The thyroid is a small gland in the front of the neck. It makes hormones that help regulate how the body uses energy, so thyroid problems can affect heart rate, temperature, digestion, mood, menstrual cycles, fertility, and many other body functions.
Hypothyroidism: An Underactive Thyroid
Hypothyroidism happens when the thyroid does not make enough thyroid hormone. Common causes include Hashimoto’s disease, thyroid surgery, radiation treatment, thyroiditis, and some medicines.
Possible symptoms include fatigue, feeling unusually cold, constipation, dry skin, thinning hair, weight gain, muscle or joint discomfort, a slower heart rate, depression, and heavy or irregular periods. These symptoms are common in many conditions, so symptoms alone cannot diagnose hypothyroidism.
How Is Hypothyroidism Diagnosed and Treated?
Diagnosis usually involves blood tests, including thyroid-stimulating hormone (TSH) and thyroid hormone levels. Additional tests may be used when needed to identify the cause.
Hypothyroidism is commonly treated with levothyroxine, a replacement form of thyroid hormone. The dose needs to be individualized and monitored with blood tests. Do not start, stop, or change thyroid hormone based only on an online article.
Hyperthyroidism: An Overactive Thyroid
Hyperthyroidism occurs when the thyroid produces more hormone than the body needs. Graves’ disease is a common cause. Symptoms can include a fast or irregular heartbeat, nervousness, tremor, heat intolerance, sweating, weight loss, frequent bowel movements, muscle weakness, and sleep problems.
Treatment may involve antithyroid medicines, radioiodine, surgery, and sometimes beta-blockers to control symptoms. The best option depends on the cause, age, pregnancy status, heart health, and other individual factors.
Thyroid Nodules and Cancer
Thyroid nodules are common, and most are not cancer. Evaluation may include a physical examination, thyroid blood tests, ultrasound, and—when indicated—a fine-needle biopsy. A lump, persistent hoarseness, difficulty swallowing, or a growing neck mass should be assessed by a healthcare professional.
Thyroid Disease, Menstrual Cycles, and Fertility
Both underactive and overactive thyroid conditions can affect menstrual cycles and fertility. If you have irregular periods, difficulty becoming pregnant, or thyroid symptoms, your clinician may recommend thyroid testing as part of the evaluation.
Thyroid conditions can also overlap with other metabolic concerns. For related information, see our guide to diabetes in women.
Thyroid Disease During Pregnancy
People with thyroid disease can have healthy pregnancies, but monitoring is important. NIDDK notes that thyroid hormone is important for fetal brain and nervous-system development. People already taking thyroid medicine should contact their clinician promptly after becoming pregnant because monitoring and dose adjustment may be needed.
For hypothyroidism during pregnancy, levothyroxine is commonly used. Hyperthyroidism during pregnancy requires individualized treatment and close medical supervision. Radioactive iodine is not used during pregnancy.
For broader pregnancy safety information, see our pregnancy essentials guide.
Postpartum Thyroiditis
Postpartum thyroiditis can occur after childbirth and may cause a temporary hyperthyroid phase, a hypothyroid phase, or both. Symptoms can overlap with normal postpartum changes, so persistent palpitations, marked fatigue, heat or cold intolerance, or unexplained mood changes should be discussed with a clinician.
When Thyroid Symptoms Are Urgent
Seek urgent medical care for severe confusion, fainting, very high fever with a racing heartbeat, severe breathing difficulty, or extreme drowsiness with signs of severe illness. Rare complications of untreated thyroid disease can be life-threatening.
Bottom Line
Thyroid symptoms can be nonspecific, so diagnosis depends on medical evaluation and blood testing rather than symptoms alone. Treatment is usually effective when the cause is correctly identified and medicines are monitored appropriately.
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Last reviewed for medical accuracy: September 2026.