It stands for “thyroid stimulating hormone” and it is produced in the pituitary gland which is located in the brain.
This gland signals the thyroid to make and release thyroid hormones into the blood.
Thyroid-stimulating hormone, or TSH, is a glycoprotein hormone secreted by the anterior pituitary lobe, which plays an important physiological role in the regulation of the hypothalamus-pituitary-thyroid by stimulating the production and secretion of thyroid hormone from the thyroid gland.
Thyroid-stimulating hormone binds to the TSH receptor which is a member of the G-protein-coupled receptor superfamily of integral membrane proteins.
The thyroid stimulating hormone is responsible for regulating the release and levels of thyroid hormones in response to some variables and increased metabolic needs.
Thyroid-stimulating hormone regulates vital functions of the body, such as:
- The breathing.
- The heart rate.
- The central and peripheral nervous system.
- Body weight.
- Muscle strength
- Menstrual cycles
- Body temperature
- Cholesterol levels
The function of the thyroid and pituitary gland can be assessed by measuring thyroid-stimulating hormone.
Elevated thyroid stimulating hormone levels combined with decreased thyroxine (T4) levels indicate hypothyroidism and thyroid gland dysfunction.
In general, decreased levels of thyroid stimulating hormone and T4 indicate secondary congenital hypothyroidism and hypothalamic pituitary dysfunction.
A normal level of thyroid stimulating hormone and a depressed level of T4 may indicate:
- Hypothyroidism due to a congenital defect in T4 globulin binding.
- Transient congenital hypothyroidism due to hypoxia or prematurity.
Early diagnosis and treatment in the neonate are crucial for the prevention of congenital hypothyroidism (cretinism).
The thyroid is the gland that controls how fast the body uses energy, makes proteins, and controls the body’s sensitivity to other hormones.
It participates in these processes by producing thyroid hormones, the main ones being triiodothyronine (T3) and thyroxine, sometimes called tetraiodothyronine (T4).
Triiodothyronine (T3) and thyroxine (T4) are enzymes produced by the thyroid gland. T4 is believed to be a more metabolically active pro-hormone for T3.
T4 is converted to T3 in tissues as required by deiodinase enzymes.
These hormones regulate the rate of metabolism and affect the growth and rate of functioning of many other systems in the body.
T3 and T4 are synthesized from iodine and tyrosine.
Calcitonin is another hormone released by the thyroid gland that is responsible for modulating calcium levels in the blood along with parathyroid hormone, which is released from the parathyroid.
Thyroid hormone production is regulated by thyroid stimulating hormone, produced by the anterior pituitary, which in turn is regulated by thyrotropin-releasing hormone produced by the hypothalamus.
Thyroid gland anatomy
The thyroid gland is a butterfly-shaped organ and is made up of two cone-shaped lobes or wings, the dexter lobe or right lobe and the sinister lobe or left lobe, connected through the isthmus.
The isthmus (the bridge between the two lobes of the thyroid) lies below the cricoid cartilage.
This organ is located on the anterior side of the neck, extends over and around the larynx and trachea, subsequently reaching the esophagus and the carotid sheath.
It begins cranially on the oblique line of the thyroid cartilage (just below the laryngeal prominence or Adam’s Apple), and extends downward to about the fifth or sixth tracheal ring.
It is difficult to delineate the upper and lower limits of the gland with vertebral levels because it moves in position relative to these structures during swallowing.
Thyroid hormones: formation, storage and release
The hormones T4 and T3 are originated by the follicular cells of the thyroid and are regulated by TSH.
The thyroid hormones thyroxine (T4) and triiodothyronine (T3) are produced from thyroid follicular cells within the thyroid gland, a process regulated by thyroid-stimulating hormone secreted by the anterior pituitary gland.
Thyroglobulin, the precursor to T4 and T3, is produced by thyroid follicular cells before being secreted and stored in the follicular lumen.
Iodine is actively absorbed from the bloodstream through a process called iodide entrapment.
In this process, sodium is transported together with iodide from the basolateral side of the membrane into the cell, and is then concentrated in the thyroid follicles to approximately thirty times its concentration in the blood.
Effects of iodine deficiency
If there is a deficiency of iodine in the diet, the thyroid will not be able to produce thyroid hormone.
