Thyroid functions and disorders presentation

THYROID FUNCTIONS AND
DISORDERS
By
Dr. Basil, B. C – MBBS (Nig),
Department of Chemical Pathology/Metabolic Medicine,
Benue State University Teaching Hospital, Makurdi.
February 2016.

OUTLINE:
• Introduction
• Anatomy
• Hormone synthesis/secretion
• Metabolism/Mechanism of action
• Functions and Control
• Disorders and Biochemical findings
• Biochemical Screening and Evaluation of thyroid
disease
• Treatment of Thyroidal Diseases
• Reference.

INTRODUCTION:
• The thyroid gland – largest single endocrine
gland in the body (15 – 20g)
• Produces the Thyroid hormones and
Calcitonin
• Thyroid hormones are critical in regulating
body metabolism, neurologic, developmental
and other body functions
• Calcitonin involved in calcium homeostasis –
see calcium and phosphate disorders.

ANATOMY
• Consist of 2 lobes and a central isthmus –
anterior to the trachea (shaped like a bow-tie)
• Posteriorly – parathyroid glands
• Originate from Ratke’s pouch at the base of the
tongue at 4-8wks IUL, and secretes hormones by
11wk – critical to neurologic development
• Consist of many small spherical follicles
(functional unit) surrounded by basement
membrane, containing colloid and interspersed
by para-follicular C cells

HORMONE SYNTHESIS / SECRETION
• Thyroid hormone synthesis includes the
following steps:
– Iodide (I-
) trapping via Na-I-
symporter (active
transport) by follicular cells (Rate limiting step)
– Diffusion of iodide to the apex of the cell and
transport into the colloid
– Oxidation of inorganic iodide to iodine (by
thyroperoxidase) and incorporation of iodine into
tyrosine residues within the thyroglobulin
molecule in the colloid

• Steps contd:
– Combination of two DIT molecules to form T4 or
of MIT and DIT to form T3
– Uptake of thyroglobulin from the colloid into the
follicular cell by endocytosis, fusion of the
thyroglobulin with a lysosome, and proteolysis
and release of T4 and T3
– Release of T4 and T3 into the circulation
– MIT and DIT are split into iodine and tyrosine, and
recycled

• T4 & T3 Circulate predominantly (>99%) bound to
serum proteins
– Thyroxine binding globulin (TBG) – 75%
– Thyroxine binding prealbumin (TBPA) – 15%
– Albumin – 10%
• The bound hormone is not biologically active
• Only 0.04% of T4 and 0.4% of T3 - free
• Changes in the concentration of serum binding
proteins alter the total, but not the free hormone
levels

METABOLISM
• 100nm of T4, 5nm each T3 and rT3 secreted daily
• T3 – active form; T4 – prehormone; rT3 –
metabolically inactive; Thyroglobulin –
prohormone.
• T4 >> T3: outer ring 5’monodeiodination (80% of
T3) by iodothyronine deiodinase (3 types) –
inhibited by Propylthiourasil etc
• Inner ring deiodination >> rT3 (3,3’,5’
triiodothyronine)
• 10% T4 cleared: 80% deiodinated, 20%
glucuronidation in liver and sulfation in liver and
kidneys
• Half-life: T4= 7days, T3=1 day

MECHANISM OF ACTION
• T4 dissociates with its carrier protein and
diffuse though cellular membrane into the
cytosol where it is converted to T3 by
deiodinase enzyme
• T3 binds with Nuclear receptor protein >>
Hormone-Receptor protein complex >> binds
to the hormone receptor element (or Thyroid
Response Element) on the DNA >> initiation of
transcription and protein synthesis

FUNCTIONS
• General metabolism:
– Increase the metabolic rate and oxygen consumption
of most of tissues of the body (exceptions include
testes, uterus, lymph nodes, anterior pituitary)
– Increase synthesis and activity of many intracellular
enzymes e.g. Na+
-K+
ATPase utilizes energy, increase
heat production
– Increase the size, number & activity of mitochondria
>> increase the rate of formation of ATP to energise
cellular function

