HYPOTHYROIDISM: SYMPTOMS AND CAUSES

HYPOTHYROIDISM

Symptoms: 
Dry skin, slow speech, constipation, decrease GI motility, weight gain despite of reduced appetite, dry hair and fallout, facial puffiness, angina, hyperlipidemia, etc.

Signs: 
Pale, dry skin, goiter, cool peripheries, alopecia, bradycardia, ascites, hydrocele, cretinism in children, etc.

This is the most common disease occurring in 5-15% women after 65 years of age. Myxedema is a severe form of hypothyroidism in which there is accumulation of mucopolysaccharides in the skin and other tissues, leading to a 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.

Fig. Hypothyroidism examination

CAUSES
Primary hypothyroidism: 
This occurs due to extrinsic factors that affect thyroid gland or due to disease of the thyroid gland. As a compensatory mechanism there is release of TRH and TSH which cause thyroid hyperplasia causing enlargement (goiter). Non goiterous hypothyroidism occurs due to functional loss of thyroid gland despite of increased production of TSH and TRH. Primary hypothyroidism is frequently associated with circulating Antithyroid antibodies (autoantibodies).

1. Hashimoto thyroiditis – This is characterized by autoimmune destruction of thyroid gland. Autoantibodies are directed against TPO, Tg and other tissues. Initially there is hyperthyroidism due to Overactivity of thyrocytes where stored thyroid hormones are released about for 6-8 weeks, gland becomes enlarged, palpable and tender i.e. goiterous.  But further gland damage leads to permanent hypothyroidism and goiter regresses.
2. Riedel’s syndrome – Gland become fibrosed
3. Iodine deficiency – Lowered gland iodine; impaired hormone synthesis. This is the most common cause of goiterous hypothyroidism.
4. Acute iodine excess – Transient inhibition of hormone synthesis may become permanent in the presence of coexisting thyroid destructive autoimmune activity

Fig. Growth retardation : effect of Hypothyroidism (All the men showing are of same age) 

 Congenital hypothyroidism

• Caused by structural abnormalities like absent gland, ectopic site, enzymes defects like iodide transport, Organification, peroxidase, deiodinase, mutation in T3, transporter, maternal antibodies. Screening of this condition is done in almost all countries of the world. Here T4 and TSH are measured.
• Drug induced defect – Lithium, glucocorticoid, propranolol, iodine 

Secondary hypothyroidism: 
Due to extrathyroidal disease. Here TSH is low or normal and T3 and T4 are low due to inadequate tropic hormones.
• Anterior pituitary failure – Loss of TSH stimulation of thyroid
• Hypothalamic dysfunction – Loss of TRH stimulation of anterior pituitary

In hypothyroidism serum TSH is elevated (due to lack of feedback regulation) whereas thyroid hormone level is suppressed in case of primary hypothyroidism. In secondary hypothyroidism due to defect in pituitary or hypothalamus, TSH is undersecreted which causes reduced synthesis of thyroid hormones.
In many patients of primary hyperthyroidism no goiter or history of goiter is found.

Subclinical hypothyroidism
In this condition TSH level is raised (but <10 mU/L) but thyroid hormone level are normal.

ATA Guidelines for hypothyroid screening

Measurement of TSH
• At age 35
• Every 5 years after 35 yrs.
• More frequently with risk factor or symptoms: goiter, family history, lithium use, amiodarone use.

After giving thyroxine as medication to treat hypothyroidism it is important to wait at least five half-lives (7x5 = 35 days, since levothyroxine has half life of 7 days) before rechecking thyroid function tests in order to achieve a new steady state.

HYPERTHYROIDISM: CAUSES AND SYMPTOMS

Hyperthyroidism is a hypermetabolic condition caused by excessive production of thyroid hormones. This is also called thyrotoxicosis. The prevalence is low 0.3 to 0.6% in population.

Symptoms: Increase irritability, sweating, palpitation, SOB, loss of weight despite increase appetite (classical feature), increase bowel movement, malabsorption, loss of appetite, etc.

Signs: Tachycardia, tremor, warm and moist peripheries due to increased cutaneous blood flow and sweating, arrhythmias, etc.

