Thyroid hormones

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Thyroid hormones

• M.Prasad Naidu
• MSc Medical Biochemistry, Ph.D,.

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• Thyroid gland produces two principal hormones
  thyroxine tri iodo thyronine which
  regulate the metabolic rate of the body.
• Iodine is essential for the synthesis of thyroid
  hormones
• More than half of the bodys total content is
  found in the thyroid gland

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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.

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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

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Mechanism of thyroid hormone receptor action
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Actions of thyroid hormones

• Brain----growthdevelopment of nervous system
• Bonetissue growth linear growth maturation of
  bones
• CVS-- increased contractility,heart rate
  cardiac output
• GUTincreased absorption of nutrients, increased
  motility
• Liver -increased gluconeogenesisglyco genolysis

• Adipose tissue increased lipolysis
• Muscle increased protein catabolism in skeletal
  muscle
• Kidney -increased erythropoietin synthesis
• Respiration- increased central stimulation of
  respiration
• Energy metabolism -increased BMR,increased oxygen
  consumption,increased heat production stimulation
  of Na-K-ATP ase

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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

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The functional unit of thyroid is thyroid
follicle. Normal follicle
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• Thyroid follicle with out TSH

• Thyroid follicle with high TSH stimulation

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High T3 or T4 gives

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

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• 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

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• Primary hypo thyroidism

• Secondary hypotyroidism

• Iodine deficiency
• Hasimotos thyroiditis
• Thyroidectomy
• Radiation therapy
• Drugs-lithium,antithyroid drugs and PAS
• Absent or ectopic thyroid gland
• Dyshormonogenesis
• TSH receptor mutation

• Hypopituitarism
• Tumors,pituitary surgery, irradiation/infiltration
  , sheehans syndrome isolated TSH deficiency
• Hypothalamic disease
• Trauma infiltration

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cretinism

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

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myxedema

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

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• Primary hyperthyroidism

• Secondary hyper thyroidism

• Grave disease
• Toxic multinodular goitre
• 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

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hyperthyroidism

• Graves Disease
• tall stature, hyperactivity
• high rate of metabolism
• high body temp
• high heart rate

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Thyroid function in pregnancy

• Four factors alter thyroid function in pregnancy
• Transient increase in hcG during first trimester
  which stimulates TSH-R
• The estrogen induced rise in TBG during the first
  trimester which is sustained during pregnancy
• Alterations in the immune system ,leading to
  onset, exacerbation ,or amelioration of an
  underlying auto immune thyroid disease
• Increased urinary iodide excretion ,which can
  cause impaired thyroid hormone production

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• Iodine supplementation is considered to be
  important in women with precarious iodine intake
• Maternal hypothyroidism occurs in 2 to 3 of
  women of child bearing age is associated with
  increased risk of developmental delay in the
  offspring
• Thyroid hormone requirements are increased by 25
  to 50µg/day during pregnancy

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Thyroid function tests
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Thyroid function tests

• Estimation of thyroid hormones
• 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

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Thyroid function tests

• Serum binding proteins
• 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

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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.

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Sandwich ELISA
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Radioimmunoassay
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Determination of free thyroid hormones

• Direct assays currently serve as reference
  methods
• Indirect assays - more widely available for
  general laboratory use

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• Direct methods
• Direct measurement of FT4FT3 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

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• Methods
• Equilibrium dialysis
• 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

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• Indirect methods
• More convenient less expensive than direct
  methods
• Automated immuno assay instuments
• Two step immunoassay
• One step immunoassay
• These methods estimate free hormone conc. by
  using antibody extraction techniques

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• 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

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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)

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• 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

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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
  abnormal hormone production or due to abnormal
  protein binding
• An FT4 index is sometimes directly calculated
  using the percentage T-uptake
• FT4I total T4(µg/dl) x thyroid uptake/ 100

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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

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TSH stimulation test

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

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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

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• 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

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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
• Estrogen induced TBG excess and congenital TBG
  deficiency are important abnormalities that
  affect the test results

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Calculation of T4TBG T3TBG ratios