FUNCTIONS
• Growth and Development:
– Essential for normal growth of soft tissue and
skeleton
– Required for production & action of growth
hormones and IGF
• Respiration:
– Increased respiration due to increased rate
– Dissociation of oxygen from hemoglobin by
increasing the amount of 2,3 DPG in RBC

FUNCTIONS
• Cardiovascular:
– Increase number & affinity of beta-1, adrenergic
receptors in the heart, also increase its sensitivity
to catecholamine
– Increases rate of blood flow in the skin for heat
elimination
– Increased cardiac output >> ↑systolic BP, but ↓
diastolic blood pressure thus >> ↑ pulse pressure

FUNCTIONS
• Gastrointestinal:
– Increase secretion of digestive juices and motility
>> increase appetite and food intake
– Necessary for the hepatic conversion of carotene
to Vit.A, so if thyroid hormones are decreased,
carotenaemia result
• Reproduction:
– Essential for normal menstrual cycles and fertility
– Increase milk secretion in lactating women

FUNCTIONS
• Nervous System:
– Promotes growth and development of the brain
(fetal life & for the first few years)
– Essential for normal myelination & development
of the nervous system in infant
– Increased response of the brain to catecholamines
& increases activation of RAS

FUNCTIONS
• Carbohydrate metabolism:
– Increases the rate of absorption of CHO from GIT,
thus blood glucose level increases after meal but
fall again as rate of glucose utilization is also
increased
– Enhance gluconeogenesis & glycolysis
– Increases cell metabolic enzymes acting on CHO

FUNCTIONS
• Protein metabolism:
– Small doses of thyroid hormone increase the rate
of formation of proteins by the ribosomes
– It increase RNA synthesis by the genes
– Large doses of thyroid hormone lead to excess
catabolism of muscle protein

FUNCTIONS
• Lipid metabolism:
– All aspects of fat metabolism increased
– Thyroid hormone accelerates the oxidation of
fatty acids by the cells
– Lowers level of cholesterol, phospholipid &
tryglycerides in the blood
– Stimulate formation of LDL receptors

CONTROL OF THYROID FUNCTION
• TRH >> (+)TSH >> (+)T4/T3 >> (-)TSH/TRH
• T4 released by the thyroid is mostly converted
to T3 by the liver and kidney by type 1
iodothyronine 5’-deiodinase
• Type 2 iodothyronine 5’-deiodinase found in
the brain and pituitary maintain constant
levels of T3 in the CNS

• TSH/Thyrotropin – Primary Regulation
– Glycoprotein hormone
– Made up two units α and β (2 different
chromosomes)
– α subunit is identical to that of LH, FSH, hCG
– β subunit confers specificity
– t½ is 60 minutes
– Acts via the TSH receptor – G protein coupled (7
transmembrane domain)

Thyroid functions and disorders presentation

• Actions of TSH:
– Increased proteolysis of the thyroglobulin (occurs
within minutes)
– Increased activity of the iodide pump
– Increased iodination of tyrosine
– Increased size and increased secretory activity of
the thyroid cells
– Increased number of thyroid cells

DISORDERS OF THYROID
• Hyper-functioning states
– Overt
– Subclinical
• Hypo-functioning states
– Overt
– Subclinical
• Others
– Inflammatory
– Neoplastic disease
– Non-thyroidal illnesses (Euthyroid Sick Syndrome)
– Euthyroid hyperthyroxinaemia

HYPER-FUNCTIONING STATES:
• Definitions:
– Thyrotoxicosis: Hypermetabolic state produced
due an excess of thyroid hormones
– Hyperthyroidism: Hypermetabolic state due to an
excessive activity of the thyroid gland
– Hyperthyroidism is a cause of thyrotoxicosis but
Thyrotoxicosis is not always as a result of
hyperthyroidism
• Women are more prone to develop hyper-
thyroidism than men

