Thyroid

Production, Regulation, and Action of Thyroid Hormones

The Thyroid Gland

Thyroid Hormones

There are two biologically active thyroid hormones: - tetraiodothyronine (T4; usually called thyroxine) - triiodothyronine (T3) Derived from modification of tyrosine.

Differences between T4 and T3

The thyroid secretes about 80 microg of T4, but only 5 microg of T3 per day.However, T3 has a much greater biological activity (about 10 X) than T4.An additional 25 microg/day of T3 is produced by peripheral monodeiodination of T4 (stay tuned….). T4
thyroid
I-
T3

Why is Iodine Important in Thyroid Hormone Production?

Thyroid hormones are unique biological molecules in that they incorporate iodine in their structure. Thus, adequate iodine intake (diet, water) is required for normal thyroid hormone production. Major sources of iodine: - iodized salt - iodated bread - dairy products - shellfish Minimum requirement: 75 micrograms/day US intake: 200 - 500 micrograms/day

Iodine Metabolism

Dietary iodine is absorbed in the GI tract, then taken up by the thyroid gland (or removed from the body by the kidneys). The transport of iodide into follicular cells is dependent upon a Na+/I- cotransport system. Iodide taken up by the thyroid gland is oxidized by peroxide in the lumen of the follicle:
peroxidase
I- I+
Oxidized iodine can then be used in production of thyroid hormones.

The Next Step: Production of Thyroglobulin

Pituitary produces TSH, which binds to follicle cell receptors.The follicle cells of the thyroid produce thyroglobulin.Thyroglobulin is a very large glycoprotein.Thyroglobulin is released into the colloid space, where it’s tyrosine residues are iodinated by I+. This results in tyrosine residues which have one or two iodines attached (monoiodotyrosine or diiodotyrosine).

The Thyroid Gland – HistologyGland is composed of hollow spheres, called colloid follicles. Squamous epithelial cells, cuboidal cells (follicle cells)
Follicle cells produce thyroglobulin ---- TH I
Colloid fills the follicle cavities

Thyroid Follicles

Thyroid Follicles

Thyroid Hormone Synthesis

Transport of Thyroid Hormones
Thyroid hormones are not very soluble in water (but are lipid-soluble). Thus, they are found in the circulation associated with binding proteins: - Thyroid Hormone-Binding Globulin (~70% of hormone) - Pre-albumin (transthyretin), (~15%) - Albumin (~15%) Less than 1% of thyroid hormone is found free in the circulation. Only free and albumin-bound thyroid hormone is biologically available to tissues.

Conversion of T4 to T3

T3 has much greater biological activity than T4.A large amount of T4 (25%) is converted to T3 in peripheral tissues.This conversion takes place mainly in the liver and kidneys. The T3 formed is then released to the blood stream.In addition to T3, an equal amount of “reverse T3” may also be formed. This has no biological activity.

One Major Advantage of this System

The thyroid gland is capable of storing many weeks worth of thyroid hormone (coupled to thyroglobulin). If no iodine is available for this period, thyroid hormone secretion will be maintained.

Regulation of Thyroid Hormone Levels

Thyroid hormone synthesis and secretion is regulated by two main mechanisms:1-- an “autoregulation” mechanism, which reflects the available levels of iodine2-- regulation by the hypothalamus and anterior pituitary

Autoregulation of Thyroid Hormone Production

The rate of iodine uptake and incorporation into thyroglobulin is influenced by the amount of iodide available:- low iodide levels →increase iodine transport into follicular cells - high iodide levels →decrease iodine transport into follicular cellsThus, there is negative feedback regulation of iodide transport by iodide.


Neuroendocrine Regulation of Thyroid Hormones: Role of TSH
Thyroid-stimulating hormone (TSH) is produced by thyrotroph cells of the anterior pituitary. TSH is a glycoprotein hormone composed of two subunits: - alpha subunit (common to LH, FSH, TSH) - TSH beta subunit, which gives specificity of receptor binding and biological activity
a
LHb
FSHb
TSHb
 LH FSH TSH

Action of TSH on the Thyroid

TSH acts on follicular cells of the thyroid. - increases iodide transport into follicular cells - increases production and iodination of thyroglobulin - increases endocytosis of colloid from lumen into follicular cells
Na+
I-
thyroglobulin
follicle cell
gene
I-
endocytosis
thyroglobulin
T3 T4
colloid droplet
I-
I+
iodination
thyroglobulin
Na+
K+
ATP

Mechanism of Action of TSH

TSH binds to a plasma membrane-bound, G protein-coupled receptor on thyroid follicle cells. Specifically, it activates a Gs-coupled receptor, resulting in increased cAMP production and PKA activation.
TSH
Gsa
Adenylyl Cyclase
ATP cyclic AMP
Protein kinase A
Follicle cell

Regulation of TSH Release from the Anterior Pituitary

TSH release is influenced by hypothalamic TRH, and by thyroid hormones themselves. Thyroid hormones exert negative feedback on TSH release at the level of the anterior pituitary. - inhibition of TSH synthesis - decrease in pituitary receptors for TRH
hypothalamus
TRH
TRH receptor
TSH synthesis
pituitary
T3/T4
+
-
-

Influence of TRH on TSH Release

Thyrotropin-releasing hormone (TRH) is a hypothalamic releasing factor which travels through the pituitary portal system to act on anterior pituitary thyrotroph cells. TRH acts through G protein-coupled receptors, activating the IP3 (Ca2+) and DAG (PKC) pathways to cause increased production and release of TSH.
TRH phospholipase C
G protein-coupled receptor
IP3 calcium
DAG PKC
calmodulin
Thyroid hormones also inhibit TRH synthesis.

