Thyroid function tests 2

.5 T4T3 Suppression Test

      The high uptake I131 can be further investigated by given T3(1000µg daily) or T4(0.1 mg TDS) for 10 days

*     Iodine deficiency related high uptake will be suppressed

*     While hyperactivity related high uptake due to Grave’s

      disease will not be affected

5. I131 Scan

Fig 31.4a & b

      Scanning helps to establish the functioning ability of nodules in comparison to adjoining tissue, excessive autonomous function of a nodule will result in suppression of rest of the thyroid (Hot Nodule), while decreased function will show up as a hollow in the scan contours (Cold nodule). These scans can also be done using techneium this scan can be done the same day thus saves patient’s time.

6. F.N.A.C

      Fine needle aspiration cyology is in practice today to obtain information on solitary thyroid nodule cell morphology.

7. Ultrasound Scan

      Ultrasonography can be used in children and pregnant women to obtain thyroid scan morphology without radiation hazards

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Thyroid function tests 1

TESTS OF THYROID FUNCTION

      T4, T3 estimations by radioimmunoassay are direct assessment of total T4, T3. These values correlate well with the functional status of the patient in most cases. These estimation can now also be done by ELISA technique.

      In case the thyroxine binding globulin (TBG) is high i.e. in those on contraceptive pills, there may be high values with normal function. Normal value of

-     T4 is 4-12 µg/100 ml

-     T3 is 100-200 µg/100 ml

-     Free T4+T3 can also be of value

4.1 Serum TSH Estimation

      This is a very sensitive test of thyroid function, since it responds to circulating levels of T4+T3. In cases of hypothyroidism, the TSH value is high.

Normal values range from 0-5 micro-units/ml

4.2 Iodine Studies

      Since iodine is the principal element required in the biosynthesis of T4,T3 any study of iodine content of serum and iodine handling by the thyroid would provide an indirect evidence regarding thyroid function.

4.3 Protein Bound Iodine

      In the past in the absence of reliable estimations of T4+T3 Protein Bound Iodine (PBI) provided indirect evidence regarding thyroid function.

*     Nearly 99% of thyroxine (T4) and othe iodothyronines in blood are bound to TBG and thus measuratleas PBI.

*     The normal range of PBI is 3.5-7.5 µg/100 ml

*       Unfortunately the level of TBG increases with the increase of oestrogens (Pill, Pregnancy and Liver disease). Oral intake of iodides (Expectorants, Iodo-hydroxy quinilones and X-ray Contrast Media) can also raise PBI, thus rendering this test unreliable

4.4 I131 Uptake

      The percent of I131 taken up by the gland at 2 and 24 hours can be compared with the uptake in normal subjects in any given country.

*     High uptake can be due to iodine deficient diet or hyperthyroidism

*       Unfortunately, this test result can be altered by intake of iodides (Expectorants and othe
rs)

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Thyroid hormones synthesis regulation

2. SYNTHESIS OF THYROID HORMONES

      The metabolism of iodine leading to the biosynthesis of thyroid hormones occurs in sequential stages:

*     The first step is active transport of iodine into the thyroid (iodine trap)

*       Oxidation of iodide and iodination of tyrosil residues within thyroglobulin to yield the hormonally inactive iodotyrosines mono and diodotyrosines (MIT and DIT) by thyroid peroxidase enzyme.

*     The coupling of these MIT and DIT to form hormonally active iodothyroines, notably T4and T3.

3.REGULATION OF HYPOTHALAMIC-

PITUITARY THYROID COMPLEX

      TSH is a glycoprotein hormone secreted by a specific cell type, the thyrotropic cell, located principally in the anteromedial portion of the anti pituitary.

*     TSH is the major regulator of the morphological and functional state of the thyroid.

*     Removal of TSH stimulation is followed by hypovascularity and atrophy of the thyroid gland accompanied by decreased sythesis and secretion of hormone (T4, T3)

*     The effects are reversed by stimulatory doses of TSH

*     Regulation of TSH secretion results from a complex interaction, mainly or entirely at the level of the pituitary thyrotropic cell.

*     By which Thyrotrophin releasing hormone (TRH) produced in the hypothalamus acts to stimulate the release and later the synthesis of TSH.

*     While high or low thyroid hormone levels (T4, T3) inhibit and stimulate these functions through Negative and positive feed back action 

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Endocreine system Thyroid anatomy IODINE METABOLISM

THYROID

ANATOMY

      The thyroid is developed from the thyroglassal duct which starts at the base if the tongue (foramen cerium) the rest of the duct disappear leaving the two lobes the isthmus and pyramidal lobby.

      The bilobed gland weighs 20-25gms.  The gland is placed on either side of the larynx on the thyroid cartilage and the upper rings of the trachea a lobule has 20-40 follides lined with epthelium. The enclosed space contains thyroglobalin. The gland has an extensive network of lymphatics arteries and veins.

1. METABOLISM OF IODIDE

-     Formation of normal quantities of thyroid hormone ultimately depends upon the availability of adequate quantities of exogenous iodine.

-     Normally iodine balance is maintained through dietary sources.

-        Daily dietary intake of iodine depends on the iodine content of soil and water through which iodine enters our food chain i.e. vegetables, animals and drinking water.

-     Iodine is ingested in both the inorganic and the organically bound forms.

-     Eventually it is made available as inorganic iodide

-     Iodide itself is rapidly and efficiently absorbed from the gastrointestinal tract, and little is lost in the stool.

-     Iodine used in the synthesis of thyroid hormone is drawn from the inorganic iodide of the extracellular fluid.

-     The iodide that is cleared in the urine is partly replenished both by iodide lost from the thyroid into the blood from unconjugation of tyrosine and by iodide liberated through deiodination of thyroid hormones in peripheral tissues.

      Normal iodide intake in the west is about 500 µg of iodine is cleared into the urine daily, in the inorganic form. The second major site of removal of iodine from the extracellular fluid is the thyroid. Iodide removed from plasma by the thyroid is not lost, since ultimately it is secreted into the circulation either as iodinated thyronines T4 and T3.

      Under normal circumstances, approximately 8000µg of iodide is in the form of iodinated amino acids.

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