Anatomy & Function

To produce thyroid hormone which is released into the blood and then circulates to all the body’s organs.

Thyroid hormone helps to control metabolism, effects growth and development, controls body temperature, and helps to regulate the cardiovascular system among many other important functions in the body.

• Iodine is essential for production of thyroid hormone. This is why iodine is added to salt (iodinated), to ensure we get adequate amounts in our diet to produce sufficient thyroid hormone.
  • Recommended daily dose is 150 mcg, ingestion of greater amounts may cause thyroid dysfunction.
    • It is very uncommon to be iodine deficient in developed countries.
  • In iodine deficient regions of the world, endemic goiter (enlarged thyroid gland) can form.
    • With less iodine available to the gland, the thyroid becomes less efficient at producing thyroid hormone and subsequently enlarges to compensate for this.
  • The Midwest of the United States was once considered a ‘goiter belt’ due to iodine deficient soil as the result of glacier effects during the Ice Age.
    • In the 1920’s, salt began to be iodinated in the US and the rate of endemic goiter in this region fell considerably.

There are two forms of thyroid hormone that are produced: T4 (thyroxine) and T3 (triiodothyronine), accounting for 80% and 20% of thyroid hormone in the blood respectively.

• The production of thyroid hormone is controlled by the brain (specifically the pituitary gland).
• The brain continuously measures thyroid hormone levels in the blood, and produces a hormone called TSH (thyroid stimulating hormone) when more thyroid hormone is needed.
  • Levels of TSH and T4 / T3 in the blood have an inverse relationship, or ‘teeter totter’ effect:
    • When T4 and T3 levels go down, TSH levels will go up to compensate, and vice versa.
    • This is a tightly regulated feedback loop (or hormone axis in the body).
  • TSH level measurements can be affected by biotin supplements (commonly present in hair and nail supplements), and therefore these supplements should be stopped 1 week prior to blood draw.

Hypothyroidism is defined by low T4 and elevated TSH levels.

Subclinical hypothyroidism is defined by T4 levels in the low-normal range and TSH levels that are elevated.

• Symptoms of hypothyroidism include: fatigue, cold sensitivity, constipation, dry skin, unexplained weight gain, brittle nails, hair loss, delayed puberty or slow growth.
• Hashimoto’s thyroiditis the main cause of hypothyroidism.
  Other rare causes include post-partum thyroiditis and viral (aka subacute) thyroiditis.

Hyperthyroidism (aka thyrotoxicosis) is defined by elevated T4 and low TSH levels.

Subclinical hyperthyroidism is defined by high-normal T4, and a low TSH.

• Symptoms of hyperthyroidism include: unexplained weight loss, rapid/irregular heartbeat, tremor, sweating, irritability, heat intolerance, insomnia, diarrhea, nervousness, anxiety.
• Graves’ disease and toxic thyroid nodule are the most common reasons for hyperthyroidism.
  • Other rare reasons include amiodarone induced thyrotoxicosis.