Hypothyroidism – An Overview
What is hypothyroidism?
This is when the thyroid produces less thyroid hormone than it should which causes the metabolism to run too slow. This is called hypothyroidism, myxoedema or an underactive thyroid. It may also be called Hashimoto’s disease.
Subclinical hypothyroidism (borderline hypothyroidism) is when patients have symptoms of hypothyroidism but only one of the thyroid tests is abnormal – the thyroid stimulating hormone (TSH) test.1
The pituitary produces a hormone called thyroid stimulating hormone (TSH) which stimulates the thyroid to produce thyroxine (T4). Thyroxine is inactive and needs to be converted by the tissues and organs of the body into the active hormone, triiodothyronine (T3).
Hypothyroidism is mostly seen in women between the ages of 40-50 and is seen in women ten times more often than men.2 It often occurs during the menopausal years and symptoms are often ignored during the early stages by both patients and doctors if the patient is at this age.
The prevalence of hypothyroidism is 1-2% (approximately 2 in every 100 people) but some doctors feel that the rate is much higher than this.3
Subclinical hypothyroidism is found in 8–10% of the population, is more common in women and increases with age.4
What causes it?
The main causes of an underactive thyroid are:
- Hashimoto’s disease – an autoimmune disease where the antibodies may first stimulate the thyroid and then destroy it 4
- genetic dysfunction – the thyroid may be dysfunctional at birth (congenital hypothyroidism) 5, or there is a polymorphism (a gene that is different to usual) that isn’t found until the person is an adult 6
- pituitary or hypothalamic failure, causing secondary hypothyroidism (central hypothyroidism) 5
- iodine or selenium deficiency 5,7
- environmental challenges or deficiencies – chemicals and some foods and drinks can cause problems for the thyroid 8,9
- previous thyroid surgery (thyroidectomy) or radioactive iodine treatment 5
- drug treatment for hyperthyroidism 5
- certain drugs can cause hypothyroidism 5
- inability to absorb synthetic levothyroxine adequately 10
- lack of conversion from T4 to T3 10
- receptor resistance 12
What are the symptoms?
There are many signs and symptoms of an underactive thyroid, the main ones being: 13,14,15,16,17,18
- weight gain, even though you eat less
- cold intolerance
- muscle weakness, arthralgia, myalgia
- menstrual problems
- impaired concentration and memory
- hair loss body, scalp and eyebrows
- hoarse voice/deepening of voice
- goitre, difficulty swallowing
- delayed relaxation of deep tendon reflexes
- loss of libido
- fertility problems
- raised cholesterol
- dry/gritty eyes
See Signs and Symptoms of Hypothyroidism for full list.
What tests are used to diagnose hypothyroidism?
Blood tests should reveal whether the thyroid gland is underactive although some doctors may not do all the blood tests available.
There are several thyroid tests that can be done to diagnose hypothyroidism: 19
- TSH (thyroid stimulating hormone)
- FT4 (FreeT4)
- FT3 (FreeT3)
- TPO (thyroid peroxidase antibody)
- TgAb (thyroglobulin antibody)
However, the TSH test is often the only test performed by doctors which means there is a possibility that autoimmune hypothyroidism and secondary hypothyroidism may be missed.
Some doctors think that this test is not suitable on its own because it is an indirect test and not a true thyroid hormone test. The (now archived) guidance, “UK Guidelines for the Use of Thyroid Function Tests”20 state that although a TSH test “may be cost effective for a wide range of clinical purposes it may be inappropriate in patients being tested for the first time, and in some specific clinical settings.” These guidelines say that TSH and FT4 should be done when optimising levothyroxine therapy in newly diagnosed patients with hypothyroidism, diagnosing and monitoring thyroid disorders in pregnancy and monitoring patients with hyperthyroidism in the early months after treatment.
