Thyroid
Medically reviewed by Hormone Journal Editorial Team · Last reviewed 2026-05-17
The thyroid is a butterfly-shaped gland in the neck that produces hormones controlling metabolism, energy, growth, and many other body functions.
What it is
The thyroid is a butterfly-shaped gland in the neck that produces hormones controlling metabolism, energy, growth, and many other body functions. Sitting at the front of the neck below the larynx (voice box), it weighs roughly 20–30 grams in adults. Its two main hormones — thyroxine (T4) and triiodothyronine (T3) — reach nearly every cell in the body and influence heart rate, body temperature, weight, mood, fertility, and more.
Causes and mechanism
The thyroid operates within the hypothalamic-pituitary-thyroid (HPT) axis — a feedback loop involving three glands. The hypothalamus releases thyrotropin-releasing hormone (TRH), which signals the pituitary gland to secrete thyroid-stimulating hormone (TSH). TSH then tells the thyroid to produce T4 and T3. When thyroid hormone levels rise, they signal back to the hypothalamus and pituitary to slow TRH and TSH output, keeping levels in a narrow range.
T4 is the more abundant hormone but is largely inactive; most of it is converted to the active form, T3, in peripheral tissues such as the liver and kidneys. Iodine — obtained from iodized salt, seafood, and dairy — is a required building block for both hormones. Iodine deficiency remains the leading global cause of preventable thyroid dysfunction.
Symptoms and diagnosis
Thyroid disorders fall into two broad categories:
- Hypothyroidism (underactive): fatigue, weight gain, cold intolerance, constipation, slowed speech, impaired memory, and irregular or heavy periods.
- Hyperthyroidism (overactive): unintentional weight loss, rapid or irregular heartbeat, heat intolerance, tremor, anxiety, diarrhea, and lighter or absent periods.
Both conditions can also affect fertility and ovulation. Older adults often present with subtler signs — unexplained weight loss or mild depression — making diagnosis easier to miss.
Diagnosis starts with a blood TSH test. A TSH outside the reference range (typically 0.4–4.0 mIU/L, though labs vary) prompts follow-up free T4 and, sometimes, free T3 or thyroid antibody tests. Imaging (ultrasound) is added when a nodule or goiter is felt on exam.
Treatment options
Treatment depends on the specific disorder:
- Hypothyroidism: Levothyroxine (synthetic T4) taken daily is the standard of care. Dose is adjusted by TSH monitoring, usually every 6–12 weeks until stable.
- Hyperthyroidism: Options include anti-thyroid medications (methimazole, propylthiouracil), radioactive iodine ablation, or surgery to remove part or all of the gland. Choice depends on cause, severity, age, and pregnancy status.
- Autoimmune thyroid disease: Hashimoto's thyroiditis (the most common cause of hypothyroidism in iodine-sufficient countries) and Graves' disease (the most common cause of hyperthyroidism) are managed with the same categories above; there is no approved therapy that reverses the underlying immune process.
Evidence on combination T4/T3 therapy for hypothyroidism is mixed. Most professional guidelines, including those from the American Thyroid Association, recommend levothyroxine monotherapy as first-line, while acknowledging that a minority of patients report persistent symptoms despite normal TSH.
When to see a clinician
Seek evaluation if you notice unexplained weight change, persistent fatigue, a visible neck swelling, heart palpitations, or menstrual irregularities. Pregnant people or those planning pregnancy should have TSH checked early, as untreated thyroid dysfunction raises the risk of miscarriage and developmental problems. Anyone already on thyroid medication should have TSH rechecked at least annually, or sooner after a dose change, illness, or new medication that can affect absorption.
FAQs
What is the difference between T3 and T4?
T4 (thyroxine) is the hormone the thyroid produces in the largest quantity, but it is mostly inactive. The body converts about 80% of T4 into T3 (triiodothyronine), the form that actively acts on cells to regulate metabolism and energy. Most standard thyroid blood panels measure TSH and free T4; free T3 is added when conversion problems are suspected.
What TSH level is considered abnormal?
Most laboratories flag TSH below 0.4 mIU/L as low (suggesting hyperthyroidism) and above 4.0 mIU/L as high (suggesting hypothyroidism), though reference ranges differ slightly between labs. Subclinical hypothyroidism — TSH between 4.0 and 10 mIU/L with normal T4 — affects roughly 5–10% of adults and may or may not require treatment depending on symptoms and other factors. Your clinician interprets results in the context of your full picture, not the number alone.
Can thyroid disease affect fertility?
Yes. Both hypothyroidism and hyperthyroidism can disrupt ovulation and menstrual cycles, reducing the chance of conception. Untreated hypothyroidism during pregnancy is linked to a higher risk of miscarriage, preterm birth, and impaired fetal brain development. The American Thyroid Association recommends TSH be kept below 2.5 mIU/L in the first trimester for people on levothyroxine who are pregnant.
Is thyroid disease a lifelong condition?
It depends on the cause. Hashimoto's thyroiditis and post-surgical hypothyroidism typically require lifelong levothyroxine. Some cases of hyperthyroidism — particularly those triggered by pregnancy (postpartum thyroiditis) or certain medications — resolve on their own within 12–18 months. Graves' disease has a remission rate of roughly 30–50% after 12–18 months of anti-thyroid drug therapy, but relapse is common.
Is thyroid hormone replacement covered by insurance?
Generic levothyroxine is on most insurance formularies in the United States and is available for as little as $4–$10 per month at major pharmacies without insurance. Brand-name versions (e.g., Synthroid, Tirosint) cost more and may require prior authorization. Diagnostic TSH testing is generally covered as preventive or diagnostic care, though cost-sharing varies by plan. Check your specific plan's formulary for exact out-of-pocket costs.