Hypothyroidism is a condition characterised by low levels of the thyroid hormones T3 and T4 in the bloodstream, where the thyroid gland is described as being underactive. Hypothyroidism can be either primary or secondary. In primary hypothyroidism, there is an issue with the thyroid and its ability to produce the thyroid hormones. Secondary hypothyroidism is indicative of a problem in the hypothalamus or the pituitary; the two endocrine glands which control the actions of the thyroid gland. Hypothyroidism is one of the most common endocrine problems in Australia and most common in middle-aged and older women; but is also a significant issue for infants. The signs and symptoms of hypothyroidism include:
- Unexplained weight gain
- Fatigue and lethargy
- Muscle aches
- Hoarse voice
- Slow heart rate (bradycardia)
- Problems maintaining concentration
- Thinned hair
- Intolerance to cold temperatures
- Menstrual irregularities
Initially, signs and symptoms of hypothyroidism are mild or absent. If untreated, the symptoms increase in severity and can produce an array of complications. Long-term complications include infertility, goitre (swelling of the thyroid due to long-term stimulation) and hypotension (low blood pressure). Hypothyroidism during pregnancy is detrimental and can produce birth defects, such as cretinism in the child.
There are several causes of hypothyroidism. In certain regions of the world, iodine deficiency is the principal cause of hypothyroidism. Iodine is necessary for the synthesis of the thyroid hormones, T3 and T4. Due to addition of iodine to table salt, iodine deficiency hypothyroidism is less of an issue in Australia now. Hashimoto’s thyroiditis is an autoimmune disease where the body produces antibodies which inhibit the thyroid-stimulating hormone (TSH) receptor. TSH is the hormone which provides the stimulus for the thyroid to produce T3 and T4; hence blocking its receptor removes this trigger, resulting in reduced thyroid hormone production. Hyperthyroidism treatments are another cause of hypothyroidism. Hyperthyroidism treatments aim to reduce production of thyroid hormones. As a result, this can result in production of too little thyroid hormone, producing the opposite problem. Hypothyroidism is common after thyroid surgery and treatment with radioactive iodine.
Hypothyroidism is treated with thyroid hormone replacement therapy. In most cases, T4 is the hormone that is replaced because it lasts for a longer time in the circulation than T3, and can be taken orally.T4 also acts on the pituitary and hypothalamus to reduce the secretion of TSH, which is elevated in hypothyroidism. T3 may also be replaced, but is not generally the first-line treatment. T3 does not last as long in the bloodstream as T4, and can also produce toxic effects on the heart. In most cases, thyroid hormone replacement therapies are taken once daily. Thyroid hormone replacement therapy is monitored closely, particularly in the early stages to ensure that the correct dose of hormone is being administered to maintain normal thyroid function. As with all drug treatments, thyroid hormone replacement therapy can produce its own set of side effects and may also interact with other medications. Side effects of thyroid hormone replacement therapy include:
- Increased heart rate
- Weight loss
- Gastrointestinal symptoms (nausea, vomiting, diarrhoea)
Pregnant women receiving thyroid hormone replacement therapy need to have their dosing regimen monitored, as they are likely to require a slightly higher dose during pregnancy to prevent problems related to hypothyroidism. Elderly patients may need to take a lower dose of thyroid hormone as requirements decrease with age. If you have questions or concerns about thyroid problems see your local doctor who will arrange for you to see a thyroid surgeon.