Lack of thyroid hormone will lead to decreased negative feedback in the pituitary gland, which in turn will lead to increased production of thyroid-stimulating hormone, which causes the thyroid to enlarge (goiter).
This enlarged endemic colloid goiter has the effect of increasing the thyroid’s ability to trap more iodide, compensate for iodine deficiency, and allow it to produce adequate amounts of thyroid hormone.
Effect of thyroid hormones on metabolism
The main activity of the thyroid hormones T3 and T4 is to increase the basal metabolic rates of proteins, fats and carbohydrates, as well as vitamins.
Effect of thyroid hormones on body temperature
Thyroid hormones affect the dilation of blood vessels, which in turn affects the rate at which heat can escape from the body.
The more dilated the blood vessels are, the faster the heat can escape.
A person suffering from hyperthyroidism (an overactive thyroid) will experience a fever; conversely, a person suffering from hypothyroidism (a less active thyroid) will experience a decrease in body temperature.
Action of thyroid hormones in the developing fetus
Developing brain cells are a prime target for T3 and T4.
Thyroid hormones play a particularly crucial role in brain maturation during fetal development by regulating actin polymerization during neuronal development.
Action of thyroid hormones in the blood
In the blood, T4 and T3 are partially bound to thyroxine-binding globulin (TBG), transthyretin, and albumin.
Only a very small fraction of the circulating hormone is free T4: 0.03% and T3 0.3%. Only the free fraction has hormonal activity.
As with steroid hormones, thyroid hormones are lipophilic and can cross the cell membrane and bind to intracellular receptors, which act alone as transcription factors or in association with other factors to modulate DNA transcription.
Calcitonin works to lower blood calcium levels in three ways:
- Inhibition of osteoclast-mediated bone breakdown.
- Stimulate osteoblastic activity to produce new bone tissue.
- Inhibition of calcium reabsorption in the kidneys.
Control of thyroid hormone release
The production of thyroxine and triiodothyronine is regulated by thyroid-stimulating hormone (TSH) that is released from the anterior pituitary.
The production of thyroxine (T4) and triiodothyronine (T3) is mainly regulated by thyroid stimulating hormone (TSH) which is released from the anterior pituitary gland.
The release of thyroid-stimulating hormone, in turn, stimulates the hypothalamus to secrete thyrotropin-releasing hormone.
This results in increased metabolism, growth, development, and the activation of many other systems controlled by thyroid hormones.
Thyroid hormones also provide negative feedback to the hypothalamus and the anterior pituitary gland.
If thyroid levels in the blood rise, thyroid-stimulating hormone and thyrotropin-releasing hormone production are reduced.
Excess TRH can also inhibit the production of additional TRH.
Thyroid stimulating hormone test
The thyroid stimulating hormone test measures the amount of this hormone in the blood and simply involves drawing a little blood from the body, to be analyzed later in a laboratory.
The best time to do this is in the morning as thyroid stimulating hormone levels can fluctuate throughout the day.
Thyroid-stimulating hormone exhibits diurnal variation, peaking between midnight.
No preparation is needed such as an overnight fast, however if you take certain medications, such as dopamine and lithium, you may need to stop before having the test.
A thyroid stimulating hormone test is done to determine if the thyroid gland is working as it should or if there are certain fluctuations.
Increased thyroid stimulating hormone
A decrease in thyroid hormone levels activates the feedback loop to increase thyroid stimulating hormone production.
- Congenital hypothyroidism in the neonate (filter paper test).
- Thyroid-stimulating hormone-producing ectopic tumors (in the lung, in the breasts).
- Primary hypothyroidism (related to a dysfunctional thyroid gland).
- Secondary hyperthyroidism due to pituitary hyperactivity.
- Thyroid hormone resistance.
- Thyroiditis (Hashimoto’s autoimmune disease).
Thyroid-stimulating hormone decreased
An increase in thyroid hormone levels activates the feedback loop to decrease TSH production.
- Excessive thyroid hormone replacement.
- Graves’ disease.
- Primary hyperthyroidism.
- Secondary hypothyroidism (related to pituitary involvement that decreases TSH production).
- Tertiary hypothyroidism (related to hypothalamic involvement that decreases TRH production).