Causes:
• Primary hyperthyroidism:
– Graves’ disease (Diffuse toxic non-nodular goitre)
– Toxic multinodular goiter(Plummer disease)
– Toxic adenoma
– Functioning thyroid carcinoma metastases
– Activating mutation of the TSH receptor
– Activating mutation of G (McCune-Albright syndrome)
– Struma ovarii
– Drugs: iodine excess (Jod-Basedow phenomenon)

Causes:
• Thyrotoxicosis without Hyperthyroidism:
– Subacute thyroiditis
– Silent thyroiditis
– Other causes of thyroid destruction: amiodarone,
radiation, infarction of adenoma
– Ingestion of excess thyroid hormone
(thyrotoxicosis factitia)

Causes:
• Secondary hyperthyroidism:
– TSH-secreting pituitary adenoma
– Chorionic gonadotropin-secreting tumours e.g
Trophoblastic tumor (Hydatidform mole)
– Gestational thyrotoxicosis

• Grave’s disease:
– accounts for more than 60% of all thyrotoxicosis
– diffuse glandular hyperplasia
– 10 times more common in women
– Autoimmune basis: thyroid stimulating
immunoglobulins (TSI) that bind to the TSH-Receptor
• Toxic Adenoma:
– solitary hyper-functioning nodules
– acquired somatic activating mutations in the TSH-R.
which induce constitutive receptor coupling to Gs
– clinical features are milder than above

• Multi-nodular goiter:
– multiple nodules, maybe polyclonal or monoclonal in
origin
– usually do not have TSH-R mutations
– may present with subclinical or mild thyrotoxicosis
• Subacute Thyroiditis (De Quervain’s, Bacterial,
Granulomatous, or Viral thyroiditis):
– follicular destruction releases thyroid hormones
– later phase may be hypothyroid, with depletion of
thyroid hormones

Biochemical findings:
• In primary hyperthyroidism – T4 and T3 are elevated,
while TSH is suppressed to undetectable
concentrations
• Secondary hyperthyroidism is characterized by
elevated levels of TSH, free T4 and/or free T3 levels.
• T3 concentration is often elevated to a greater degree
than is T4 in the early stages of Graves’ disease and in
patients with T3 thyrotoxicosis (solitary or multinodular
toxic goiters)
• In subclinical hyperthyroidism, there is normal
concentration of serum T3 and FT4 and persistently
suppressed serum TSH

HYPOFUNCTIONING STATES
• Primary Hypothyroidism:
– Autoimmune hypothyroidism: Hashimoto’s
thyroiditis, atrophic thyroiditis
– Iatrogenic: 131
I treatment, subtotal or total
thyroidectomy,
– External irradiation of neck for lymphoma or cancer
– Drugs: antithyroid drugs, amiodarone, lithium
– Congenital hypothyroidism
– Iodine deficiency
– Infiltrative disorders

Causes:
• Transient Hypothyroidism:
– Silent thyroiditis
– Subacute thyroiditis
– Withdrawal of thyroxine treatment in individual
with an intact thyroid
– After 131
I treatment or subtotal thyroidectomy for
Graves’ disease

Causes:
• Secondary hypothyroidsm:
– Hypopituitarism: tumours, pituitary surgery or
irradiation,
– Infiltrative disorders, Sheehan’s syndrome, trauma
– Isolated TSH deficiency or inactivity
– Hypothalamic disease: tumours, trauma, infiltrative
disorders,
– Idiopathic
– Drugs: Dopamine, Corticosteroids
– Congenital TRH deficiency (rare)

• Iodine deficiency:
– Commonest cause of worldwide
– Responsible for endemic goiter and cretinism (mental and
growth retardation in children)
– Incidence also impacted by thiocynates in cassava and
selenium deficiency
• Hashimoto’s thyroiditis:
– Autoimmune based disease
– Destruction of thyroid cell by CD8+ cytotoxic T cells
– Antibody markers include Anti TPO, TG and TSH-R blocking
Ab
– Clinical course: subclinical (or mild) to clinical (or overt)

• Congenital hypothyroidism
– thyroid agenesis
– inborn errors of thyroid hormone synthesis
– TSH-R mediated
• Iatrogenic
– Radio iodine therapy
– Subtotal/total thyroidectomy
• Pituitary disease (Secondary hypothyroidism)