CAUSES:
Causes of thyrotoxicosis are divided into two types: (I) those associated with frank hyperthyroidism and increased production and secretion of thyroid hormones from the gland,
and (2) those that are not.

TSH measurement shows suppressed level with highly increased thyroid hormones in all cases (except in pituitary adenomas where TSH is inappropriately secreted). For follow up of treatment measurement of free T4 and T3 should be done with TSH until TSH returns to normal.

Graves ‘disease
There is presence of TSH mimicking autoantibodies to TSH receptor. There is diffused goiter due to thyroid hyperplasia, Opthalmopathy, myxedema and hyperthyroidism. It predominately affects female. There is staring eyes with forward protrusion of eyeball and lid lagging behind globe. During this condition there is long period of hyperthyroidism and again hypothyroidism ensues due to excess damage of thyroid cells. Iodine-131 can be used therapeutically to treat hyperthyroidism due to Graves’ and other thyroid disorders, this radioiodine interferes with Organification of iodine, inhibits thyrocytes replication by inducing radiation damage thus controlling thyroid Overactivity. Male to female ratio is 5:1 in having this disease. Laboratory test shows very high level of T3, T4 with low or undetectable TSH except in those rare conditions where there is TSH secreting pituitary adenoma or pituitary resistance to thyroid hormones. TSH within the euthyroid reference range eliminates the diagnosis of hyperthyroidism. When TSH is low and T4 within normal range T3 should be measured as it is highly increased during Graves’s disease and in multinodular toxic goiter (T3-toxicosis).

Toxic Nodular Goiter (Plummer’s disease)
This occurs by autonomously functioning thyroid tissue without requiring TSH. Here multiple sites within thyroid gland autonomously produce thyroid hormones. The biochemical diagnosis of hyperthyroidism is suppression of TSH but thyroid hormones lies at URL. Radioiodine is the treatment of choice. MNG usually results from a low-grade, probably intermittent stimulus to the thyroid gland from iodine deficiency, goitrogens (foods that induce hypothyroidism and goiter in the diet such as cabbage, broccoli, cauliflower, and brussels sprouts), decreased thyroid hormone production, or an autoimmune disease, which causes multiplication and growth of small groups of thyroid cells.

TSH-secreting pituitary tumour
Rarely adenomas of pituitary gland secreting TSH (TSHomas) may produce hyperthyroidism. There is persistence of TSH secretion despite overproduction of thyroid hormones. There is thyroid gland Overactivity and hyperplasia leading to goiter.

Iodine Induced
In individual with goiter due to previous iodine deficiency, chronic administration of excess iodine in the diet can induce a hyperthyroid state. This phenomenon (sometimes called the Jod-Basedow phenomenon) and occurs in patients who already have pre-existing thyroid autonomy, expression of which may have been masked by lack of iodine.

hCG secreting Trophoblastic tumor
Here hCG functions as TSH since it has common alpha subunit and its receptor and TSH receptor have similar ligand binding domain.

[Amiodarone is anti-arrhythmic drug that contain 1-12 mg iodine, this prevent peripheral conversion of T4 to T3 with increased production of rT3 and it also inhibits both iodine uptake by thyroid and entry of T4 into cells and can cause both iodine induced hypothyroidism and hyperthyroidism]. This is also a wolf chaikof effect. Some develop hyperthyroidism if the medication leads to inflammation of the thyroid gland (subacute thyroiditis) and subsequent leakage of stored thyroid hormone into the circulation.

Subclinical hyperthyroidism
Here TSH is suppressed but normal concentration of T3 and T4, usually in URL.

Hyperthyroidism or NTI
Suppressed TSH but elevated fT4 is common picture but it’s difficult to differentiate whether this is due to NTI or hyperthyroidism. A raised T4 is uncommon in NTI. The typical signs and symptoms of hyperthyroidism are absent in NTI. Laboratory pattern shows low T4 and TSH. Here illness decreases 5’ monodeiodinase activity, less T4 is converted to T3. This leads to decreased T3 but high rT3.

