Physiology of Thyroid Hormones

What are the two primary functions of the thyroid gland?

The thyroid gland secretes thyroid hormones to maintain the level of metabolism and calcitonin to regulate circulating calcium levels.

What are the main components of thyroid hormones?

Thyroid hormones are mainly composed of triiodothyronine (T3) and tetraiodothyronine / thyroxine (T4).

Where are thyroid hormones synthesized?

Thyroid hormones are synthesized in the thyroid follicles.

What is the essential component required for thyroid hormone synthesis?

Iodine is an essential component required for thyroid hormone synthesis.

How are thyroid hormones transported in the blood?

Thyroid hormones are transported in the blood bound to thyroid binding proteins.

What is the difference in potency between T3 and T4?

T3 is more potent than T4.

What are the clinical features of hypothyroidism?

The clinical features of hypothyroidism include myxoedema and cretinism.

What are the clinical features of hyperthyroidism?

The clinical features of hyperthyroidism include Graves disease.

What is the regulation of thyroid hormone secretion?

The regulation of thyroid hormone secretion is complex and involves the hypothalamus, pituitary gland, and thyroid gland.

What is the mechanism of action of thyroid hormones?

The mechanism of action of thyroid hormones involves binding to nuclear receptors and regulating gene expression, leading to increased metabolism.

What is the effect of thyroid hormone on metabolism?

Thyroid hormone increases metabolism.

What are the factors that affect the concentration of thyroid binding proteins?

The factors that affect the concentration of thyroid binding proteins include pregnancy, estrogen therapy, liver disease, and genetic variations.

What are thyroid hormones mainly composed of?

Thyroid hormones are mainly composed of triiodothyronine (T3) and tetraiodothyronine / thyroxine (T4).

What is the difference between T3 and T4 in terms of biological activity?

T3 is more biologically active, while T4 is a less active molecule.

What is the mechanism of action of thyroid hormones?

Thyroid hormones bind to intracellular receptors - nuclear receptors.

What is the precursor/prohormone of thyroid hormones?

Thyroglobulin is the precursor/prohormone - a large iodinated, glycosylated protein the storage form of T4 and T3 in the colloid.

What is the process of formation of thyroid hormones?

The process of formation of thyroid hormones involves oxidation, organification, coupling, and hydrolysis.

What is the process by which T4 is converted into Tyrosine and I-?

Deiodination by the enzyme Deiodinase.

What are the proteins that thyroid hormones bind to in the plasma?

Mainly thyroxine-binding globulin (TBG), transthyretin/thyroxine-binding pre-albumin, and albumin.

What is the reason for the slow onset of action of thyroid hormones?

Due to their binding with proteins both in the plasma as well as in tissues.

Why do we measure Free Thyroxin rather than measuring Total thyroxin level?

Free Thyroxin is the active form of the hormone, while Total thyroxin level includes both bound and unbound forms.

What factors affect the concentration of thyroid binding to proteins?

Condition of binding protein, Total Plasma T4, T3, RT3, Free Plasma T4, T3, RT3, Plasma TSH, and Clinical state.

What is TSH?

TSH is a glycoprotein made up of two subunits α and β.

What stimulates the secretion of TSH?

The secretion of TSH is stimulated by hypothalamic hormone TRH.

What inhibits the secretion of TSH?

The secretion of TSH is inhibited by negative feedback from circulating free T4 and T3, stress (Dopamine and somatostatin inhibit TSH), and glucocorticoids.

What is TRH?

TRH is a tripeptide.

What stimulates the secretion of TRH?

The secretion of TRH is stimulated by cold.

What is the mechanism of action of thyroid hormones?

Thyroid hormones stimulate every step in thyroid hormone synthesis and secretion, including increased proteolysis of the thyroglobulin, releasing the thyroid hormones into the circulating blood, and increased activity of the iodide pump, which increases the rate of “iodide trapping”.