HYPOFUNCTIONING STATE
• Peripheral Resistance to Thyroid Hormones:
– The most common cause of the syndrome are
mutations of the β (beta) form (THRB gene) of the
thyroid hormone receptor
– Mutations in MCT8 and SECISBP2 have also been
associated with this condition
– Also referred to as impaired sensitivity to thyroid
hormone

• Myxedema is a severe form of hypothyroidism in
which there is accumulation of mucopolysacchrides
in the skin and other tissues, leading to thickening of
facial features and a doughy induration of the skin
• Cretinism is the term used to describe severe
hypothyroidism that develops in the newborn period
• Congenital hypothyroidism: may be as a result of a
complete absence of thyroid gland (athyreosis) or
defects in thyroid hormone synthesis, which can lead
to irreversible neurological damage without early
treatment

Biochemical Findings:
• Primary hypothyroidism:
– Low T4 and T3 lead to pituitary hypersecretion
evidenced by elevated serum TSH concentration
an important finding particularly in early detection
of thyroid failure
– In mild or subclinical cases, thyroid hormone
concentrations remain within the reference range
but TSH is elevated.
– Presence of a circulating autoantibodies especially
TPOAb in serum

Biochemical Findings:
• Secondary hypothyroidism:
– Serum concentration of thyroid hormone is low,
but TSH is either low or within the healthy
reference interval
• Peripheral resistance to thyroid hormones:
– T4 and T3 are elevated but TSH is not suppressed
(or not as suppressed as expected)

OTHER DISORDERS
• Non-Thyroidal Illnesses/Sick Euthyroid Syndrome
– Any acute or chronic severe illness causing
abnormalities in circulating TSH or thyroid hormone
levels in the absence of underlying thyroid disease.
– Reduced T4 ->T3 conversion but normal/increased rT3
– Importance is because of misleading information from
thyroid testing during periods of acute illness.
– Unless a thyroid disorder is strongly suspected, routine
testing of thyroid function should be avoided in acutely
ill patients

OTHER DISORDERS
• Biochemical Findings in Non-Thyroidal Illness/
Euthyrois sick syndrome:
– Low serum T3 level
– High serum rT3 level
– Normal or Low serum T4 levels
– If serum TBG levels are low, serum T4 would be
low
– Serum TSH levels are usually normal, but may
show mild depression during acute phase and
mild elevation during recovery phase

OTHER DISORDERS
Thyroid Malignancies – 4 types:
• Papillary carcinoma (PC):
– Well differentiated, very good prognosis; 90% 20 year
survival post treatment.
•Follicular carcinoma (FC):
– Well differentiated, very good prognosis; 90% 20 year
survival post treatment.
•Medullary carcinoma of thyroid (MCT):
– Poorly differntiated. 50% 10 year survival
– High calcitonin is a marker
•Anaplastic carcinoma:
– Poorly differentiated. Mortality approaches 100%

OTHER DISORDERS
Thyroid Neoplasia:
•Solitary Thyroid Nodules:
– As many as 50% of these are benign
– Occasionally, thyroid nodules can take on
characteristics of malignancy and require either a
needle biopsy or surgical excision

OTHER DISORDERS
• Biochemical findings in Euthyroid Hyper-
thyroxinaemia:
– Serum total T4 level is high
– Serum T3, TSH and TRH response are normal
• FT4 level compares well with the functional
condition of the thyroid than Total T4
• Causes: Bisalbuninaemia, increase oestrogen,
pregnancy, liver disease, acutely hospitalized
psychiatric pxt, familial dysalbuminaemia

BIOCHEMICAL SCREENING AND
EVALUATION OF THYROID DISEASE
• NEONATAL SCREENING:
• When to screen:
– Normal hospital delivery at term – (filter-paper
collection ideally at 2 – 4days of age or at the time
of discharge)
– NICU/Preterm home birth – (within 7days of birth)
– Maternal history of Thyroid medication/family
history of Congenital hypothyroidism – (cord
blood for screening)

