Thyroid Gland Overview and Hormone Functions

Study Notes

The thyroid gland is butterfly-shaped and located in the lower neck, beside the trachea.
It's composed of two lobes connected by an isthmus.
Thyroid hormones (T3 and T4) regulate metabolism and neurologic development.
Calcitonin, another thyroid hormone, controls calcium homeostasis.

The hypothalamic-pituitary-thyroid axis regulates thyroid hormone production.
Thyrotropin-releasing hormone (TRH) stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH).
TSH then stimulates the thyroid gland to produce T3 and T4.
T4 is primarily produced and converted to T3 in the liver and kidneys.
T3 and T4 act through nuclear receptors within cells.

T4 is initially released, and then primarily converted to the active form, T3, within peripheral tissues.
The conversion of T4 to T3 is regulated via monodeiodinases.
Thyroid hormones bind to specific plasma proteins for transport.

TSH (thyroid-stimulating hormone) is the most useful test for evaluating thyroid function.
Different generations of TSH assays exist, with third-generation usually preferred for its accuracy.
Total and free T3 and T4 levels can also be measured.
Radioimmunoassay (RIA) and Chemiluminescent assays are common methods for measuring these hormones.
T3 and T4 are usually measured after separating them from the binding proteins.

Conditions like Graves' disease and Hashimoto's thyroiditis are caused by autoimmune responses directed against the thyroid gland.

Thyroid ultrasound can visualize and evaluate palpable thyroid nodules.
Fine-needle aspiration biopsy is a vital tool to diagnose the nature of thyroid abnormalities.
In fine-needle aspiration biopsy, thyroid nodules are evaluated according to six categories: non-diagnostic for malignancy, suspicious for malignancy, intermediate or suspicious for neoplasm, follicular lesion of undetermined significance, and benign.
A radioactive iodine uptake (RAIU) test is part of nuclear medicine evaluation used to assess thyroid function.

Hypothyroidism: A condition where thyroid hormone production is insufficient.
Causes of hypothyroidism encompass primary, secondary, or tertiary conditions, such as chronic lymphocytic thyroiditis (Hashimoto's thyroiditis).
Hyperthyroidism: An overactive thyroid that produces excessive amounts of thyroid hormones, common causes are Graves' disease, toxic adenomas (benign nodules on the thyroid).
Thyrotoxicosis: Peripheral tissues experience excess thyroid hormone which triggers symptoms and complications.

For hypothyroidism, levothyroxine - a synthetic thyroid hormone - is used for replacement therapy.
Treatment for hyperthyroidism varies depending on the underlying cause.

The four parathyroid glands are located behind the thyroid gland, near the thyroid capsule.
They regulate calcium homeostasis by producing parathyroid hormone (PTH).

Primary hyperparathyroidism: Caused by a functioning parathyroid adenoma resulting in elevated PTH and calcium.
Secondary hyperparathyroidism: Characterized by a decrease in serum calcium, diffuse hyperplasia of the parathyroid gland, often seen in diseases like vitamin D deficiency or chronic kidney failure.
Tertiary hyperparathyroidism: Develops when secondary hyperparathyroidism is prolonged and uncontrolled.