How to identify Thyroid dysfunction causes ?

Test to identify cause of thyroid dysfunction

a. Antibodies to thyroid peroxidase
These antibodies are found in almost 95% patients with autoimmune hypothyroidism secondary to Hashimoto’s thyroiditis early in the course of the disease and in some patients with other autoimmune thyroid disease. Their target antigen is thyroid peroxidase enzyme. These antibodies can fix the complement and play a major pathogenic role in autoimmune thyroiditis leading to impaired formation of T3 and T4.

b. Antibodies to thyroglobulin
This occurs with lower frequency and do not fix complement and are not known to play a direct pathogenic role in aetiology of autoimmune thyroid disease in man. These antibodies can interfere in Tg measurement giving falsely decreased concentration in IMA. The main reason to measure them to indicate possible assay interference in Tg assay.

c. Antibodies to TSH receptor
This is called Grave’s disease where autoantibodies (IgG) against TSH receptor are formed and these antibodies mimic the function of TSH leading to overt hyperthyroidism, in this condition the TSH level is reduced. There is thyroid gland Hyperfunction. Other TSH receptor antibodies (blocking antibodies) although infrequently encountered can inhibit gland function and lead to hypothyroidism.

Opthalmopathy in Grave’s disease is poorly understood. Orbital muscle, connective tissue and adipose tissue become infiltrated with lymphocytes and macrophages. The extracellular compartment of extraocular muscle and orbital fibro-adipose tissue becomes oedematous owing to water deposition caused by production of glycosaminoglycans by orbital fibroblasts. TSH receptor is also expressed in orbital connective tissue, orbital fat and extraocular muscle fibers which are also the target of these autoantibodies.

99mTc given intravenously as pertechneate, is concentrated within the gland but not organified into thyroid hormones and therefore diffuses out of the gland with time, and the functioning thyroid tissue can be obtained by imaging technique and determining the total dose taken per time, activity of gland can be determined.

Use of 123I being best isotope to use for imaging thyroid tissue, this provides the idea on Organification process. Here radioactive isotope of iodine is given and proportion of total administered dose of isotoe concentrated within the thyroid gland during a given time period is quantified to estimate the activity of gland.

Thyroid profile testing or Thyroid function test (TFT)

Thyroid blood tests:

The blood tests that may be done as part of a thyroid diagnosis include the following:

1. Thyroid Stimulating Hormone (TSH) Test
2. Total T4/ Total Thyroxine
3. Free T4 / Free Thyroxine
4. Total T3 / Total Triiodothyronine
5. Free T3 / Free Triiodothyronine
6. Thyroglobulin/Thyroid Binding Globulin/TBG
7. T3 Resin Uptake (T3RU)
8. Reverse T3
9. Thyroid Peroxidase Antibodies (TPOAb) / Antithyroid Peroxidase Antibodies
10. Antithyroid Microsomal Antibodies / Antimicrosomal Antibodies
11. Thyroglobulin Antibodies / Antithyroglobulin Antibodies
12. Thyroid Receptor Antibodies (TRAb)
13. Thyroid-Stimulating Immunoglobulins (TSI)

Test for thyroid dysfunction

a. Measurement of TSH
TSH is measured by sandwich ELISA method by using anti TSH antibodies (Ab against β subunit of TSH) as primary and HRP enzyme conjugated secondary antibody, here TSH is sandwiched between these two antibodies and color is produced when chromogenic substrate like TMB (Tetramethyl Benzidine) is added. Substrate A containing TMB and substrate B containing H2O2 is mixed and added. HRP will release [O] by hydrolysis of H2O2 which will oxidize TMB producing color.

Reference range for TSH is 0.39 – 6.16 µIU/L.

b. Measurement of free T4 and T3
The plasma concentration of free thyroid hormones are extremely small and as most of them are protein bound and especially in NTI or under medication, there is alteration in protein level or hormone itself making their measurement less informative. So, free hormone estimate is used for quantification. The binding of T4 to TBP is overcome by using barbital buffer which will selectively inhibit the binding. Similarly Anilino naphthalene sulfonic acid (ANS) is also used for this purpose. These agents displace T4 from TBG. This is in case of measurement of total T3 and T4.