BIOCHEMICAL SCREENING
Type of Screening:
•Primary TSH + Backup T4
– May miss:
• TBG deficiency
• Hypothalamic-pituitary Hypothyroidism
• Hypothyroxinemia with delayed TSH elevation
– For better sensitivity – use sensitive TSH assay and age
adjusted TSH cutoff (20 – 25mU/L at 24hr of age)
•Primary T4 + Backup TSH:
– Will miss delayed TSH elevation with initial normal T4
•Primary T4 and TSH:
– Ideal screening approach

• EVALUATION OF THYROID DISEASE:
• TSH:
– The most useful test for assessing thyroid function
– Used to monitor and adjust thyroid hormone
replacement therapy as well as screen for both
hyperthyroidism and hypothyroidism
– Have been deployed in the management of subclinical
thyroid disease
– For pregnant women with previously diagnosed
hypothyroidism, serum TSH levels should be
measured every 3-4 weeks during the first half of
pregnancy and every 6-10 weeks thereafter

Low free T4 Normal free T4 High free t4
Low
TSH
Secondary
hypothyroidism
Severe NTI
Subclinical
hyperthyroidism
NTI
Hyperthyroidism
Normal
TSH
Secondary
hypothyroidism
Severe NTI
Normal Artifact
Pituitary
hyperthyroidism
Lab draw within 6hrs
of thyroxine dose
High
TSH
Primary
hypothyroidism
Subclinical
Hypothyroidism
Test artifact
Pituitary
hyperthyroidism
Thyroid hormone
resistance

• Thyroglobulin:
– Ideal tumor marker for thyroid cancer patients and its
detection is a proof of thyroid tissue either benign or
malignant
– The accuracy of the assay is primarily dependent on
the specificity of the antibody used and the absence
of antithyroglobulin autoantibodies which interfere
with measurements
– Its prevalence is noted to be 3% of the general
population, 12 – 30% of patients with Graves’ disease,
and 35 – 60% with Autoimmune hypothyroidism

• Thyroid Autoimmunity:
– In autoimmune thyroid disease, antibodies are
directed at thyroid tissue with variable responses
– In Graves’ disease (the most common cause of
hyperthyroidism), the antibody is directed to the TSH
receptor, stimulating it and leading to growth of the
thyroid gland and production of an excess amount of
thyroid hormone
– Tests for TSH receptor antibodies detect any antibody
(stimulating or blocking) against TSH receptor

• Thyroid Autoimmunity – contd
– Both stimulating and blocking antibody assays will
be positive in most patient with Graves’ disease
– In Hashimoto’s thyroiditis, Thyroid peroxidase
antibody mediate decreased thyroid hormone
production

• Urinary Iodine measurements
– Iodine essential for normal thyroid gland function
– Iodine deficiency responsible for endemic goitre
and cretinism
– Majority of ingested iodine is excreted in the urine
– Only reflects recent dietary pattern
– Major use is for epidemiologic purposes
– Use 24 hour urine sample

• OTHER TOOLS:
– Nuclear Medicine Evaluation – RAIU Assay
• High uptake – metabolically active gland
• Concurrent TSH assay done
• Low uptake -
– Thyroid Ultrasound – detects very small nodules
– Fine-Needle Aspiration Biopsy – first step and
most accurate tool in the evaluation of thyroid
nodules

TREATMENT OF THYROIDAL DISEASES
HYPERTHYROIDISM:
•Use of beta-blockers for symptomatic relief:
– To be commenced after oral rehydration if necessary, and
avoided in known asthmatics
– If beta-blockers are contraindicated or poorly tolerated,
calcium channel blockers (verapramil or diltiazem) can be
administered
•Use of anti-thyroid drugs(e.g Carbimazole, Methimazole,
Propylthiouracil):
– Carbimazole, initial dose 30-40mg/day; maintainance 15-20
mg/day
– Preferred in younger age group, especially children
– More useful in Small diffuse gland and mild disease and in
cases of expected remission, especially immunological
background
– Therapy needs to be continued up to 12 months. Remission
rate is 40-50%