• These ratios correlate with FT4 or FT3 conc. And
  are particularly useful in sera with altered TBG
  conc.
• failuresThey may fail however to compensate for
  TBG variants with reduced T4 affinity for
  abnormal albumin binding
• Ref . Interval is 3.8 to 4.5

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Determination of thyroglobulin

• Method immunometric assay method
• These assays are based on the use of two or more
  monoclonal antibodies directed to different
  portions of the Tg molecule
• Difficulty interference with anti-Tg antibodies
  as seen in pts with thyroid cancer
• Heterophilic antibody interference(HAMA)
• Ref interval is 3 to 42 µg/dl

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• Thyroglobulin is used primarily as tumor marker
  in pts carrying a diagnosis of differentiated
  thyroid carcinoma
• Tg levels are elevated in
• Thyroid follicular papillary carcinoma
• Certain non neoplastic conditions like..,
• Thyroid adenoma
• Subacute thyroiditis
• hashimotos thyroiditis
• Graves disease

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• Serum Tg conc. are not increased in pts with
  medullary thyroid carcinoma
• Serial measurements of Tg is most useful in
  detecting recurrence of diff. thyroid carcinoma
  following surgical resection

• Tg determination is used as an adjunct to
  ultrasound and radio iodine scanning
• Assessment of serum Tg also aids in management of
  infants with congenital hypo thyroidism
• In hyperthyroidism-Tg
• Low conc.-thyrotoxicosis factita

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Determination of antithyroid antibodies

• Anti thyroid antiodies are found in autoimmune
  diseases and certain malignancies
• These autoantibodies are directed against several
  thyroid and thyroid hormone antigens
• Tg (Tg Ab)
• Thyroid peroxidase(TPO Ab)
• Thyroid receptor(TR Ab)
• TSH,T4,T3

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• The presence of TPO antibodies is a risk factor
  for autoimmune thyroid dysfunction
• However there is a high prevalence of anti-TPO
  antibodies in the elderly
• With sensitive assays,low conc of TPO antibodies
  may be detected in some healthy individualsthey
  may have occult or subclinical thyroid
  dysfunction

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• Method
• RIA
• CHEMILUMINISCENCE based immunometry
• Radioimmunometric technique
• Reference value is 2U/ml(with sensitive
  chemiluminiscence assay)
• Detectable conc. Of TPO Ab are seen in
  hashimotos thyroiditis,idiopathic myxedema,
  graves disease, Type 1 IDDM

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Determination of thyrotropin receptor antibodies

• Thyrotropin receptor antibodies are a group of
  related immunoglobulins that bind to TSH
  receptors
• Seen in pts with Graves disease other auto
  immune thyroid disorders
• These Ab s demonstrate substantial heterogeneity
• Some cause thyroid stimulation , where as others
  have no effect or decrease thyroid secretion by
  blocking the action of TSH

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• Invitro bioassays assess the capacity of
  immunoglobulins to stimulate functional activity
  of thyroid gland such as..,
• 1.adenylatecyclase stimulation
• 2. c AMP formation
• 3.colloid mobilization
• 4.iodothyronine release
• TSI s are present in 95 of pts with
  untreated Graves disease
• TSI measurement is also used for
  following the course of therapy predicting
  relapse remission

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Radio active iodine uptake(RAIU)

• Radioactive iodine uptake by thyroid gland and
  thyroid scanning with Tc 99 are of diagnostic
  value.

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calcitonin

• Calcitonin is secreted by the para follicular or
  C cells ,which arise from the neural crest are
  distributed through out the thyroid gland
• A marker for medullary thyroid carcinoma (tumor
  of C cells)
• Ref range 25pg/m L in men and 20 pg/m L
  

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Normal ranges

• T3 120-190 ng/dl
• r T3 10-25 ng/dl
• T4 5-12 µg/dl
• Thyroglobulin3-5 µg/dl
• TRH 5-60 ng/L
• TSH 0.5-5 µU/ L
• Thyroxine binding globulin 1-2 mg/dl

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Thank you