What is the thyroid gland
The thyroid gland consists of two lobes and is found below the larynx. It is responsible for the control of energy expenditure and metabolism. It carries out its functions by the production of hormones called thyroxine (T4), and tri -iodothyronine (T3). The production of these hormones is controlled by the hypothalamus, pituitary gland and circulating levels of the two hormones.
The hypothalamus releases thyrotrophin releasing hormone (TRH) in response to low T4 and T3 levels in the bloodstream. TRH acts on the pituitary gland enhancing the secretion of thyroid stimulating hormone (TSH). TSH then stimulates the thyroid gland to produce T4 and T3. This increases the blood levels of T4 and T3, and negative feedback to both the hypothalamus and pituitary gland stops the further stimulation of T4 and T3 production. In health when levels of the two hormones fall below the reference range again the whole process repeats itself.
Causes of hypothyroidism
There are many causes of an underactive thyroid gland including:-
- Hashimoto's thyroiditis-
This is the most common cause of hypothyroidism and is an autoimmun disease where the body produces antibodies which attack the thyroid gland and thyroid cells. The result of this process is a decrease in thyroid stimulating hormone, and therefore hypothyroidism surfaces.
- Lithium Treatment-
Lithium is given to treat manic depression and bipolar disorder, but it is known to inhibit the production of T3 and T4. Therefore people on this drug need their TSH profile monitored on a regular basis.
- Iodine containing drugs such as amiodarone-
This drug has also been shown to inhibit the formation of the thyroid hormones by the thyroid gland.
- Hyperthyroidism Treatment
In this condition there is a lack of TSH and an excess of the thyroid hormones. Treatment with radioactive iodine or removal of the thyroid gland reduces the amount of the thyroid hormones produced, and gives rise to hypothyroidism.
- Postpartum Thyroiditis
This is a rare cause of hypothyroidism which occurs in the 6 months after pregnancy. It is represented by an inflammed thyroid gland, and around a third of sufferers will go on to develop permanent hypothyroidism.
A small number of cases of hypothyroidism are actually to do with problems in the functioning of the hypothalamus and pituitary gland.
Symptoms of hypothyroidism
If you are experiencing some of the following symptoms then you may have an underactive thyroid gland. However many of these symptoms are non-specific and may be present in a wide range of conditions.
- Excessive tiredness, sluggishness and general exhaustion
- Weight changes either loss or gain and the inability to lose weight through diet and exercise
- Disturbances in sleep
- Brittle nails
- Brittle hair with some hair loss
- Changes in skin texture,not limited to dryer more coarse skin
- Changes in voice so that it appears more hoarse
- More periods of constipation than normal, which are not caused by a change in diet or poor diet.
- Carpal tunnel syndrome, as well as aches in hands and feet.
- Mood swings and feeling more restless
- Personality changes including feeling worthless and appearing depressed and forgetful.
- Slow heart beat.
Other symptoms such as heavy menstrual periods and a poor sex drive occur more frequently in those that have hypothyroidism.
If you are experiencing some or all of these symptoms you should visit your clinician for proper diagnosis.
The diagnosis is typically made by taking a blood sample and analysing the levels of Thyroid Stimulating Hormone (TSH) and thyroxine (T4), specifically free T4. In subclinical hypothyroidism the level of TSH is above the reference range but the level of free T4 is within the range. As time passes the level of free T4 may drop further below the reference range, resulting in an increase in TSH giving rise to full hypothyroidism.
Hypothyroidism is treated by replacing T4 with synthetic T4 known as levothyroxine. Initially a small dose should be prescribed as large doses may cause hyperthryoidism. After a month the patient should have their TSH levels re checked and if the level is still high then an increase in dose should be considered. This frequent monitoring must continue until the patient's TSH levels are stabilised within the normal range. When this happens TSH levels can be monitored less closely, and a blood test every three months is suffice. Everyone differs in the dose required and period of time taken to stabilise their levels.