Negative Feedback Actions of Thyroid Hormones on TSH Synthesis and Release

hypothalamus
TRH
TRH receptor
TSH synthesis
pituitary
T3/T4
+
-
-
-
TRH synthesis
Thyroid gland follicle cell receptors
TSH binds

Other Factors Regulating Thyroid Hormone Levels

Diet: a high carbohydrate diet increases T3 levels, resulting in increased metabolic rate (diet-induced thermogenesis).Low carbohydrate diets decrease T3 levels, resulting in decreased metabolic rate.Cold Stress: increases T3 levels in other animals, but not in humans.Other stresses: increased or decreased?Any condition that increases body energy requirements (e.g., pregnancy, prolonged cold) stimulates hypothalamus  TRH  TSH (Pit)

Actions of Thyroid Hormones

Thyroid hormones are essential for normal growth of tissues, including the nervous system. Lack of thyroid hormone during development results in short stature and mental deficits (cretinism). Thyroid hormone stimulates basal metabolic rate. What are the specific actions of thyroid hormone on body systems?

Actions of Thyroid Hormone

Required for GH and prolactin production and secretion Required for GH action Increases intestinal glucose reabsorption (glucose transporter) Increases mitochondrial oxidative phosphorylation (ATP production) Increases activity of adrenal medulla (sympathetic; glucose production) Induces enzyme synthesis Result: stimulation of growth of tissues and increased metabolic rate. Increased heat production (calorigenic effect)

Effects of Thyroid Hormone on Nutrient Sources

Effects on protein synthesis and degradation: -increased protein synthesis at low thyroid hormone levels (low metabolic rate; growth) -increased protein degradation at high thyroid hormone levels (high metabolic rate; energy) Effects on carbohydrates: -low doses of thyroid hormone increase glycogen synthesis (low metabolic rate; storage of energy) - high doses increase glycogen breakdown (high metabolic rate; glucose production)

Expression and Regulation of Thyroid Hormone Receptors

Thyroid hormone receptors are found in many tissues of the body, but not in adult brain, spleen, testes, uterus, and thyroid gland itself. Thyroid hormone inhibits thyroid hormone receptor expression.

Thyroid hormones:Key Points

Held in storage Bound to mitochondria, thereby increasing ATP production Bound to receptors activating genes that control energy utilization Exert a calorigenic effect

Thyroid Hormone Actions which Increase Oxygen Consumption

Increase mitochondrial size, number and key enzymes Increase plasma membrane Na-K ATPase activity Increase futile thermogenic energy cycles Decrease superoxide dismutase activity

Effects of Thyroid Hormones on the Cardiovascular System

Increase heart rate Increase force of cardiac contractions Increase stroke volume Increase Cardiac output Up-regulate catecholamine receptors

Effects of Thyroid Hormones on the Respiratory System

Increase resting respiratory rate Increase minute ventilation Increase ventilatory response to hypercapnia and hypoxia

Effects of Thyroid Hormones on the Renal System

Increase blood flow Increase glomerular filtration rate

Effects of Thyroid Hormones on Oxygen-Carrying Capacity

Increase RBC mass Increase oxygen dissociation from hemoglobin

Effects of Thyroid Hormones on Intermediary Metabolism

Increase glucose absorption from the GI tract Increase carbohydrate, lipid and protein turnover Down-regulate insulin receptors Increase substrate availability


Effects Thyroid Hormones in Growth and Tissue Development
Increase growth and maturation of bone Increase tooth development and eruption Increase growth and maturation of epidermis,hair follicles and nails Increase rate and force of skeletal muscle contraction Inhibits synthesis and increases degradation of mucopolysaccharides in subcutaneous tissue

Effects of Thyroid Hormones on the Nervous System

Critical for normal CNS neuronal development Enhances wakefulness and alertness Enhances memory and learning capacity Required for normal emotional tone Increase speed and amplitude of peripheral nerve reflexes

Effects of Thyroid Hormones on the Reproductive System

Required for normal follicular development and ovulation in the female Required for the normal maintenance of pregnancy Required for normal spermatogenesis in the male

Thyroid Hormone Deficiency: Hypothyroidism

Early onset: delayed/incomplete physical and mental developmentLater onset (youth): Impaired physical growthAdult onset (myxedema) : gradual changes occur. Tiredness, lethargy, decreased metabolic rate, slowing of mental function and motor activity, cold intolerance, weight gain, goiter, hair loss, dry skin. Eventually may result in coma.Many causes (insufficient iodine, lack of thyroid gland, lack of hormone receptors, lack of TH binding globulin….)

How is Hypothyroidism Related to Goiter?

During iodine deficiency, thyroid hormone production decreases. This results in increased TSH release (less negative feedback). TSH acts on thyroid, increasing blood flow, and stimulating follicular cells and increasing colloid production.

Midwest – the Goiter Belt If goiter is due to decreased I, then thyroid gland enlarges – called endemic or colloidal goiter.Pituitary gland  TSH to stim. thyroid gland to produce TH, but the only result is that the follicles accumulate more and more unusable colloid.Cells eventually die from overactivity and the gland atrophies.

Thyroid Hormone Excess: Hyperthyroidism

Emotional symptoms (nervousness, irritability), fatigue, heat intolerance, elevated metabolic rate, weight loss, tachycardia, goiter, muscle wasting, apparent bulging of eyes, may develop congestive heart failure.Also due to many causes (excessive TSH release, autoimmune disorders,…)


How is Goiter Related to Hyperthyroidism?
Due to excessive stimulation by TSH (thyroglobulin production, enlarged follicles…).In this case, excessive stimulation of the thyroid gland by TSH ,result in thyroid hormone secretion, since iodine is available.