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Physiology

Endocrine

Question 137 of 180

Regarding the thyroid hormones, which of the following statements is CORRECT:

Answer:

Thyroid tissue is made up of colloid which contains iodinated thyroglobulin. Thyroglobulin is synthesised by the surrounding follicular cells and is the large molecule from which thyroid hormones are made. The thyroid gland produces two thyroid hormones, thyroxine (T4) and triiodothyronine (T3). More T4 than T3 is synthesised -the average plasma concentration of T3 is roughly one-sixth that of T4 - however, T4 is converted peripherally to the more potent and shorter-acting T3. Most of the thyroid hormones in the blood are bound tightly to proteins in the circulation - thyroxine-binding protein (TBG), transthyretin and albumin - and are thus unavailable to their receptors which are located inside target cells.

Thyroid Function

Anatomical Location

The thyroid gland is attached to the anterior surface of the trachea just below the larynx. It has a right and left lobe joined by a central isthmus. Thyroid lesions can be distinguished from other neck lumps by their movement on swallowing. The recurrent laryngeal nerve lies laterally on each side and the parathyroid glands lie posteriorly - both may be damaged during thyroid surgery. The thyroid gland has a rich vascular supply from the inferior and superior thyroid arteries.

By OpenStax College [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

Anatomy of the Thyroid Gland. (Image by OpenStax College [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons)

Production and Transport of Thyroid Hormones

Thyroid tissue is made up of colloid which contains iodinated thyroglobulin. Thyroglobulin is synthesised by the surrounding follicular cells and is the large molecule from which thyroid hormones are made.

The thyroid gland produces two thyroid hormones, thyroxine (T4) and triiodothyronine (T3). More T4 than T3 is synthesised -the average plasma concentration of T3 is roughly one-sixth that of T4 - however, T4 is converted peripherally to the more potent and shorter-acting T3.

The hormones are released under the control of thyroid-stimulating hormone (TSH) from the anterior pituitary, which itself is regulated by thyrotropin-releasing hormone (TRH) from the hypothalamus in response to thermoreceptor and metabolic signals, and circulating levels of T3 and T4 detected by both the hypothalamus and pituitary (negative feedback).

Most of the thyroid hormones in the blood are bound tightly to proteins in the circulation - thyroxine-binding protein (TBG), transthyretin and albumin - and are thus unavailable to their receptors which are located inside target cells. Only the small amounts of free T3 and T4 in plasma can readily cross the cell membranes to bind to thyroid hormone receptors. Thyroid receptors are present in almost all tissues, with particularly high levels in the liver.

By Mikael Häggström (All used images are in public domain.) [Public domain], via Wikimedia Commons

Thyroid Hormones. (Image by Mikael Häggström [Public domain], via Wikimedia Commons)

Physiological Effects of Thyroid Hormones

Basic levels of thyroid hormone release are essential to maintain a normal metabolic rate. Situations requiring increased heat production, for example when the core temperature falls, leads to enhanced activation of the thyroid axis. The effects take up to 4 days to reach a maximum.

Physiological effects of thyroid hormones include:

  • Heat production (thermogenesis)
  • Increased basal metabolic rate
  • Metabolic effects
    • Increase in protein turnover (both synthesis and degradation are increased, although overall effect is catabolic)
    • Increase in lipolysis
    • Increase in glycogenolysis and gluconeogenesis
  • Enhanced catecholamine effect
    • Increase in heart rate, stroke volume and thus cardiac output
  • Important role in growth and development

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  • Biochemistry
  • Blood Gases
  • Haematology
Biochemistry Normal Value
Sodium 135 – 145 mmol/l
Potassium 3.0 – 4.5 mmol/l
Urea 2.5 – 7.5 mmol/l
Glucose 3.5 – 5.0 mmol/l
Creatinine 35 – 135 μmol/l
Alanine Aminotransferase (ALT) 5 – 35 U/l
Gamma-glutamyl Transferase (GGT) < 65 U/l
Alkaline Phosphatase (ALP) 30 – 135 U/l
Aspartate Aminotransferase (AST) < 40 U/l
Total Protein 60 – 80 g/l
Albumin 35 – 50 g/l
Globulin 2.4 – 3.5 g/dl
Amylase < 70 U/l
Total Bilirubin 3 – 17 μmol/l
Calcium 2.1 – 2.5 mmol/l
Chloride 95 – 105 mmol/l
Phosphate 0.8 – 1.4 mmol/l
Haematology Normal Value
Haemoglobin 11.5 – 16.6 g/dl
White Blood Cells 4.0 – 11.0 x 109/l
Platelets 150 – 450 x 109/l
MCV 80 – 96 fl
MCHC 32 – 36 g/dl
Neutrophils 2.0 – 7.5 x 109/l
Lymphocytes 1.5 – 4.0 x 109/l
Monocytes 0.3 – 1.0 x 109/l
Eosinophils 0.1 – 0.5 x 109/l
Basophils < 0.2 x 109/l
Reticulocytes < 2%
Haematocrit 0.35 – 0.49
Red Cell Distribution Width 11 – 15%
Blood Gases Normal Value
pH 7.35 – 7.45
pO2 11 – 14 kPa
pCO2 4.5 – 6.0 kPa
Base Excess -2 – +2 mmol/l
Bicarbonate 24 – 30 mmol/l
Lactate < 2 mmol/l

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