Hypothyroidism

What it is

Hypothyroidism is an underactive thyroid; in the U.S., nearly 5 out of 100 people age 12 or older have it. It means the thyroid gland does not make enough thyroid hormone to meet the body’s needs. Because thyroid hormones help regulate energy use, temperature, heart rate, digestion, skin and hair turnover, and brain function, low hormone levels can slow many body processes.

Most cases are primary hypothyroidism, where the problem is in the thyroid gland itself. A much smaller number are central hypothyroidism, caused by disease of the pituitary or hypothalamus. More than 99% of cases are primary in most series. In iodine-sufficient countries, the most common cause is autoimmune thyroiditis, often called Hashimoto thyroiditis. Globally, iodine deficiency remains the leading cause.

Common symptoms include fatigue, feeling cold, constipation, dry skin, weight gain, hoarse voice, slowed thinking, depression, heavy or irregular periods, muscle cramps, and high cholesterol. Symptoms can develop gradually and may be mild at first, so blood testing is often needed to confirm the diagnosis.

Main type	What it means	Typical lab pattern
Primary hypothyroidism	Thyroid gland underproduces hormone	High TSH, low free T4
Subclinical hypothyroidism	Early or mild thyroid failure	High TSH, normal free T4
Central hypothyroidism	Pituitary or hypothalamic problem	Low or inappropriately normal TSH with low free T4

India-relevant context matters because iodine intake strongly affects thyroid disease patterns. India has long used universal salt iodization to reduce iodine deficiency disorders, but both inadequate and excessive iodine exposure can influence thyroid function in some settings.

How it works

The thyroid mainly makes thyroxine (T4) and smaller amounts of triiodothyronine (T3). The pituitary gland controls thyroid hormone production by releasing thyroid-stimulating hormone (TSH). When thyroid hormone levels fall, TSH usually rises to push the thyroid to work harder.

In primary hypothyroidism, the thyroid cannot respond adequately. This may happen because of autoimmune destruction, thyroid surgery, radioactive iodine treatment, some medicines, thyroid inflammation, congenital thyroid disorders, or long-term iodine deficiency. In central hypothyroidism, the pituitary or hypothalamus fails to send the right signal.

Low thyroid hormone slows metabolic activity. That is why people may notice tiredness, slower pulse, constipation, puffy face, dry skin, hair thinning, menstrual changes, infertility, and reduced exercise tolerance. In severe untreated disease, complications can include worsening cholesterol levels, heart problems, nerve symptoms, and rarely myxedema coma, a life-threatening emergency.

Evidence and uses

The standard treatment is levothyroxine, a synthetic form of T4. It is the recommended first-line therapy because it is effective, inexpensive, and usually restores normal thyroid hormone levels when taken correctly. The goal is to relieve symptoms and normalize thyroid blood tests, especially TSH in primary hypothyroidism.

Treatment is clearly indicated for overt hypothyroidism, meaning a raised TSH with a low free T4 in primary disease. It is also important in pregnancy, where untreated hypothyroidism can increase risks for the mother and fetus. People who have had thyroid surgery or radioactive iodine treatment often need lifelong replacement.

For subclinical hypothyroidism, the decision is more individualized. Some people benefit from treatment, especially if TSH is clearly elevated, thyroid antibodies are positive, symptoms are present, pregnancy is planned, or cardiovascular risk is a concern. Evidence is mixed for symptom improvement in mild cases, particularly in older adults, so clinicians often weigh lab values, age, symptoms, and comorbidities before starting therapy.

Combination therapy with T4 plus T3 is an area of ongoing research. Some patients continue to report symptoms despite a normal TSH on levothyroxine alone, but trials have not shown consistent benefit of combination therapy for most people. Current evidence does not support routine use, though specialists may consider it in selected cases after careful review.

Diet alone does not treat established hypothyroidism. If iodine intake is low, correcting deficiency is important, but taking extra iodine when intake is already adequate can worsen thyroid dysfunction in some people. In India, using adequately iodized salt is the usual public health measure rather than self-prescribing iodine supplements.

Diagnosis / how it's measured

Diagnosis is based on symptoms, examination, and blood tests. The key tests are:

1. TSH: usually the best first test for suspected primary hypothyroidism.
2. Free T4: helps confirm whether hypothyroidism is overt or subclinical.
3. Thyroid peroxidase (TPO) antibodies: may support autoimmune thyroiditis.

Typical interpretation:

• High TSH + low free T4: primary overt hypothyroidism
• High TSH + normal free T4: subclinical hypothyroidism
• Low/normal TSH + low free T4: consider central hypothyroidism

Doctors may also check lipids, sodium, complete blood count, or other tests depending on the situation. Imaging is not routinely needed unless there is a goiter, thyroid nodule, or concern for pituitary disease.

Testing should be interpreted carefully in pregnancy, severe illness, and in people taking medicines that affect thyroid tests. Biotin supplements can interfere with some thyroid assays, so patients are often advised to tell the lab and clinician about supplement use.

When to see a clinician

See a clinician if you have persistent fatigue, constipation, cold intolerance, dry skin, unexplained weight change, menstrual irregularity, fertility problems, or a neck swelling. You should also seek evaluation if you have a personal or family history of thyroid disease, autoimmune disease, prior thyroid surgery, or treatment with radioactive iodine or neck radiation.

Urgent medical care is needed for severe drowsiness, confusion, low body temperature, shortness of breath, or marked swelling in someone with known or suspected hypothyroidism, because these can be signs of severe decompensation.

If you are already taking levothyroxine, follow-up blood testing is important because both under-treatment and over-treatment can cause harm. Dose needs can change with pregnancy, aging, major weight change, gastrointestinal disease, and new medicines.

Limitations and open questions

Hypothyroidism is usually straightforward to diagnose and treat, but some areas remain uncertain. Symptoms are nonspecific, so fatigue, weight gain, low mood, and hair loss may have other causes even when thyroid tests are only mildly abnormal. That is one reason treatment decisions in subclinical hypothyroidism can be difficult.

Another open question is why some patients continue to feel unwell despite a normal TSH on levothyroxine. Research is examining tissue-level thyroid hormone signaling, genetic differences in hormone conversion, and whether a small subgroup may benefit from alternative replacement strategies. Evidence in humans is still limited, and routine use of newer or combination approaches is not established.

There is also ongoing debate about the best TSH targets for older adults and for people with coexisting heart disease. Finally, iodine nutrition remains a public health issue worldwide: deficiency can cause hypothyroidism, but excess iodine can also trigger thyroid dysfunction in susceptible people. For most patients, the safest approach is evidence-based testing, standard treatment, and regular follow-up with a clinician rather than self-treating with supplements or thyroid products.