What is calorigenesis?

Calorigenesis is the increase in metabolic rate caused by thyroid hormones.

What is the effect of thyroid hormones on metabolism?

Thyroid hormones increase the oxygen consumption in almost all metabolically active tissues.

What is calorigenesis?

Calorigenesis is the increase in metabolic rate.

What are the exceptions to the increase in oxygen consumption caused by thyroid hormones?

The exceptions are the adult brain, testes, uterus, lymph nodes, spleen, and anterior pituitary (to inhibit TSH).

What is the effect of thyroid hormones on the metabolism of fatty acids and proteins?

Thyroid hormones increase the metabolism of fatty acids and proteins.

What is the effect of thyroid hormones on the need for vitamins?

The need for vitamins increases, and vitamin deficiencies may precipitate.

What is the necessary role of thyroid hormones in hepatic conversion of carotene to Vitamin A?

Thyroid hormones are necessary for hepatic conversion of carotene to Vitamin A, and carotenemia may occur in hypothyroidism.

How are the actions of catecholamines and thyroid hormones interrelated?

The actions of catecholamines and thyroid hormones are interrelated.

What are the non-calorigenic effects of thyroid hormones?

The non-calorigenic effects of thyroid hormones include stimulating the CNS and CVS.

What is the diabetogenic effect of thyroid hormones?

The diabetogenic effect of thyroid hormones is an increase in the rate of absorption of carbohydrates from the gastrointestinal tract, potentiation of the effects of catecholamines and hepatic glycogen depletion, gluconeogenesis, and acceleration of the degradation of insulin.

What happens to plasma glucose levels after a carbohydrate meal in hyperthyroidism?

Plasma glucose levels rise rapidly after a carbohydrate meal in hyperthyroidism.

What is the effect of thyroid hormones on cholesterol metabolism?

Thyroid hormones increase the formation of LDL receptors in the liver and the removal of cholesterol from the circulation, leading to a decrease in plasma cholesterol levels.

What is the effect of hypothyroidism on plasma cholesterol levels?

It leads to hypercholesteremia with a risk of atherosclerosis.

What is the mechanism of action of hyperthyroidism on cardiovascular function?

It increases beta-adrenergic receptors and enhances the action of catecholamine, leading to increased heart rate and stroke volume.

What is the effect of hyperthyroidism on peripheral resistance?

It decreases peripheral resistance due to cutaneous vasodilation, leading to increased levels of renal Na+ and water absorption, expanding blood volume.

What is the consequence of severe hyperthyroidism on cardiac function?

It can lead to cardiac failure due to increased workload and protein catabolism.

What is the effect of hyperthyroidism on blood pressure?

It increases systolic blood pressure and pulse pressure, leading to systolic hypertension with normal diastolic BP due to reduced TPR.

What is the effect of increased metabolism in hyperthyroidism on blood flow?

It causes vasodilation, thus increasing blood flow, especially in the skin due to the increased need for heat elimination from the body.

What is the effect of increased need for heat elimination on blood flow in the skin?

It increases the rate of blood flow in the skin.

What are the characteristics of cold and dry skin?

Cold and dry.

What are the characteristics of warm and moist skin?

Warm and moist.

What is the effect of thyroid hormones on cerebration?

It increases cerebration.

What are the consequences of slow mentation?

Extremely nervous, psychoneurotic tendencies - anxiety complexes, extreme worry, and paranoia.

What is the effect of thyroid hormones on brain development?

They promote growth and development of the brain during fetal life and for the first few years of postnatal life.

What is cretinism?

A condition caused by congenital lack of thyroid hormone.

What is the effect of thyroid hormones on reflexes?

They shorten the reaction time of stretch reflexes.

What is the effect of thyroid hormones on sympathetic nervous system?

They increase catecholamine sensitivity and β adrenergic receptor numbers in heart, skeletal muscle, adipose cells, and lymphocytes.