EVALUATION OF THYROID FUNCTION

CLINICAL EVALUATION

This is done by evaluating sign and symptoms of thyroid disorders or by palpation. Normal thyroid is rubbery feel; in Graves’s disease and diffused colloidal goiter it has soft consistency. In Hashimoto’s disease it is firm, in thyroid carcinoma and Riedel’s thyroiditis it is rock hard and irregular outline.

Doctors performing ultrasound of thyroid

Hyperthyroidism

Symptoms: Increase irritability, sweating, palpitation, SOB, loss of weight, increase bowel movement, loss of appetite, etc.

Signs: Tachycardia, tremor, warm and moist peripheries, arrhythmias, etc.
Hypothyroidism:

Symptoms: Dry skin, slow speech, constipation, weight gain, dry hair and fallout, facial puffiness, angina, etc.

Signs: Pale, dry skin, goiter, cool peripheries, alopecia, bradycardia, ascites, hydrocele, etc.
 

(Courtesy: About.com, Thyroid disease)

IN VITRO TEST OF THYROID ACTIVITY 

The tests used to investigate thyroid dysfunction can be grouped into:

1. Test for thyroid dysfunction: TSH, T4, T3 measurement in serum
2. Test to identify cause of thyroid dysfunction: E.g. autoantibodies and serum TBG, thyroid enzyme activities, biopsy, etc.
3. Test to monitor treatment and detect recurrences of follicular carcinoma: TBG measurement

Factors affecting thyroid function

FACTORS THAT AFFECT THYROID FUNCTION

a. Age
The level of TSH and thyroid hormones are higher in neonates and children which is required for growth and development. In old age slight decrease is seen.

b .Pregnancy
In pregnancy due to effect of estrogen and diminished clearance there is increase in TBG. Also there is increase in deiodination of thyroid hormones in developing placenta. So, during pregnancy there is increase in requirement for iodine (200µg/day) and more T4 and T3 is produced to compensate for overutilization. In early pregnancy due to thyroid stimulating action of hCG there is slight rise in fT3 and fT4, and this suppresses TSH but as pregnancy progress this pattern subsides since hCG also falls thus TSH rises.

c. Non-thyroidal illness
Patients in hospital with NTI have abnormalities in thyroid function tests. A low T3 may be found even though patients are clinically euthyroid; this has been termed as sick euthyroid syndrome. Several mechanisms are involved, including:

1. Hypothalamic-pituitary-thyroidal malfunction leading to decrease store of TRH and  suppression of TSH due to increased concentration of dopamine, cytokine, cortisol, etc. 
2. Alteration in plasma concentration and affinity of binding proteins (usually reduced concentration and affinity). High concentration FFA can compete with thyroid hormone binding to plasma proteins so there may be slight rise in T4.
3. Impaired uptake of thyroid hormones in tissue
4. Decreased conversion of T4 to T3 in peripheral tissue and receptor dysfunction. This cause marked decline in T3 and slight increase in T4. Excess T4 is converted to rT3 which is metabolically inactive and there is marked increase in rT3.

TSH is the most reliable test of thyroid function in hospitalized patients. Normalization of thyroid parameters occurs during recovery from NTI or refeeding after starvation. These changes in TSH helps in differential diagnosis of thyroid disorder which lead to hyper and hypothyroidism. In NTI plasma proteins are altered, since thyroid hormones are bound by proteins, so measurement of T3, T4 would not indicate exact picture.

In non-thyroidal illness 5’-mono-deiodination (D1) is impaired leading to a decreased production of T3 but increased rT3 due to impaired clearance; however total T4 remains unchanged.

Assessment of thyroid illness in ill patient should be postponed until the illness resolves.

