THYROID HORMONES

1. Thyroid hormones M.Prasad Naidu MSc Medical Biochemistry, Ph.D,.

2. Thyroid gland produces two principal hormones … thyroxine & tri iodothyronine which regulate the metabolic rate of the body. • Iodine is essential for the synthesis of thyroid hormones • More than half of the body’s total content is found in the thyroid gland

10. Hypothalamo pituitary axis The hypothalamo-pituitary axis is a classical negative feedback regulatory mechanism in which secretion of TSH is modulated by thyroid hormones. Release of TSH from the pituitary gland is stimulated by thyrotropin releasing hormone (TRH) from the hypothalamus.

11. Hypothalamo pituitary axis • A small increase in T3 and T4 produces a diminished TSH response to TRH at the pituitary level. • T3 and T4 act at the hypothalamic level by inhibiting mRNA for TRH synthesis. • Only unbound fractions of hormone are metabolically active and only this free hormone has an inhibitory effect on the secretory activity of the thyroid. • dopamine physiologically inhibits TSH secretion • glucocorticoids have been shown to dull the response of the pituitary to TRH • oestrogens increase the sensitivity of thyrotrophs to TRH

12. Mechanism of thyroid hormone receptor action

13. Actions of thyroid hormones • Brain----growth&development of nervous system • Bone&tissue growth– linear growth & maturation of bones • CVS-- increased contractility,heart rate &cardiac output • GUT—increased absorption of nutrients, increased motility • Liver -increased gluconeogenesis&glycogenolysis • Adiposetissue –increased lipolysis • Muscle –increased protein catabolism in skeletal muscle • Kidney -increased erythropoietin synthesis • Respiration- increased central stimulation of respiration • Energymetabolism -increased BMR,increased oxygen consumption,increased heat production stimulation of Na-K-ATP ase

14. Wolff-chaikoff effect • Iodine deficiency increases thyroid blood flow & upregulates the NIS , stimulating more efficient uptake. • Excess iodide transiently inhibits thyroid iodide organification ,a phenomenon known as the wolff-chaikoff effect

15. The functional unit of thyroid is thyroid follicle. Normal follicle

16. Thyroid follicle with out TSH Thyroid follicle with high TSH stimulation

18. High T3 or T4 gives • decreased TSH subunit synthesis • inactive thyrotrophs may lose the capacity to respond to reduced T3 or T4 levels

19. somatostatin TRH • inhibits TSH release • potentiates the effect of thyroid hormones on thyrotrophs, ie thyroid hormone has inhibitory effects on TSH release • derives from the median eminence of the hypothalamus • thyrotropin releasing hormone, ie stimulates TSH release

20. Primary hypo thyroidism Secondary hypotyroidism • Iodine deficiency • Hasimoto’sthyroiditis • Thyroidectomy • Radiation therapy • Drugs-lithium,antithyroid drugs and PAS • Absent or ectopic thyroid gland • Dyshormonogenesis • TSH receptor mutation Hypopituitarism • Tumors,pituitary surgery, irradiation/infiltration, sheehan’s syndrome & isolated TSH deficiency Hypothalamic disease • Trauma & infiltration

21. cretinism • congential absence of T3 and T4 or chronic iodine deficiency during childhood • retarded growth • sluggish movements • mental deficiencies

22. myxedema • low rate of metabolism and lethargy • decreased body temp • decreased heart rate • outer skin becomes scaley • myxodema – swelling of sub-cu connective tissues

23. Primary hyperthyroidism Secondary hyper thyroidism • Grave’ disease • Toxic multinodulargoitre • Toxic adenoma • Functioning metastatic thyroid carcinoma • TSH receptor mutation • Struma ovarii • Iodine excess • TSH secreting pituitary adenoma • Thyroid hormone resistance syndrome • Chorionic gonadotropin secreting tumours • Gestational thyrotoxicosis

24. hyperthyroidism • Grave’s Disease • tall stature, hyperactivity • high rate of metabolism • high body temp • high heart rate

27. Thyroid function in pregnancy Four factors alter thyroid function in pregnancy • Transient increase in hcG during first trimester which stimulatesTSH-R • The estrogeninduced rise in TBG during the first trimester which is sustained during pregnancy • Alterations in the immunesystem ,leading to onset, exacerbation ,or amelioration of an underlying autoimmunethyroiddisease • Increasedurinaryiodideexcretion ,which can cause impaired thyroid hormone production

28. Iodinesupplementation is considered to be important in women with precarious iodine intake • Maternalhypothyroidism occurs in 2 to 3% of women of child bearing age & is associated with increased risk of developmentaldelay in the offspring • Thyroid hormone requirements are increased by 25to50µg/day during pregnancy

