Physiology
Endocrine
Regarding the thyroid hormones, which of the following statements is CORRECT:
Answer:
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.Thyroid Function
Physiology / Endocrine / Thyroid Function
Last Updated: 2nd May 2019
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](https://mrcemsuccess.com/wp-content/uploads/2018/08/Thyroid.png)
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](https://mrcemsuccess.com/wp-content/uploads/2018/08/Thyroid-System.png)
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 |
