Thyroid hormones are synthesised in the thyroid gland by the pairing of two molecules of amino acid tyrosine. This procedure is reliant on the body having an adequate supply of iodine, which is achieved by absorption from diet in the small intestine. In thyroxine there are four atoms of iodine in each molecule and in triiodothyronine there are three atoms of iodine in each molecule. (Zilva et al, 1988). It is believed T4 becomes active when it is converted by the liver to T3. This is achieved by the removal of one atom of iodine. Normally, a hormone called thyroid stimulating hormone (TSH) or thyrotrophin maintains amounts of T3 and T4 in the blood within very narrow limits. TSH is secreted by the anterior pituitary gland. When the levels of thyroid hormones fall in the blood the pituitary increases secretion of TSH, therefore increasing the amount of hormone the thyroid produces. Once thyroid hormone levels rise TSH secretion is stopped. (Toft, 2006)
When thyrotoxicosis or hyperthyroidism occurs, usually as the result of an antibody in the blood that stimulates production of thyroid hormones and occasionally increases the size of the thyroid gland (producing a goiture) it is often the result of Graves’ disease, which in some cases may be hereditary. Patients with Graves’ disease may also suffer from bulging eyes (exopthalmos), and possibly raised red areas on the skin of the limbs and possibly thickening of the skin (pretebial myxoedema). It is believed the thyroid stimulating antibodies and symptoms may be caused by an abnormality in the patients’ immune system. (Toft, 2006). The clinical symptoms of an overactive thyroid include increased appetite, due to the fact that the excessive amounts of thyroid hormone will burn off calories (energy) very quickly.
As metabolism increases the body will also produce excessive heat therefore causing an increase in sweating as the body tries to cool itself. Most people with Graves’ disease become irritable and have difficulty concentrating. It can be seen that a person may suffer from palpitations, and in the elderly may further develop into irregular heartbeat, atrial fibrillation and even heart failure. Tremors of the hands, along with general muscle weakness and itching all over are some more common symptoms. It may be seen that hair becomes thinner and nails become brittle. Graves’ disease can occur at any age but is more common in females. (Zilva et al, 1988). Symptoms of hyperthyroidism can be acute or chronic depending on the severity. Hyperthyroidism itself tends to be a chronic condition that may come and go in bouts until a successful treatment is found.
Thyrotoxicosis can be diagnosed by a blood test which measures the actual levels of T3, T4 and TSH in the blood. This is known as a thyroid function test. In the blood 99% of thyroid hormones are bound to proteins, usually α-globulin, thyroxine-binding globulin (TBG) and thyroxine binding prealbumin are inactive. The 1% of unbound hormones are therefore active and free to control metabolism in the body. (Zilva et al,1988)
Measurements are taken of both free and bound hormones T3 and T4 this is known as total T3(TT3) and total T4(TT4). A measure is also taken of the unbound hormones T3(fT3) and T4(fT4). It is the measurement of fT3 and fT4 that could indicate a thyroid disorder. In people with thyrotoxicosis readings of TT4 would be about 190nmol/l (nanomols per litre). TT3 readings would show a reading of approximately 4 nmol/l. Unbound/free hormone would be approximately 12 pmol/l (picomol per litre) of fT3 and 40 pmol/l fT4. A patient with hyperthyroidism would have such a low level of TSH in the blood that it would be undetectable. (Toft, 2006)
Treatment of Graves’ disease is dependent on the severity and accompanying symptoms. Treatment of Graves' disease aims to keep thyroid hormone levels within the normal range and prevent eye problems due to exposure of the delicate eye tissues which can be very difficult. Treatment for the eye problems includes drug treatments and eye drops to reduce swelling and close lids, steroids tablets if the eye muscles are paralysed or badly swollen and even surgery to reduce swelling or close lids. (Zilva et al, 1988)
When treating raised hormone levels drugs for immediate and then long term continuation are used. (Zilva et al,1988) The drugs used may be beta blockers which are used for acute symptomatic treatment, and antithyroid drugs for long term treatment. Antithyroid drugs such as carbimazole can be prescribed for either short-term control of hyperthyroidism prior to having a definitive treatment, or in the hope of achieving remission in patients with Graves' disease. It works by reducing the amount of hormone the thyroid gland makes. A course 6-18 months is recommended especially in those presenting who are under the age of 40. The probability of achieving long-term remission is less than 50%. It can also be seen that rates of remission are lower in males, in the elderly, and in those patients with severe thyrotoxicosis. (Laurence et al, 1987)
Surgical intervention may be necessary if the patient is suffering from a large goiture that is unlikely to resolve with antithyroid drugs. Surgery to remove up to three quarters of the thyroid gland may be performed on a person under the age of 50 who are having their second bout of thyrotoxicosis. Before surgery can take place normal levels of thyroid hormones must be achieved by taking carbimazole, and an iodine medication is usually given to shrink the size of the gland and reduce its blood flow. For about eighty percent of patients the operation works well, although in fifteen percent of people, surgery would result having too much gland removed and suffer hypothyroidism. The remaining five percent will not have enough gland removed and still suffer hyperthyroidism. (Toft, 2006).
Another treatment option is radioactive iodine (RAI), which tends to be reserved as a treatment for people over the age of 45 years. It can be taken as a capsule or drink which is administered in hospital. It works by destroying some of the cells in the thyroid and not allowing remaining cells to divide to form new cells. It has to be noted that it is hard to find a dose of RAI that will give a good cure rate for thyrotoxicosis without leading to hypothyroidism, which currently can affect as many as 20 percent of those treated within the first two years after treatment, and three to five percent more each year after that. Those affected may have to take supplements of thyroid hormones (thyroxine) in order to get enough thyroid hormone. (Toft, 2006)
To conclude, most diseases of the thyroid gland can be treated and will not reduce life expectancy if treated properly. Diagnosis is simple, results from blood tests are reliable and treatment is usually successful. The body needs to have a normal balance of hormones in order to function adequately and any disturbance can lead to a variety of illness and disease.
References
Laurence D, Bennett P. (1987) Clinical Pharmacology, 6th Ed, London:Churchill Livingstone
Thibodeau G, Patton K.(2002) The Human Body in Health and Disease, 3rd Ed, London:Mosby
Toft A. (2006) Understanding thyroid disorders, Poole:Family Doctor Publications
Wilson K, Waugh A. (1996) Ross and Wilson – Anatomy and Physiology in Health and Illness, 8th Ed, London:Churchill Livingstone
Zilva J, Pannall P, Mayne P. (1988) Clinical Chemistry in Diagnosis and Treatment. 5th Ed, London:Hodder & Stoughton
Bibliography
Laurence D, Bennett P. (1987) Clinical Pharmacology, 6th Ed, London:Churchill Livingstone
Thibodeau G, Patton K.(2002) The Human Body in Health and Disease, 3rd Ed, London:Mosby
Toft A. (2006) Understanding thyroid disorders, Poole:Family Doctor Publications
Wilson K, Waugh A. (1996) Ross and Wilson – Anatomy and Physiology in Health and Illness, 8th Ed, London:Churchill Livingstone
Zilva J, Pannall P, Mayne P. (1988) Clinical Chemistry in Diagnosis and Treatment. 5th Ed, London:Hodder & Stoughton