What is the effect of thyroid hormones on cardiac α adrenergic receptors?

They decrease cardiac α adrenergic receptors.

What are the effects of hypothyroidism on the musculoskeletal system?

Muscle weakness, cramps, and stiffness.

What are the effects of hyperthyroidism on the musculoskeletal system?

Muscles become sluggish, and they relax slowly after a contraction.

What are the effects of hypothyroidism on the reproductive system?

Poor libido, Menorrhagia, Amenorrhea, oligomenorrhoea.

What are the effects of hyperthyroidism on the reproductive system?

Normal menstrual cycle and fertility.

What are the effects of hypothyroidism on the gastrointestinal system?

Constipation.

What are the effects of hyperthyroidism on the gastrointestinal system?

Diarrhea.

What is the effect of thyroid hormones on growth and development?

Promote growth and development of the brain during fetal life and for the first few years of postnatal life.

What are the effects of hypothyroidism on bone growth?

Slowed bone growth, delayed epiphysial closure.

What are the effects of hyperthyroidism on bone growth?

Excessive skeletal growth, causing the child to become considerably taller at an earlier age.

What is the effect of thyroid hormones on erythropoietic activity?

Accelerate erythropoietic activity.

What are the effects of hypothyroidism on blood?

Anemia.

What are the effects of hyperthyroidism on blood?

Polycythemia.

What is the effect of hypothyroidism on cardiac α adrenergic receptors?

Decrease.

What is the effect of hyperthyroidism on cardiac α adrenergic receptors?

No effect mentioned in the text.

What are the effects of increased erythropoietic activity?

Anemia and polycythemia.

What are the effects of increased glucose metabolism?

Increased secretion of insulin.

What is the effect of increased metabolic activity of bone formation?

Increased need for PTH.

What is the effect of increased inactivation of glucocorticoids by the liver?

Feedback increase of ACTH production and glucocorticoid secretion by the adrenals.

What are the symptoms of hypothyroidism?

Decreased sympathetic activity, myopathy, decreased cardiac output, symptoms of hyperprolactinemia, and nonpitting edema.

What is the effect of decreased transcription of sarcolemmal genes?

Myopathy and decreased cardiac output.

What is the effect of TRH stimulation?

Prolactin and TSH release.

What are the symptoms of hyperprolactinemia?

Amenorrhea or menorrhagia, galactorrhea, erectile dysfunction, infertility in men, and decreased libido.

What is the cause of nonpitting edema in hypothyroidism?

Excessive tissue gel in the interstitial spaces caused by decreased clearance of complex glycosaminoglycans and hyaluronic acids from the reticular layer of the dermis.

What is myxedema?

Myxedema is a condition characterized by preorbital edema, facial puffiness, decreased rapidity of cerebration, and slow mentation.

How does hypothyroidism affect LDL formation and blood cholesterol levels?

Hypothyroidism reduces LDL formation, which increases blood cholesterol levels and leads to increased atherosclerosis.

What is carotenemia, and how is it distinguished from jaundice?

Carotenemia is the accumulation of carotene in the bloodstream, which is responsible for the yellowish tint of the skin. It can be distinguished from jaundice because in carotenemia, the sclera are not yellow.

What are the symptoms of congenital hypothyroidism?

Congenital hypothyroidism can cause poor brain development.

How is congenital hypothyroidism diagnosed?

Congenital hypothyroidism can be diagnosed through screening.

What is hyperthyroidism?

Hyperthyroidism is a condition characterized by an overactive thyroid gland, which leads to an excess of thyroid hormones in the body.

What are the consequences of untreated congenital hypothyroidism?

Poor brain development.

How should congenital hypothyroidism be screened?

Need screening.

What clinical features would you see in hyperthyroidism?

Various clinical features.

How can you explain the clinical features of hyperthyroidism based on your knowledge of thyroid hormone physiology?

By understanding the effects of excess thyroid hormone on the body.