitre
• Toxic adenoma
Secondary hyperthyroidism :Raised T3/T4 and Raised TSH
Due to stimulation of the thyroid gland by excessive thyroid-
stimulating hormone (TSH)
Causes
• TSH-secreting tumours
• Chorionic-gonadotropin secreting tumours (hCG secreting)