Hypothyroidism in euthyroid sick syndrome shows a reduced total T4 and a slightly subnormal FT4. Serum TSH is probably the best single test to distinguish between euthyroid sick syndrome and hypothyroidism (in the absence of suspected pituitary or hypothalamic disease or medications, such as dopamine or glucocorticoids). A clear elevation of the TSH concentration (>20 mIU/L) would indicate hypothyroidism. Lesser TSH elevations may be seen transiently in euthyroid sick syndrome patients during recovery. If the question of hypothyroidism in acutely ill patients cannot be resolved with TSH and FT4 testing, measurement of rT3 may help (rT3 being low in hypothyroidism and normal or high in euthyroid subjects). Documentation of a normal serum cortisol may help-distinguish euthyroid sick syndrome patients from those with hypothalamic or pituitary hypothyroidism.

Hyperthyroidism in euthyroid sick syndrome shows subnormal TSH values often associated with the acute phase of illness or with glucocorticoid or dopamine therapy. In these ill patients TSH level is mildly suppression in 0.05 to 0.1 mIU/L range as compared to hyperthyroid patients where TSH is highly suppressed.

d. Drugs
Dopamine, glucocorticoid, cytokine decreases TSH secretion. Lithium, iodide decrease or increase thyroid hormone secretion; Propylthiouracil, carbimazole decrease thyroidal synthesis; Oestrogens increases TBG whereas androgen, glucocorticoid decreases TBG. NSAIDs, Phenytoin, carbamazepine, furosemide and salicylate compete with thyroid hormone binding to plasma binding proteins and may increase plasma fT4 concentration.

How Thyroid hormone synthesis and secretion are controlled ?

CONTROL OF THYROID HORMONE SYNTHESIS AND SECRETION

The most important regulator is TSH. This dimeric peptide hormone comprises a specific beta subunit, that bind to receptor and alpha receptor which is common to gonadotrophins; both subunits along with associated carbohydrate moieties are required for bioactivity. TSH secretion has circadian rhythm, plasma concentration being highest between midnight and 4.00h and lowest at midday.  The classic hypothalamo-pituitary-thyroidal axis for regulation of thyroid hormone synthesis is shown below.

Fig. Regulation of Thyroid hormones

(Source:Bishop's clinical chemistry) 

TSH itself auto regulates its release from hypothalamus and pituitary. Pituitary have TSH receptors, on binding of TSH its secretion is inhibited. Interaction of TSH receptor antibodies with TSH receptor in pituitary explains why patients with Grave’s disease may continue to have suppressed TSH weeks or months after normal thyroid hormone concentration achieved after therapy.







Other mechanisms like release of cortisol under stress condition, release of cytokines like IL-1, TNF during illness, somatostatin released during malnutrition all these inhibits TSH release. Thus these factors are important during non thyroidal illness (NTI).

TSH acts via cell surface receptor which is coupled to G protein. This receptor has extracellular N-terminal domain for hormone binding, 7 transmembrane domains and short intracellular C-terminal domain involved in activation of G protein modulators of adenylate cyclase-protein kinase A system.  These characteristics are shared with receptors for gonadotropins which has 40% homology in extracellular domain. This may explain the weak thyroid stimulating activity of hCG. Binding of TSH results in activation of AC and accumulation of cAMP. The calcium and phosphoinositol signaling pathways may also be activated by TSH.

Fate of T4 and T3 hormone

TRANSPORT OF T4 AND T3 IN CIRCULATION

 In circulation most of T3 and T4 are carried by plasma carrier proteins. They are,

1.Thyroxine binding globulin (TBG) – bind 70% of circulating thyroid hormones. It binds 80% T3 and 68% T4.
2.Thyroxine binding prealbumin or transthyretin (TBPA) – binds 20% of circulating thyroid hormones. It binds 9% T3 and 11% T4.
3.Albumin – binds 10% of circulating hormones. It binds 20% T3 and 11% T4.

Thus about 99.97% of T4 and 99.80% of T3 is protein bound. Low-density lipoprotein (LDL) specifically binds and transports <1% of total circulating T4-LDL facilitates entry of T4 into cells by forming a T4-LDL complex that is recognized by the LDL receptor

T4 is more tightly bound than T3 to all these proteins. Approximately 0.2% of T3 and 0.03% of T4 are in free form and are active. So although total T4 is 40 times more than T3 but in free form T4 is only 3 times that of fT3. The half life of T3 is 1-2 days and that of T4 is 5-7 days. In circulation normal T4:T3 ratio is 7:1. This protein bound T4 is the reservoir of T4 and accounts for constant supply of free T4 as well as free T3.

