thyroids

The thyroids, or thyroid glands, are a pair of large endocrine glands found in most vertebrates. In humans and many mammals they lie on the anterior lower neck, just below the larynx, with two lobes connected by an isthmus. In other species the shape and arrangement vary, but the glands share a common role in metabolism and development.

The thyroids produce hormones via follicular cells: thyroxine (T4) and triiodothyronine (T3), which regulate basal metabolic rate, heat production, and macronutrient metabolism. C cells secrete calcitonin, which helps regulate blood calcium in some species. Hormone synthesis requires iodine.

Regulation: The hypothalamus releases thyrotropin-releasing hormone (TRH), stimulating the pituitary to secrete thyroid-stimulating hormone (TSH). TSH stimulates the thyroids to synthesize and release T4 and T3. Negative feedback from circulating T4/T3 modulates TRH and TSH release.

Clinical aspects: Hypothyroidism occurs when thyroid hormone production is insufficient; symptoms include fatigue, cold intolerance, and weight gain. Hyperthyroidism involves excess hormone, causing weight loss, heat intolerance, anxiety, and tachycardia. Goiter is thyroid enlargement, often due to iodine imbalance, nodular disease, or autoimmune thyroiditis such as Hashimoto's or Graves' disease. Nodules are common; most are benign, but some may harbor cancer.

Diagnosis and treatment: Evaluation uses TSH and free T4 tests, thyroid antibodies, ultrasound, and sometimes fine-needle aspiration biopsy. Treatments depend on the condition: levothyroxine for hypothyroidism; antithyroid drugs, radioactive iodine, or surgery for hyperthyroidism; management of nodules or cancer may include biopsy, surgery, and targeted therapies.

In many species thyroids play a similar role in development and metabolism, though anatomy and disease prevalence vary. Iodine deficiency remains a public health issue in some regions, whereas iodized salt programs have reduced related disorders.