29. Thyroid function tests

30. Thyroid function tests Estimationofthyroidhormones • Total T4 • Total T3 Estimation of free hormone fraction • Free T4 fraction %FT4 • Free T3 fraction %FT3 • THBR Estimates of free hormone concentration • FT4E (T4 X %FT4) • FT3E (T3 X % FT3) • FT4I (T4 X THBR) • FT3I (T3 X THBR) • T4: TBG ratio

31. Thyroid function tests Serumbindingproteins • Thyroxine binding globulin • Thyroxine binding prealbumin Tests for auto immune thyroid disease • Anti thyroglobulin Abs • Anti microsomal Abs • Anti TPO antibodies • TSH receptor anti bodies Other hormones & thyroid related proteins • TRH • Thyroglobulin • calcitonin

32. Measurement of T4,T3 &rT3 • METHOD • Immunoassay • Chemiluminiscence • The major clinical role for T3 measurements are in the diagnosis & monitoring of hyperthyroid pts with suppressed TSH &normal FT4 • r T3 test is not always elevated with illness.It is seldom used in pts with euthyroid sick syndrome • Specifially,renal failure is associated with low r T3 conc.

33. Sandwich ELISA

34. Radioimmunoassay

35. Determination of free thyroid hormones • Direct assays – currently serve as reference methods • Indirect assays - more widely available for general laboratory use

36. Directmethods • Direct measurement of FT4&FT3 is a technical challenge as free hormone conc. are low in serum healthy individuals • Assays for free thyroid hormones must be capable of measuring sub picomole amounts • Only minimal dilution of serum specimens is allowed as dilution alters the binding of drugs, FFAs and other substances to serum proteins

37. Methods • Equilibriumdialysis • Ultra filtration techniques these techniques physically separate free hormone from protein bound hormone (before direct measurement of the free fraction with a sensitive T4 or T3 immunoassay) These methods are unaffected by variations in SBPs or thyroid hormone auto antibodies

38. Indirect methods • More convenient & less expensive than direct methods • Automatedimmunoassayinstuments • Twostepimmunoassay • Onestepimmunoassay • These methods estimate free hormone conc. by using antibodyextractiontechniques

39. FT4 is 0.03% of total serum T4 • FT3 is 0.3% of total serum T3 • Because T3 is less firmly bound by TBG than is T4 the dialyzable fraction of T3 is appreciably greater (by almost 10 times) than that of T4

40. Free hormone estimates • FT4E = total T4 X %FT4 • The free hormone fraction as measured dialysis or ultra filtration of diluted serum containing tracer T4 or t3 is multiplied by the respective total hormone concentration to obtain indirect estimates • THBR = %uptake(patient serum)/% uptake (reference serum)

41. Invitro I –T3resin uptake by Resin • A known amount of I-T3 is added to a standard volume of serum from a patient • The amount of I-T3 which binds to the serum proteins varies inversely with the endogenous thyroid hormones already bound to serum proteins(TBG) • Residual free I-T3 then adsorbed by resin is removed from the sample and then adsorbed/bound I is measured

42. FT4 index • Unlike direct free T4 methods , index methods measure both the serum total T4 & the free T4 fraction • They have an advtantage that they can define whether an abnormal FT4 estimate is due to abnormalhormoneproduction or due to abnormalproteinbinding • An FT4 index is sometimes directly calculated using the percentage T-uptake • FT4I =total T4(µg/dl) x % thyroid uptake/ 100

44. Plasma TSH Method- Immunoassay -chemiluminiscence Secretion of TSH occurs in a circadian fashion Primary Hypothyroidism-TSH increased Secondary hypothyroidism-TSH ,T3 ,T4 are low Primary hyper thyroidism –TSH decreased Secondary hyperthyroidism-TSH,T3,T4 high

45. TSH stimulation test Measurement of serum T4 after TSH injection • No response - primary • Increase of T4- secondary • Useful for distinguishing primary from secondary hypothyroidism

46. TRH response test • TRH administration will stimulate the production of TSH • Useful for differentiating hypothalamic from a pituitary hypotyroidism • There is increase of TSH after TRH in hypothalamic disorder

47. If the hypothalamo pituitary axis is normal .the T3 and T4 secretions will be increased An abnormal response is seen in Hyperthyroidism – T4 elevated • Hypopituitarism- T4 Levels subnormal • Primary hypothyroidism-exaggerated response

50. Determination of thyroid binding globulin • TBG is the thyroid binding globulin with the greatest affinity for T4 • TBG is very important for regulating the conc. And availability of the FT4 hormone. • Method - immunoassay - commercial kit methods available - chemiluminiscence • EstrogeninducedTBGexcess and congenitalTBGdeficiency are important abnormalities that affect the test results