Thyroid hormones are amine hormones produced by the thyroid gland. There are two types of Thyroid hormones: triiodothyronine (T3) and thyroxine (T4). The hormones are stored in thyroid follicles as a component of thyroglobulin. Once released in the blood 2/3 of the hormones are transported while bound to Thyroxine-Binding Globulin.

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a space-filling model (right) of (S)-thyroxine (T4.)

Image: “a space-filling model (right) of (S)-thyroxine (T4.)” by Benjah-bmm27. License: Public Domain

Functions of Thyroid Hormones

Thyroid hormones generally work to increase body’s metabolic rate by increasing the rate of the chemical reactions occurring inside cellsEffects of thyroid hormones on the following metabolisms are:

  • Protein metabolism stimulates protein synthesis by increasing transcription process on ribosome and translation process in nucleus. This especially results in the increase of cellular enzymes.
  • Carbohydrate metabolism stimulates glucose absorption from the gut, increases secretion of insulin, increase glucose uptake by body cells and increase in glycolysis as well as gluconeogenesis.
  • Fat metabolism increases fatty acid concentration in the plasma by mobilizing fatty acids in the adipose tissues. This process is  known as lipolysis.
  • Vitamin metabolism: By increasing formation of cellular enzymes, thyroid hormones subsequently increase body’s need for vitamins. Vitamins are crucial components of enzymes and co-enzymes in metabolic reactions.

Organs which are exempted from the metabolic effects of thyroid hormones are brain, spleen and kidneysEffects of thyroid hormones on the following body systems:

  • Cardiovascular system: Thyroid hormones directly increase heart rate; heart rate also increases due to increased cardiac output. There’s an increase in cardiac output as a result of increased blood flow which, in turn, is due to increased metabolism in the body.
  • Respiratory system: Thyroid hormones increase body’s metabolism which in turn increases the demand and utilization of Oxygen. Increased Oxygen demand leads to increase in respiratory rate and depth.
  • Central Nervous System: Maternal levels of thyroid hormones is the most important when it comes to CNS development during the perinatal period. Proper maturation of the central nervous system is highly dependable on thyroid hormone in the perinatal period. Low thyroid levels in the pregnant mother can lead to permanent mental retardation.
Thyroid System

Image: “Diagram of the hypothalamic–pituitary–thyroid axis. The hypothalamus secretes TRH (green), which stimulates the production of TSH (red) by the pituitary gland. This in turn stimulates the production of thyroxine by the thyroid (blue). Thyroxine levels decrease TRH and TSH production by a negative feedback process.” by Mikael Häggström. License:

Thyroid stimulating hormone

This glycoprotein hormone is produced by the thyrotrope cells in the anterior pituitary and works through the adenylate cyclase-cAMP mechanism to increase synthesis and secretion of thyroid hormones from the follicular cells of the thyroid gland. Thyroid releasing hormone, produced by the hypothalamus, causes the release of thyroid stimulating hormone from the pituitary gland.

Thyroid binding globulin

This binding globulin has the highest affinity for thyroid hormones in plasma. Its levels in the blood can be used to test for thyroid diseases, especially in the case of elevated endogenous thyroid hormones.

Maternal Thyroid Changes During Pregnancy

Early in pregnancy, the mother’s thyroid hormone production increases by 50 %. Discussed below are the reasons why that happens.

Estrogen is a primary female sex hormone which also plays a major part in pregnancy. Estrogen contributes to the development of many fetal parts, especially stimulating the fetus’s adrenal glands to produce hormones and working to maintain the uterus to accommodate the pregnancy as well as responding to Oxytocin.

One of the functions of estrogen is to cause increased levels of thyroid binding globulin synthesis and release from the liver. This significantly increases its blood concentrations during pregnancy.

TBG has a higher affinity for T4 than T3, hence increase in the blood concentration of TBG leads to lowered levels of free T4 in the blood. This triggers a negative feedback reaction which results in increased production of TSH from the anterior pituitary, which in turn causes an increase in thyroid hormone production. Hence the final effect of increased TBG is to cause increased production of thyroid hormones which meets the demands of the pregnant mother’s body.

The structure of Human chorionic gonadotropin

Image: “The structure of Human chorionic gonadotropin” by Borislav Mitev. License: Public Domain

Furthermore, a hormone called human chorionic gonadotropin which is produced by the syncytiotrophoblast in the placenta of pregnant women also works to stimulate the thyroid gland due to its similarity to TSH. hCG levels are the highest during the last days of the first trimester, therefore, there is a fall in TSH levels during this time.

Increased demand of Iodine and pregnancy-induced goiter

Thyroid, Diffuse Hyperplasia

Image: “Thyroid, Diffuse Hyperplasia” by Ed Uthman . License: CC BY 2.0

Iodine plays a pivotal role in thyroid hormone synthesis. There’s an increased demand of Iodine in a pregnant mother not only because of increases synthesis of thyroid hormones, but also because there’s an increased glomerular filtration rate in the kidneys which results in loss of Iodine to the urine as well as the fetus taking a lot of maternal Iodine for its own development. If the mother does not take adequate supply of Iodine supplements, a goiter can form.

Goiter is a swelling in the lower part of the neck due to an enlarged thyroid gland. Due to iodine deficiency, thyroid hormones cannot be synthesized in an adequate amount, this results in increased TSH from the anterior pituitary as a feedback mechanism. TSH continuously stimulates the follicular cells in the thyroid gland to make more thyroid hormone, this persistent stimulation results in follicular cells growing in size and proliferating. This change in the follicular cells results in diffuse hyperplasia of the thyroid gland causing a goiter.

Thyroid hormones and fetal brain development

Research has proved that thyroid hormones play the most vital part in the last stages of brain differentiation. This includes formation of axons and dendrites, myelination, formation of synapses and neuronal migration.

Congenital Hypothyroidism

Human fetus has two main sources of thyroid hormones; the mother’s and its own. The fetus does not start synthesizing its own thyroid hormones until 12 weeks of gestation.

Congenital hypothyroidism, or Cretinism causes:

  1. Image: “3 month old infant with untreated CH; picture demonstrates hypotonic posture, myxedematous facies, macroglossia, and umbilical hernia” by Rastogi and LaFranchi. License: CC BY 2.5

    Image: “3 month old infant with untreated CH; picture demonstrates hypotonic posture, myxedematous facies, macroglossia, and umbilical hernia” by Rastogi and LaFranchi. License: CC BY 2.5

    Maternal hypothyroidism: Overt maternal hypothyroidism alone cannot cause Cretinism because that would usually result in infertility. However, the subclinical type of maternal hypothyroidism has been found to cause significant developmental abnormalities in the baby.

  2. Fetal hypothyroidism: These babies do well until the mother’s supply of thyroid hormones is circulating in the body. However, sometime after birth symptoms of hypothyroidism start manifesting.

Causes of this can be anatomical defect in the thyroid gland, genetic or iodine deficiency in the mother. Symptoms include mental retardation, jaundice, hypotonia, decreased activity, small size of baby, decreased weight gain. These babies also present with a large anterior fontanelle, rough facial features, macroglossia (large tongue), pale and dry skin, goiter and umbilical hernia.

Congenital hypothyroidism is diagnosed by low levels of T4 and elevated levels of TSH. Thyroid scanning can further help identify the cause of the disease. Early diagnosis is crucial for managing this condition. Levothyroxine is used as the optimal treatment.

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