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THYROID HORMONE SECRETION. CONTROL & DIAGNOSIS. THYROID STIMULATING HORMONE (TSH). Also called Thyrotropin. Secreted by the ANTERIOR PITUITARY. A glycoprotein with a m.w: 28,000. Main function : It increases the secretion of both Tri-Iodothyronine & Thyroxine by the Thyroid Gland.

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thyroid hormone secretion

THYROID HORMONE SECRETION

CONTROL & DIAGNOSIS

thyroid stimulating hormone tsh
THYROID STIMULATING HORMONE (TSH)
  • Also called Thyrotropin.
  • Secreted by the ANTERIOR PITUITARY.
  • A glycoprotein with a m.w: 28,000.
  • Main function: It increases the secretion of both Tri-Iodothyronine & Thyroxine by the Thyroid Gland.
thyroid stimulating hormone
THYROID STIMULATING HORMONE

Mechanism of Action:

TSH + TSH receptors (a typical G-protein coupled receptor) on the thyroid follicular cell membrane of the Thyroid Gland

Adenylyl Cyclase stimulated

ATP→ cAMP

Protein kinase A is activated

Multiple phosphorylations throughout the cell

  • Immediate increase in thyroid hormone secretion
  • Stimulates growth of the thyroid glandular tissue
thyroid stimulating hormone1
THYROID STIMULATING HORMONE

EFFECTS ON THE THYROID GLAND:

  • Increased secretion of TG and proteolysis of the TG already stored in the follicular cells.
  • Increased activity of the NIS (SYMPORTER) so that Iodide Trapping is increased.
  • Stimulates the step Organification.
  • Increased number, size & secretory activity of the thyroid cells (with change into columnar from cuboidal).
thyroid stimulating hormone2
THYROID STIMULATING HORMONE

IN SUMMARY: TSH increases all the known secretory activities of the thyroid gland!

MOST IMPORTANT IS PROTEOLYSIS WHICH CAUSES RELEASE OF THE TH INTO THE BLOOD STREAM WITHIN 30 MINUTES!

slide6
ANTERIOR PITUITARY SECRETION OF TSH IS REGULATED BY THYROTROPIN-RELEASING HORMONE FROM THE HYPOTHALAMUS
thyrotropin releasing hormone trh
THYROTROPIN RELEASING HORMONE (TRH)
  • A tripeptide amide (pyroglutamyl-histidyl-proline-amide)
  • Secreted by the nerve endings in the median eminence of hypothalamus

Mechanism of secretion:

TRH + TRH receptor in the pituitary cell membrane of the Pituitary Gland

Phospholipase C second messenger system activated

TSH released

hypothalamic hypophysial pituitary axis
HYPOTHALAMIC-HYPOPHYSIAL PITUITARY AXIS

Hypothalamus

TRH

Hypothalamic-Hypophysial portal system

Anterior pituitary

TSH

Thyroid gland

Tri-iodothyronine & Thyroxine

slide9
IF TYROID HORMONE IS DEFICIENT, ITS SECRETION IS STIMULATED THROUGH THIS FEEDBACK SYSTEM!

&

VICE VERSA!

points to remember
POINTS TO REMEMBER:
  • EXCITEMENT & STRESS CAUSE A DECREASE IN THE TRH SECRETION & therefore TSH secretion.
  • Cold in infants increases the secretion of TRH by the hypothalamus & thus causes an increase in the TSH secretion!

BOTH THESE EFFECTS ARE NOT SEEN WHEN THE HYPOTHALAMIC- HYPOPHYSIAL TRACT HAS BEEN CUT SHOWING THAT THESE EFFECTS ARE MEDIATED THROUGH THE HYPOTHALAMUS

remember
REMEMBER:
  • When the rate of TH secretion increases by 1.75 times normal, the rate of TSH secretion falls essentially to zero!
  • This effect on the anterior pituitary is seen even when Anterior Pituitary has been separated from hypothalamus!
thyroid function tests1
THYROID FUNCTION TESTS

1. TESTS RELATED TO HORMONE CONCENTRATION IN BLOOD:

  • Serum total T4 conc.: normal value is 5- 12 µg/ dl
  • Serum total T3 conc.: normal value is 70-190 µg/ dl
  • Serum free T4 & T3 conc.: these values represent physiologically active forms of T4 & T3

ALL THE ABOVE VALUES ARE RAISED IN HYPERTHYROIDISM & DECREASED IN HYPOTHYROIDISM

thyroid function tests2
THYROID FUNCTION TESTS

2. DIRECT TESTS OF THYROID FUNCTION:

- RADIOACTIVE IODINE UPTAKE TEST (RAIU): A small dose of 123I is given orally & fraction taken up by the thyroid gland after 24 hours is found by taking pictures at 4 & 24 hours to measure its radioactivity.

Normally this value is about 30%.

  • It is raised in Hyperthyroidism & decreased in Hypothyroidism.
  • Also used to assess residual thyroid tissue after

thyroid gland removal.

  • Determine recurrence of thyroid cancer
thyroid function tests3
THYROID FUNCTION TESTS

3. INDIRECT TESTS OF HOMEOSTATIC CONTROL:

  • Plasma conc. Of TSH: normal level is 0.3- 3 mu/l.
    • LOW in Hyperthyroidism
    • Increased in Hypothyroidism of thyroid origin
    • LOW or normal in pituitary or hypothalamic Hypothyroidism

TSH is the Best single test to screen for thyroid disease.

  • Levels of TSH reflect the amount of free, biologically active TH.
  • TSH is the best test to monitor thyroid replacement therapy.
  • TSH can remain misleadingly high with the

initiation of thyroid replacement, “pituitary reset”.

  • Wait 6-8 weeks before repeating TSH after starting the therapy.
thyroid function tests4
THYROID FUNCTION TESTS

4. MISCELLANEOUS TESTS:

  • Antithyroid antibodies & Antithyroglobulin antibodies (TSI in Grave’s disease)
  • Antibodies against TSH receptors
  • Circulating antibodies against T3 & T4
  • Scanning images of thyroid gland (radio-iodine or Na): these techniques indicate areas of increased or decreased thyroid activity. Esp. useful with retrosternal goitre or ectopic thyroid tissue.
  • Ultrasonic examination of thyroid gland: measures the size of the thyroid gland & helps differentiate b/w solid & cystic thyroid nodules
  • BMR: raised in Hyper & lowered in Hypothyroidism.
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