Hypothyroidism can produce a broad range of symptoms because thyroid hormones influence metabolism and the function of many organ systems. Common symptoms include persistent tiredness, weight gain, increased sensitivity to cold, dry skin, thinning or dry hair, muscle or joint pain, constipation, a slower heart rate and low mood. Menstrual periods can become heavy or irregular, and fertility may be affected. Some people develop a puffy appearance, hoarse voice or an enlarged thyroid, depending on the underlying cause. Symptoms vary considerably between individuals, and the condition can be mild enough that a person notices little or nothing. Because hypothyroidism often develops gradually, symptoms may be attributed to ageing, stress, lifestyle or other medical conditions before thyroid disease is considered.
Fatigue is one of the most commonly recognised symptoms of hypothyroidism, but it is not specific to thyroid disease. Reduced thyroid-hormone activity can slow metabolic processes and contribute to reduced energy, lethargy and reduced physical capacity. Some people describe increased sleepiness or difficulty maintaining normal levels of activity. However, fatigue also occurs with anaemia, sleep disorders, infections, depression, nutritional problems and many other conditions. The same principle applies to weight gain, another commonly associated symptom. Hypothyroidism can reduce energy expenditure and can contribute to weight gain, but substantial unexplained weight gain should not automatically be attributed to the thyroid. Symptoms become more informative when several characteristic features occur together and are supported by appropriate thyroid-function tests.
Cold intolerance occurs because thyroid hormones contribute to metabolic activity and heat production. When thyroid-hormone action is reduced, the body can generate less heat and an individual may feel unusually cold compared with others or compared with their previous baseline. Dry skin and hair changes are also recognised manifestations. Reduced thyroid function can alter skin turnover and the normal growth cycle of hair, producing dry skin and dry or thinning hair.
Hair loss can have many causes, including nutritional deficiency, autoimmune disease, hormonal changes and physical stress, so it is not diagnostic of hypothyroidism by itself. The combination of cold intolerance, dry skin, fatigue, constipation and other characteristic symptoms may increase clinical suspicion, but laboratory testing is required to determine whether thyroid dysfunction is present.
Gastrointestinal and cardiovascular symptoms reflect the systemic effects of reduced thyroid-hormone activity. Constipation can occur because intestinal motility becomes slower. Heart rate can also decrease, reflecting reduced stimulation of cardiovascular activity. Some individuals experience reduced exercise tolerance, muscle aches, stiffness or weakness. Thyroid hormones influence muscle metabolism and nervous-system function, so abnormalities can affect physical performance even when there is no primary muscle disease. Severe untreated hypothyroidism can produce more pronounced slowing of physiological function. However, symptoms exist on a spectrum, and mild biochemical hypothyroidism may produce few noticeable symptoms. The severity of symptoms therefore does not necessarily correspond precisely to a single thyroid-hormone measurement.
Psychological and cognitive symptoms can occur in hypothyroidism. Low mood, reduced concentration, slowed thinking and general mental fatigue have been reported, and severe hypothyroidism can produce substantial neurological slowing. These effects arise because thyroid hormones influence brain function and energy metabolism. Nevertheless, depression, anxiety, sleep deprivation, medication effects and other neurological or psychiatric conditions can produce similar symptoms. A diagnosis of hypothyroidism should therefore not be made solely from mood or cognitive symptoms. Conversely, an individual with known hypothyroidism who develops persistent cognitive or mood changes should not automatically assume that thyroid hormone is the only explanation. Clinical assessment is necessary because multiple conditions can coexist.
Reproductive symptoms are another important feature. Hypothyroidism can cause heavy or irregular menstrual periods and can contribute to fertility problems. Thyroid hormones interact with the reproductive endocrine system, and significant thyroid dysfunction can disturb normal ovulatory and menstrual physiology. Pregnancy also changes thyroid requirements and physiology, and untreated hypothyroidism during pregnancy can affect both mother and developing baby. People experiencing difficulty becoming pregnant may therefore be assessed for thyroid dysfunction when clinically appropriate. These reproductive manifestations are not unique to hypothyroidism, however, because menstrual abnormalities and infertility have numerous possible causes. Thyroid testing is useful because an underlying thyroid abnormality can be identified and treated when present.
Physical examination can reveal additional clues, including dry skin, thinning hair, a slower pulse, changes in reflexes, swelling or a goitre, depending on the cause and severity. Hashimoto's disease can produce an enlarged thyroid, while long-standing autoimmune destruction can eventually leave a smaller gland. Some people have no obvious thyroid enlargement. The absence of a goitre therefore does not exclude hypothyroidism, and the presence of a goitre does not prove that the thyroid is underactive. Thyroid nodules and other structural abnormalities can also occur with normal thyroid function. Physical findings must therefore be integrated with medical history and laboratory results rather than interpreted in isolation.
Diagnosis requires biochemical assessment because symptoms alone are insufficiently specific. TSH is generally the principal initial blood test for suspected primary thyroid dysfunction, with free T4 used to determine the actual circulating availability of thyroid hormone. In typical primary hypothyroidism, TSH is elevated and free T4 is low. Mild or subclinical disease may have elevated TSH with free T4 still within the reference range. Thyroid antibody testing can help identify autoimmune Hashimoto's disease. Untreated severe hypothyroidism can produce serious complications, including extremely slowed physiological function known as myxoedema or myxedema coma, although this is uncommon. Appropriate diagnosis and treatment with thyroid-hormone replacement can restore hormone availability and prevent many complications.