These guidelines also say that both tests should be done in rare situations such as central hypothyroidism, end-organ thyroid hormone resistance and TSH-secreting pituitary adenomas and “if clinical details are not available that allow the identification of the above categories of patient, then it may be prudent for laboratories to measure serum TSH and FT4 on all specimens rather than embark on a first-line serum TSH testing strategy followed by a cascade to include FT4 and FT3 if indicated.”
In our experience, many doctors are not aware of either the rarer forms of hypothyroidism or these particular sections of the guidelines. Therefore, it might be an idea to discuss the guidelines with your GP even though they are archived.
Unfortunately, more recent guidance doesn’t clarify that FT4 testing should be done, even if the patient is on liothyronine (synthetic T3). 21,22
Thyroid UK feels that in many instances, patients should be referred to an endocrinologist to check for central hypothyroidism and other thyroid disorders. However, patients are very rarely referred to an endocrinologist for hypothyroidism as it is considered that the GP can deal with treatment.
What test results would give a diagnosis of hypothyroidism?
A level of FT4 below the reference range together with a TSH level above the reference range will usually give a diagnosis of hypothyroidism.
However, the “UK Guidelines for the Use of Thyroid Function Tests” state that, “There is no evidence to support the benefit of routine early treatment with thyroxine in non-pregnant patients with a serum TSH above the reference range but <10mU/L (II,B). Physicians may wish to consider the suitability of a therapeutic trial of thyroxine on an individual patient basis.” Therefore, many doctors will not diagnose hypothyroidism until a patient’s TSH level is more than 10. If your TSH is above the range but less than 10, discuss a therapeutic trial of levothyroxine with your doctor.
There is controversy in regard to when a patient is classed as hypothyroid and whether they should be treated or not.
Experts don’t all agree what level of TSH should enable patients to be treated. Some feel that a level to 2.5 23 should be the point at which patients should be treated.
If you suspect you may have hypothyroidism or subclinical hypothyroidism we have lots of other articles that you may find useful:
- Subclinical hypothyroidism
- Getting a Diagnosis
- If you are diagnosed with an underactive thyroid
- Treatment Options
- Tips on Staying Well
- The Role of Vitamins and Minerals
Check out what people are saying on our online community regarding hypothyroidism:
Thyroid UK relies on donations so that we can continue to support and campaign for people with thyroid disease and related disorders. If you have found our information helpful, please do think about donating or becoming a member.
Autoimmune – patients with autoimmune diseases frequently have antibodies circulating in their blood that target their own body tissues
Endocrinologist – a medical practitioner qualified to diagnose and treat disorders of the endocrine glands and hormones
Goitre – a swelling of the neck resulting from enlargement of the thyroid gland
Hashimoto’s disease – an autoimmune disease causing chronic inflammation and consequential failure of the thyroid gland
Hypothalamic failure – a problem (or) condition with the region of the brain known as the hypothalamus, which helps to control and regulate body functions of pituitary gland like adrenal glands, ovaries, testes, thyroid gland
Levothyroxine – a synthetic thyroid hormone commonly given to treat an underactive thyroid. It is also known as L-thyroxine
Metabolism – the chemical processes within the human body
Pituitary – of or relating to the pituitary gland
Thyroidectomy – the removal of all (total) or part (partial) of the thyroid gland
Thyroid stimulating hormone – produced by the pituitary gland its role is to regulate the production of hormones by the thyroid gland
Thyroxine – the main hormone secreted into the bloodstream by the thyroid gland. It is the inactive form
Triiodothyronine – alternative name is T3. Triiodothyronine is the active form of the thyroid hormone, thyroxine
TSH-secreting pituitary adenomas – benign tumours of the pituitary gland. They produce too much thyroid stimulating hormone (TSH), which causes the thyroid gland to enlarge and produce thyroid hormone in excess, leading to an overactive thyroid (hyperthyroidism)
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Date created: 22/08/2019 (V1.3)
Review date: 22/08/2021