Generality
Subclinical hypothyroidism is a disorder of the thyroid gland characterized by an increase in serum levels of the thyroid-stimulating hormone (TSH) associated with thyroid hormone values (thyroxine and triiodothyronine) in the norm.
In this condition, the typical symptoms of overt hypothyroidism are scarce or absent: the increase in TSH levels is able to maintain thyroid hormone values in the normal range.
The most common cause of subclinical hypothyroidism is Hashimoto's thyroiditis .
Thyroid: key points
Before defining the characteristics of subclinical hypothyroidism, it is necessary to briefly recall some notions related to the thyroid gland:
- The thyroid is a small endocrine gland, located in the anterior region of the neck, in front of and laterally to the larynx and trachea. The main hormones it produces - thyroxine (T4) and triiodothyronine (T3) - control metabolic activities and are responsible for the proper functioning of most of the body's cells.
- More in detail, thyroid hormones signal how fast the body must work and how it must use food and chemical substances to produce energy and perform its functions properly. Not only: the thyroid intervenes in the processes of growth and development of many tissues and stimulates cellular activities, optimizing, in particular, the functions of the cardiovascular system and the nervous system.
- Thyroid hormone production is activated and deactivated via a feedback system (feed-back). Among the various factors involved in this mechanism, the thyroid stimulating hormone (TSH) is responsible for maintaining the concentration of thyroid hormones stable in the bloodstream.
What is Subclinical Hypothyroidism
Subclinical hypothyroidism is a thyroid dysfunction in which:
- The serum concentrations of the thyroid-stimulating hormone increase beyond the normal threshold (high TSH);
- Thyroxine (T4) and triiodothyronine (T3) levels remain within the reference range.
Causes
Subclinical hypothyroidism may depend on multiple causes.
In most cases, this condition occurs as a result of a thyroid disease caused by an autoimmune process that targets the thyroid gland.
An example of this is:
- Hashimoto thyroiditis (main cause of subclinical hypothyroidism);
- Basedow-Graves disease .
Other causes of subclinical hypothyroidism may be:
- Prior acute phlogosis ;
- Iodic deficiency (dietetic: poor diet of iodine or rich in foods, called "gozzigeni", which hinder the assimilation; endemic: long stay in geographical areas iodocarenti, above all mountainous and far from the sea);
- Iatrogene, in particular:
- Previous ablative therapy with radioactive iodine;
- Thyroid removal surgery (thyroidectomy);
- Drugs (amiodarone, lithium, radiological contrast agents containing iodine, etc.);
- Insufficient replacement therapy;
- External radiotherapy of the head and neck (given, for example, in the case of laryngeal carcinoma, Hodgkin's lymphoma, leukemia, intracranial neoplasms, etc.).
Subclinical hypothyroidism can also occur in idiopathic form (ie due to unidentifiable causes).
Who is most at risk
Subclinical hypothyroidism is relatively frequent (the prevalence is estimated between 4 and 10% in the general population).
The condition affects mainly with advancing age and in the female sex ("critical" periods for thyroid function are pregnancy and menopause).
Subclinical hypothyroidism is particularly prevalent in those with underlying Hashimoto's thyroiditis.
The subjects most likely to develop subclinical hypothyroidism are:
- Patients with Down syndrome;
- Women in the post-partum period (within 6 months);
- Menopausal women;
- Elderly patients;
- Patients with type 1 diabetes mellitus;
- Patients with heart failure;
- Patients with a history of thyroid disease;
- Patients with other autoimmune diseases.
Symptoms and Complications
By its own definition, subclinical hypothyroidism is asymptomatic : increasing TSH levels maintains thyroid hormone values in the normal range. However, some patients report a non-specific symptomatology, which may be associated with thyroid hypofunctionality .
It should be remembered that subclinical hypothyroidism is a condition in which the alteration of thyroid function is mild to moderate . If it is neglected, however, the dysfunction can progress to full- blown hypothyroidism (the circulating levels of TSH are high and the thyroid hormone values are lower than the normal limits, so they are insufficient to maintain a state of euthyroidism).
Subclinical hypothyroidism: main symptoms
The manifestations of subclinical hypothyroidism can be blurred or mild.
The symptomatology usually occurs after a long subclinical course and may include:
- Muscle weakness;
- Asthenia;
- Daytime sleepiness;
- Cold intolerance;
- Difficulty concentrating;
- Hoarseness;
- Dry and rough skin;
- Eyelid edema;
- Memory loss;
- Constipation.
In most cases, subclinical hypothyroidism remains stable for several years and can sometimes regress.
The risk of subclinical hypothyroidism to progress towards the established form is greater in elderly patients and in those who present high values of anti-thyroid antibodies (parameter indicative of the presence of autoimmune diseases).
Problems associated with subclinical hypothyroidism
In recent years, several scientific studies have associated subclinical hypothyroidism with various clinical conditions.
In addition to the possible progression of dysfunction to overt hypothyroidism, there may be:
- Increased low density lipoprotein level;
- Increased cardiovascular risk;
- Cognitive decline (in older patients);
- Anxiety and depression.
Furthermore, patients suffering from subclinical hypothyroidism are more likely to develop:
- Hypercholesterolemia (increase in total cholesterol level);
- Atherosclerosis;
- dyslipidemia;
- coronary artery disease;
- Peripheral arterial disease.