ENTRY OF THYROID HORMONES INTO TISSUE
Specific transporters are involved in transport of thyroid hormones into cells either in peripheral tissue or to brain and neurons.

PERIPHERAL FATE OF THYROID HORMONES
Tg is a preprohormone, T4 is prohormone while T3 performs all the biological actions.  In periphery 45% of T4 is deiodinated to T3 and 45% to rT3 by deiodinases (D1, 2, 3). All these deiodinase are selenoenzymes and requires adequate dietary intake of selenium for their expression. So, normal T4, production of about 100 nmol daily, approximately 40 nmol of T3 and 45 nmol of rT3 are produced by peripheral deiodination. In euthyroid state at least 85% of T3 production and all of rT3 are produced by peripheral deiodination of T4. Peripheral 5’ deiodination of T4 to T3 and 5-deiodination to rT3 is catalyzed by D1 and D2.

D1 carries out either 5’-deiodination giving T3 or 5-deiodination giving inactive rT3 and this enzyme is most abundant found mostly in liver and kidney and is responsible for largest contribution to circulating pool of T3, especially in hypothyroid condition. During hyperthyroidism when T4 is high, T3 production is primarily derived from D1. D2 provides important source of T3 in pituitary, brain and brown adipose tissue as well as in other tissues. This maintains the constant level of T3 in CNS. D3 converts T4 to rT3 and is thought to be important extrathyroidal control mechanism or regulate T3 action. Propylthiouracil and propranolol inhibits the conversion of T4 to T3.

CATABOLISM OF THYROID HORMONES
Sulphation, glucuronidation, deamination, oxidative decarboxylation, ether cleavage and deiodination are the main routes of inactivation and degradation which are excreted via bile or urine.

What is Wolff Chaikoff effect ?

Wolff-Chaikoff effect:

Iodine deficiency leads to severe mental deterioration especially in children. The recommended dietary intake of iodine is 150µg/day for adults but increases during pregnancy and lactation. If iodine intake drops below 50µg/day then hypothyroidism results. High iodine intake (e.g. in amiodarone therapy) can induce hypothyroidism, goiter and sometimes hyperthyroidism. A large excess of iodide, when given acutely, inhibits the adenylate cyclase response to TSH and iodination of thyroglobulin also inhibit thyroglobulin hydrolysis. This is termed the Wolff-Chaikoff effect.

After few day of exposure to high iodide concentration, thyroidal uptake is very low, the intrathyroidal iodide concentration falls and the synthesis of iodinated Tg recommences. This effect can be used to prepare a thyrotoxic patient for thyroidectomy by giving excess KI.

Thyroid Hormone: Synthesis, Storage, Release and Biological action

GROSS ANATOMY
Thyroid gland lies in front of trachea, just below the larynx and is the largest gland and butterfly shaped. The gland is bilobed with central isthmus and weighs 10-20g in adult. Behind thyroid gland there are 4 parathyroid glands.

MICROANATOMY
Thyroid gland consists of thousands of follicles filled with colloid inside and cuboidal epithelial follicular cells called thyrocytes. Colloid is composed of thyroglobulin. The interfollicular stroma contains C cells (parafollicular cells) which secrete calcitonin involved in calcium homeostasis. The primary function of thyroid is to synthesize and secrete thyroid hormones.

Fig. Hypothalamus Pituitary Axis

Fig. a) Location of thyroid gland b) Histological structure of Thryoid follicle (Source: Bishop's Clinical chemistry, 6th edition)

BIOLOGICAL ACTION
Under normal circumstances 10 nmol of tri-iodothyronine (T3) and 110 nmol of thyroxine (T4) are formed per day. The actions of thyroid hormone are exerted through modulation of gene expression. Thyroid hormones promote differentiation and growth; they are essential for normal fetal and neonatal development. Thyroid hormones increases mitochondrial oxidative phosphorylation, increase calorigenesis and oxygen consumption in tissue, except brain. They stimulate protein synthesis, gluconeogenesis, liver glycogenolysis, enhance carbohydrate absorption from GI, lipolysis and accelerate insulin degradation.