• Use of radioactive iodine-131 (131 I) therapy:
– 5-7 mCi single dose 131l therapy; 1/3 will require a second
dose
– Recommended for more than 35 years of age and in cases of
Severe thyrotoxicosis, Systemic illness that prevents other
methods of treatment and Recurrence after antithyroid
drugs, surgery or radioablation
– There is minimal risk involved; symptoms abate in 3 weeks
to 3 months
• Surgical approach (thyroidectomy):
– Subtotal thyroidectomy (leave 2-3 g of tissue; prepare
suitably with antithyroid drugs) is preferred in cases where
there is
• Large size of gland
• Multinodularity
• Pressure symptoms
• Retrosternal goiter

• In thyroid crisis, treatment has to be aggressive:
– Propranolol 1-2 mg slowly IV or 40-80 mg orally is the first
step. This can be repeated 6 hourly.
– Appropriate sedation by phenobarbitone may relieve
anxiety. Barbiturates retard the peripheral deiodination of
the hormone
– Thyroid hormone release can be inhibited by IV iodine in the
form of sodium iodide 1 g IV in 24 hours or by oral Lugol’s
iodine 5 drops every 6 hours. Drugs such as amiodarone can
be used as well.
– This is followed by thionamide or thiouracil administration in
regular dosage.
– Corticosteroids (especially dexamethasone) are indicated if
there is slightest suspicion of limitation of the adrenocortical
reserve
– Fluid and electrolyte balance should be adequately
corrected.

HYPOTHYROIDISM:
•Use of Levothyroxine (LT4) for Thyroid hormone
replacement – treatment of choice:
– Started as full replacement in young individuals but in
the elderly and those with IHD, treatment with quarter
to half the required dose is initiated and raised gradually
over 4-6wks
– For mild to moderate conditions, starting dose is about
50-75µg/day
– Clinical benefits begin btw 3-5days and level off after 4-
6wks but TSH take several months to get to target range
due to delayed re-adaptation of the hypothalamic-
pituitary axis
– In central hypothyroidism, T4 levels are used to guide
treatment instead of TSH levels

• In pregnant women with hypothyroidism, the LT4 dose
should be adjusted so as to keep the serum TSH below
2.5 mIU/L. TSH and free T4 levels should be measured 3-
4 weeks after every dosage adjustment.
• In subclinical hypothyroidism, guidelines from the
American Association of Clinical Endocrinologists (AACE)
recommend treatment in patients with TSH levels higher
than 10 mIU/L and in patients with TSH levels of 5-10
mIU/L in conjunction with goiter or positive anti-TPO
antibodies;
– These patients have the highest rates of progression to overt
hypothyroidism.
– An initial LT4 dosage of 50-75 µg/day can be used, which can
be titrated every 6-8 weeks to achieve a target TSH of
between 0.3 and 3 mIU/L

• In patients with myxedema coma:
– Give 4 µg of LT4 per kilogram of lean body weight
(approximately 200-250 µg) as an IV bolus in a
single or divided dose, depending on the patient’s
risk of cardiac disease
– 24 hours later, give 100 µg IV
– Subsequently, give 50 µg/day IV, along with stress
doses of IV glucocorticoid
• Adjust the dosage on the basis of clinical and
laboratory findings

REFERENCES
• Teitz Textbook of Clinical Chemistry and Molecular
Diagnostics; 5th ed., by Burtis et al
• Clinical Chemistry – Principles, Techniques and
Correlations, 7th ed., by Micheal L. Bishop et al.
• Thyroid Function and Disorders; Abuja Revision Course,
NPMCN, Feb 2016, by Christian Ogoegbunem Isichie.
• Robbins and Cotran - Pathologic Basis of Disease 7th ed., by
Kumar et al.
• Bolarin’s Aids to Chemical Pathology, New ed., by Debayo
M. Bolarin
• Clinical Chemistry and Metabolic Medicine, 7th ed., by
Martin Crook.
• Clinical Chemistry in Diagnosis and Treatment, 6th ed., by
Philip D. Mayne

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