Factors that interfere with the test
Drugs and other substances that can increase thyroid stimulating hormone levels include: amiodarone, benserazide, erythrosine, flunarizine, iobenzamic acid, iodides, lithium, methimazole, metoclopramide, morphine, propranolol, radiographic medium, HRT, and valproic acid.
Medications and other substances that can lower thyroid stimulating hormone levels include: acetylsalicylic acid, amiodarone, anabolic steroids, carbamazepine, corticosteroids, glucocorticoids, hydrocortisone, interferon-alpha-2b, iodamide, levodopa (in hypothyroidism), levothyroxine, methergoline, nifedipine, T4 and triiodothyronine T3.
Failure to allow the filter paper sample to dry may affect the test results.
Symptoms of hyperthyroidism
Common symptoms include:
- Weight loss despite a normal appetite.
- Sweating and heat intolerance
- Shaking and tremors.
- Generalized muscle weakness.
- Menstrual abnormalities.
- Bulging eyes.
- Goiter (swelling of the front of the neck).
Symptoms of hypothyroidism
Common symptoms include:
- Lethargy and tiredness.
- Cold intolerance
- Weight gain.
- Dry skin and hair.
Problems associated with variations in thyroid stimulating hormone
Nutrition problems (related to slow metabolism)
Decreased appetite is observed with weight gain, high-calorie, high-sodium food selection, sedentary lifestyle, caloric intake greater than metabolic needs, constipation, and decreased activity. The patient should be taught to:
- Avoid foods high in sodium, saturated fat and cholesterol, teach the patient to eat a diet high in protein and low in calories to promote weight loss.
- Encourage the patient to eat small, frequent meals to avoid overeating and improve weight control.
- Encourage the consumption of foods high in fiber, such as fruits and vegetables, with peels and whole grain breads to improve gastric motility; control daily weight.
- Accurately assess appetite and measure caloric intake over a 24-hour period.
- Organize the dietary consultation.
Decreased cardiac output (related to a thyroid hormone deficiency)
Bradycardia, lethargy, hypotension, decreased thyroid hormone levels, fatigue, activity intolerance, poor peripheral perfusion, cold skin, and shortness of breath are observed. The patient should be taught to:
- Assess and detect vital signs and blood pressure.
- Monitor and study thyroid laboratory studies (TSH, T3, T4, radioactive iodine uptake).
- Evaluate indicators of cardiac status (peripheral pulses, skin color, skin temperature, dry and flaky skin),
- Evaluate periorbital edema.
- Administer prescription medications to replace thyroid hormone.
- Facilitate measures to improve the patient’s warmth (blankets, warm clothing and fluids, warmer environment).
- Monitor activity and schedule rest periods to control fatigue.
- Use pulse oximetry to monitor oxygen saturation.
- Assess the verification of the respiratory status of crackles and increased respiratory rate.
- Control fluid overload.
Altered thought processes (related to decreased cardiac output and impaired cerebral perfusion secondary to thyroid hormone deficiency)
Altered memory, mental disability, decreased concentration, depression, imprecise environmental perception, inappropriate thinking, memory deficits are observed. The patient should be taught to:
- Minimize apprehension and fear.
- Collaborate with the doctor to manage the problem associated with decreased cerebral perfusion.
- Promote understanding of current events.
- Provide a modified environment that promotes security.
- Monitor the ability to provide self-care (activities of daily living).
- Monitor the risk of injury (violence, risk of falls, risk of self-harm).
- Administer prescription medications to replace thyroid hormone.
Self-esteem (related to chronic diseases, physiological deterioration associated with thyroid hormone deficiency)
Negative self-evaluation is observed, negative evaluation of personal abilities, expressions of shame or guilt, looks for excessive reassurance, exaggerates negative feedback, indecisive, passive, non-assertive and compliant.
The patient should be taught to:
- Facilitate a positive outlook based on real rather than exaggerated factors that influence health.
- Facilitate the recognition of personal strengths and values.
- Provide a safe environment for the verbalization of concerns.
- Collaborate with the patient to design an appropriate plan of care.
- Collaborate with the patient to promote effective decision making.
- Monitor the frequency of negative self-comments.
- Promote the concept of counseling to improve the concept of self-esteem.
- Use role play and modeling to improve positive behavioral skills.
- Provide information on community resources for ongoing counseling.