Hypothyroidism cause the increase in plasma cholesterol, CK-MM, TBG, creatinine, decreased sodium. T3 also stimulate urea cycle enzyme (CPS) so that urea is excreted rather than ammonia.

SYNTHESIS, STORAGE AND RELEASE 

Synthesis of T4 and T3 occurs on thyroglobulin (Tg), a glycoprotein of molecular weight 660 kDa with 5000 amino acids, 115 tyrosine residues. Thyroglobulin is synthesized by the thyrocytes and exported to be stored within the colloid of follicular lumen. Other enzyme TPO is also synthesized in thyrocytes and present in apical membrane and involved in oxidation of Iodide to Iodine using NADP+ and H2O2. Use of radioactive iodine uptake (RIU) method has helped in elucidation of the pathway of synthesis of thyroid hormones. About 70% of iodide is present as MIT and DIT in Tg and 30% as in T3 and T4.

Trapping of iodide: 
Iodide from plasma is actively transported by sodium-iodine symporter (2Na+ and 1I- are symported inside cell) situated in basal membrane of thyrocytes. This is an ATP dependent process since iodide is taken against concentration gradient. This is also a rate limiting step in thyroid hormone synthesis. In the apical membrane a anion transporter or iodide transporter protein pendrin mediates iodide efflux into colloid. Pertechnetate (which is used for radioactive imaging of gland since it inhibits the pump for iodine but itself gets transported inside), Perchlorate, thiocyanates (found in certain foods) competitively inhibit iodide pump but are not taken up. The ratio of iodide in thyroid to in serum (T:S) is about 25:1 in normal iodine diet. Small amount of iodide also enters by diffusion so any extra iodide not incorporated in MIT, DIT (about <10%) leaves by this mechanism.

Oxidation of iodide to iodine by thyroid peroxidase: 
Oxidation of iodide only occurs in thyroid gland. This occurs in the luminal side of apical membrane and requires heme containing TPO and H2O2, which is generated by calcium dependent flavoprotein enzyme system situated at apical membrane. Antithyroid drugs like propylthiouracil, thiourea, methimazole, carbimazole inhibits the oxidation of iodide by TPO.

Iodination or Organification:
This occurs in tyrosyl residues present in Tg. MID and DIT are formed through the action of TPO. At first 3rd position of tyrosine is iodinated followed by 5th position. Free tyrosine can also be iodinated but it will not be incorporated into protein since there is no tRNA for iodotyrosine.

Fig. Structure of Thyroid hormones (Source: Tietz Textbook of clinical chemistry, 4th Edition)

Coupling of iodine into tyrosyl residue on thyroglobulin: 
This is also catalyzed by TPO and produces T3 (Tri-iodotyrosine) and T4 (Tetra-iodotyrosine) that remain linked to Tg. Here DIT + DIT forms T4 and MIT + DIT forms T3. Iodinated Tg is stored in follicular lumen. When iodine supply is limited proportion of T3 in Tg is increased.


Internalization of Tg and release of T4 and T3:
Thyroglobulin is internalized by pinocytosis and appears as colloid droplets that fuse with lysosome and undergo proteolytic degradation to release T3 and T4. Some uncoupled MIT and DIT are again deiodinated to Iodide and Tyrosine which are recycled. While in thyrocytes some amount of T4 is converted to T3 by deiodinase D1 and D2. Release into circulation occurs by fusion of endocytic vesicle containing thyroid hormones with plasma membrane.

Iodide released by deiodinase (of MIT, DIT) in thyroid cell constitute the important pool within thyroid in contrast to iodide that enters the blood.

TSH stimulates each of the above process by cAMP mediated Ca++/PI pathway by binding to cell surface G-protein coupled receptor. Prolonged TSH action cause hypertrophy and increase vascularity of thyroid gland leading to thyroid enlargement (Goiter).