Thyroid Hormone Regulation

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Questions and Answers

Which hormone promotes the release of thyroid hormones (T3 and T4) by the thyroid gland?

Adrenaline

Glucagon

Insulin

TSH (correct)

Which condition is monitored and regulated by TRH in the hypothalamus?

Adrenaline level

Insulin level

Thyroid hormone level (correct)

Cortisol level

Which of the following is a main feature of hypothyroidism?

Hand tremor

Sweating

Weight loss

Coarse and dry hair (correct)

Which symptom is commonly associated with hyperthyroidism?

Palpitations Signup and view all the answers

Which body cells are the effectors in the control of thyroid hormone secretion?

Most body cells Signup and view all the answers

Which of the following is not a diagnostic test to establish thyroid dysfunction?

CT scan Signup and view all the answers

What percentage of the community is affected by thyroid diseases, especially in women?

3-5% Signup and view all the answers

What is the diagnostic value of a low T3 concentration for hypothyroidism?

No diagnostic value Signup and view all the answers

What stimulates the production of thyroid hormones?

TSH Signup and view all the answers

Which thyroid hormone has a concentration 50 times greater than the other in the bloodstream?

T4 Signup and view all the answers

What is the normal range of TSH in the blood?

0.4-3.8 mU/L Signup and view all the answers

What percentage of T3 is secreted directly by the thyroid gland?

20% Signup and view all the answers

Which of the following is a function of thyroid hormones?

Increases bone turnover Signup and view all the answers

What is the main protein involved in the synthesis of thyroid hormones?

Thyroglobulin Signup and view all the answers

Which of the following increases due to thyroid hormone function?

Increases erythropoietin production Signup and view all the answers

What is the final step in the production of thyroid hormones?

Transport of thyroid hormones in the blood Signup and view all the answers

Which laboratory finding is consistent with primary hypothyroidism?

Low T4, Low T3, High TSH Signup and view all the answers

What is the most common cause of hyperthyroidism?

Graves' disease Signup and view all the answers

Which symptom is NOT typically associated with hyperthyroidism?

Feeling cold and shivering Signup and view all the answers

What is a common feature of Graves' disease?

Thyroid acropachy Signup and view all the answers

Which condition is characterized by thickened and waxy skin on the lower leg?

Pretibial myxedema Signup and view all the answers

What is the common diagnostic feature of thyroid cancer?

High T3 and T4 concentrations Signup and view all the answers

Which imaging technique is utilized in differential diagnosis during thyroid cancer assessment?

TFTs Signup and view all the answers

What is not a cause of secondary hypothyroidism?

Hashimoto's thyroiditis Signup and view all the answers

Study Notes

Control of Thyroid Hormone Secretion

TSH promotes the release of thyroid hormones (T3 and T4) by the thyroid gland in response to low levels of thyroid hormones and increased metabolic rate.
TRH in the hypothalamus stimulates the release of TSH from the anterior pituitary.
TSH receptors are located in the anterior pituitary.
The thyroid follicle cells are the control center, and most body cells are the effectors.
The response to TSH is an increased metabolic rate and increased levels of thyroid hormones (T3 and T4).

Laboratory Investigations

Results are typically available within 48 hours, but in cases of thyrotoxic crisis or myxoedema coma, results are available within 2 hours.
Blood tests measure TSH, FT4, and FT3 to establish if there is thyroid dysfunction.
Auto-antibody tests are used to elucidate the cause of thyroid dysfunction.

Thyroid Diseases

Thyroid diseases are especially common in women, with a prevalence of 3-5% in the community.
Thyroid diseases can be caused by excessive or inadequate amounts of thyroid hormones, inflammation or infection of the thyroid gland, or benign or malignant growth.
Thyroid diseases are easily treatable with an excellent long-term outcome.

Hypothyroidism and Hyperthyroidism

Hypothyroidism is characterized by coarse and dry hair, dry skin, puffiness of the face, swelling around the eyes, tiredness and lethargy, bradycardia, weight gain, and intolerance to cold.
Hyperthyroidism is characterized by lid retraction, sweating, anxious look, goitre, palpitations, intolerance to heat, and weight loss.

Hypothyroidism (Under-Activity)

Causes of hypothyroidism include low T4, low T3, and high TSH (primary), low T4, low T3, and low/normal TSH (secondary), thyroiditis, certain drugs, congenital causes (leading to cretinism), iodine deficiency, and disorders of the pituitary gland or hypothalamus.

Hyperthyroidism (Over-Activity)

Causes of hyperthyroidism include high T4, high T3, and undetectable TSH, T3 toxicosis, thyrotoxicosis, Graves' disease, toxic nodular goiter, thyroiditis, drug-induced hyperthyroidism, gestational hyperthyroidism, struma ovarii, and thyroid-secreting pituitary adenoma.

Goitre

Goitre is an enlarged thyroid gland, which can be small, large, or huge.

Thyroid Hormones

TSH controls thyroid cell growth and synthesis, and secretion of thyroid hormones.
TSH secretion is inhibited by T4 and T3 (negative feedback).
Thyroid hormones produced by the thyroid gland regulate metabolism and growth.
TSH is synthesized by the anterior pituitary and composed of two subunits: α and β.
Normal range of TSH is 0.4-3.8 mU/L.

Thyroid Hormones - Functions

Thyroid hormones increase basal metabolic rate (oxygen consumption and heat production), cardiac output, and amplify the catecholamine effect.
They increase hypercapnic drive in the respiratory center, production of erythropoietin, bone turnover, and hepatic gluconeogenesis and glycogenolysis.

Circulating Concentration

T4: 60-170 nmol/L, involved in digestion, brain development, and bone health.
T3: 0.8-2.7 nmol/L, involved in 20% of thyroid hormone production, and 80% is produced from the conversion of thyroxine.

The Production of Thyroid Hormones

Iodide trapping: The thyroid gland gathers iodine from the blood.
Synthesis of thyroglobulin: A protein called thyroglobulin is produced.
Oxidation of iodide: Iodine is converted to its active form.
Iodination of tyrosine: Iodine is attached to tyrosine (an amino acid) in thyroglobulin.
Coupling of T1 and T2: Iodinated tyrosine molecules link together.
Pinocytosis and digestion of colloid: The thyroglobulin containing colloid is engulfed and broken down.
Secretion of thyroid hormones: T4 and T3 are released in the blood stream.
Transport of thyroid hormones in the blood: T4 and T3 travel through the blood stream carried by proteins.

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Description

Understand how TSH promotes the release of thyroid hormones and the control mechanisms involved in regulating thyroid hormone secretion. Learn about the stimulus, controlled conditions, receptors, control center, effectors, and response in thyroid hormone regulation.