Diagnosis
Subclinical hypothyroidism is often discovered accidentally, after checking the level of thyroid hormones and TSH or during tests to ascertain the causes of non-specific symptoms (such as, for example, drowsiness, tiredness or changes in the menstrual cycle) .
The diagnosis of subclinical hypothyroidism can be formulated based on:
- Accurate medical history of the patient;
- Presence of symptoms and signs of mild hypofunction of the thyroid gland;
- Dosage of serum concentrations of TSH, free T4 (FT4) and free T3 (FT3) after a simple blood sample.
Subclinical hypothyroidism is characterized by elevated serum levels of TSH (thyroid-stimulating hormone) associated with normal levels of free thyroid hormones (FT3 and FT4) on two occasions at least 2-3 months apart.
The detection of anti-Tireoglobulin antibodies (anti-TG antibodies ) and anti-thyroid peroxidase antibodies (anti-TPO Ab) in the blood allows us to establish the autoimmune etiology of subclinical hypothyroidism and the opportunity to start replacement therapy with L-thyroxine (L-T4).
Thyroid ultrasound, scintigraphy and needle aspiration are a useful complement to the evaluation of the clinical case, as they provide information on the morphology and functional capacity of the thyroid.
What tests are needed for subclinical hypothyroidism?
The blood tests useful for the diagnosis of subclinical hypothyroidism are:
- Dosage of TSH, FT3 and FT4 (free form of T4);
- Stimulation test with TRH (thyrotropin-releasing hormone);
- Dosage of anti-thyroperoxidase (Ab anti-TPO) and anti-thyroglobulin (Ab anti-TG) antibodies;
- Dosage of total cholesterol, HDL, LDL and triglycerides.
In subclinical hypothyroidism, thyroid hormone levels typically circulate within normal limits, associated with a high serum TSH value. The dosage of anti-thyroid antibodies indicates the presence of the antibodies responsible for the most common form of hypothyroidism, ie the autoimmune one.
What to do when the high TSH is found?
The first thing to do is to repeat the TSH dosage after 2 or 12 weeks to rule out a transient abnormality. The evaluation of the FT4 is useful in defining the condition of subclinical hypothyroidism and allows us to assess the degree of gravity.
Subclinical hypothyroidism vs Transient increase of TSH
The TSH dosage is the most sensitive laboratory data regarding the diagnosis of subclinical hypothyroidism. It must be considered, however, that some physiological or pathological situations may increase TSH secretion transiently.
The causes of this phenomenon include sleep disorders, circadian rhythm abnormalities (eg night work), exposure to toxic substances (pesticides, industrial chemicals etc.), some forms of thyroiditis (subacute or post-partum), antithyroid drugs or inhibiting TSH secretion (glucocorticoids, dopamine, etc.), major surgery, severe trauma, infections and malnutrition.
Treatment
The therapy of subclinical hypothyroidism involves the administration of thyroid hormone drugs (replacement therapy with L-thyroxine, L-T4; eg levothyroxine), initially at low doses. The purpose of treatment is to restore a condition of euthyroidism.
However, before adhering to a possible replacement therapy with L-thyroxine, the doctor should monitor the dysfunction in a short period of time (indicatively for 3-6 months) and confirm the increase in TSH (it could depend on a transient abnormality) ).
If L-thyroxine is not taken (due to lack of adherence to the therapeutic protocol by the patient) or is not sufficient, a condition of hypothyroidism is created. For this reason, while taking the drug, the patient with subclinical hypothyroidism must undergo regular follow-up to check the effects of the treatment.
Subclinical hypothyroidism: scheme for monitoring
- After the first finding of elevated TSH and normal thyroid hormones, carry out the dosage of TSH, FT4 and anti-Thyroperpidash antibodies (Ab anti-TPO) in the blood after 2-3 months.
- If TSH does not normally perform further tests;
- If TSH is high (ie subclinical hypothyroidism is persistent):
- Perform an ultrasound examination of the thyroid gland;
- Evaluate thyroid function (TSH and FT4) every 6 months; after 2 years, this check can become annual.
In general, thyroid function should be evaluated in pregnant women, in those who develop symptoms of hypothyroidism or in other blood tests.
Treatment of subclinical hypothyroidism: yes or no?
Even today, the treatment or not of subclinical hypothyroidism is the subject of controversy in the various guidelines.
In general, thyroid hormone replacement therapy begins when TSH values are above 10 µU / ml . With regard to concentrations below 10 µU / ml we tend to exploit, instead, the greater stimulation of TSH on the thyroid gland, so that this still ensures a normal production of thyroid hormones. Therapy can be started for TSH values between 4 and 10 µU / ml in the case of chronic autoimmune thyroiditis or nodular thyroid disease.
The only condition in which the treatment of subclinical hypothyroidism is always indicated in adults is pregnancy, to avoid the effects of dysfunction on gestation and fetal development. The beginning of therapy can be considered by the doctor in the presence of clinical symptoms or in case of coexistent hyperlipidemia and heart failure.
Prevention
Unfortunately, there is no prevention for subclinical hypothyroidism.
The best strategy to avoid the consequences associated with the loss of thyroid gland function is to diagnose the condition as soon as possible.
Measurement of serum TSH and free T4 at regular intervals (indicatively, every 6-12 months) allows to evaluate the progression of the clinical picture (if not in treatment) or to regulate the dosage of L-thyroxine to restore a condition of euthyroidism.
The follow up also allows monitoring of the possible evolution of subclinical hypothyroidism in the established form.
