The RAAS system and thyroid hormones play an essential role in managing metabolism, blood pressure and fluid balance. These hormones regulate electrolytes, water retention and metabolic rate.
RAAS
The renin-angiotensin-aldosterone system involves several organs and plays an essential role in maintaining blood pressure. A drop in blood flow to the kidneys activates the RAAS to increase blood pressure and maintain filtration in the kidneys.
The kidneys convert circulating prorenin into renin in response to reduced renal blood flow. Renin converts angiotensinogen, produced in the liver, into angiotensin I. The lungs contain ACE (angiotensin converting enzyme) which converts angiotensin I into angiotensin II.
Angiotensin II degrades into angiotensin III which stimulates aldosterone secretion from the adrenal cortex. Angiotensin III increases the blood pressure while angiotensin II narrows the blood vessels (vasoconstriction.)
Aldosterone is a mineralocorticoid hormone produced by the adrenal cortex. Aldosterone increases sodium and water retention, as well as potassium excretion. This keeps the electrolytes (salts) in balance and keeps water in the circulation to increase blood pressure.
This system has several potential drug targets for treating hypertension (high blood pressure), heart disease, kidney failure and diabetes. Examples include ACE inhibitors, angiotensin II receptor blockers and renin inhibitors.
Activation of RAAS
Any loss in blood volume or drop in blood pressure triggers the RAAS, such as dehydration or significant haemorrhage. The system raises blood pressure and improves blood flow in the kidney’s glomeruli.
It also triggers the release of anti-diuretic hormone (ADH), also known as vasopressin which controls water reabsorption and retention in the kidneys. ADH also acts on the brain to increase appetite for salt and thirst for water.
Thyroid Hormones
The thyroid gland is located in the neck, just below the larynx. It typically has two lobes, one on each side, with a pair of parathyroid glands on each side as well. The term ‘goitre’ means an enlargement of the thyroid gland which you can feel, or even see if it is large enough. A goitre does not always mean a tumour or cancer.
The two main thyroid hormones are triiodothyronine (T3) and thyroxine (T4). They are created from tyrosine and iodine, the only function iodine has in the body. The thyroid hormones increase basal metabolic rate, absorption rate in the gut and fat breakdown. They also increase heart rate, oxygen consumption and play a role in growth and brain development.
Regulation of Thyroid Hormone Release
Like others we’ve discussed, the hypothalamus controls release of thyroid hormones. It secretes thryotropin releasing hormone (TRH) which stimulates the anterior pituitary to produce thyroid-stimulating hormone (TSH). TSH then acts on the thyroid gland to produce T3 and T4. The thyroid hormones have a negative feedback effect on the hypothalamus and pituitary gland to stop further hormone release.
Thyroid Disorders
Thyroid disorders are fairly common in humans. Hyperthyroidism (excess thyroid hormones) is extremely common in elderly cats. Hypothyroidism (underactive thyroid) is common in dogs.
Hyperthyroidism means excess production of T3 and T4. Grave’s disease and thyroid adenoma are common causes. In cats, a benign thyroid adenoma produces excess thyroid hormones, ignoring the normal regulatory system the body has.
Symptoms of hyperthyroidism include unexplained weight loss, hair loss, an increased appetite, fast heart rate and stress or restlessness. It can be treated with drugs that suppress thyroid function, radioactive iodine (I-131) therapy or surgery to remove part/all of the thyroid (partial or total thyroidectomy.)
Hypothyroidism is an underactive thyroid. Remember the terminology series: hyper- means too high, hypo- means too low. An underactive thyroid is common in dogs and in humans. Iodine deficiency is a common cause, as is an autoimmune disease such as Hashimoto’s thyroiditis.
Symptoms of hypothyroidism include weight gain that is difficult to lose, fatigue, constipation, hair loss, a slow heart rate and myxoedema. Myxoedema is abnormal swelling in limbs or the face, and is a type of non-pitting oedema. Pitting oedema means if you press it with a finger, the dent remains. Non-pitting oedema obviously means it doesn’t.
Treatment for hypothyroid conditions includes immune modulating treatments, iodine supplements and replacement thyroid hormone like levothyroxine.
Axolotls
Axolotls are odd little creatures. They exist in a state of arrested development, never moving beyond the juvenile phase of the life cycle. This happens because they lack iodine and cannot produce thyroxine. They can reproduce but they retain their gills and remain fully aquatic.
They are also capable of regenerating lost limbs. If they are treated with thyroid hormone or exposed to idone, they metamorphose into salamanders. However, they don’t survive well if this happens.
Conclusion
These complex systems are often misunderstood but they are important to maintain the body’s physiology as it should be. Next week we’ll cover the remaining systems, insulin and parathyroid hormones.
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