February 22, 2016
A Look At The Thyroid Gland
By Michael D. Shaw
This endocrine gland is located in the anterior part of the lower neck, below the larynx, and secretes hormones essential to growth and basic metabolism. Readers of a certain age will recall pictures in biology textbooks featuring victims of extreme goiter, in which the thyroid is enlarged, and thus the neck swollen—for any number of reasons, including a dietary deficiency of iodine.
The name “thyroid” derives from the Greek, and denotes a shield-shaped cartilage. These days, it would be more likely described as “butterfly-shaped,” with its two distinct and connected lobes.
Two major hormones are produced by the thyroid: Thyroxine (3,5,3′,5′-tetraiodothyronine) aka T4; and Triiodothyronine (3,5,3′-triiodothyronine) aka T3. [For better readability, we will render these without the subscripts from now on.] The underlying biochemistry and physiology are somewhat complex. The true active hormone is T3, with T4 as its so-called prohormone or precursor. In humans, the ratio of T4 to T3 released into the blood is around 20 to 1.
The regulation of T3 and T4 levels exemplify a negative feedback control system. “Negative feedback” is a regulatory mechanism whereby a stimulus causes an opposite output, in order to maintain an ideal level of the parameter being regulated. Probably, the most familiar embodiment of this mechanism is a thermostat. The regulation of T3 and T4 is part of the hypothalamic-pituitary-thyroid axis.
As such, thyroid production of the two hormones is stimulated by the hypothalamic thyrotropin-releasing hormone (TRH); and the anterior pituitary hormone thyrotropin (aka thyroid-stimulating hormone or TSH). Likewise, the thyroid hormones—at a preset level—inhibit the production and secretion of both thyrotropin-releasing hormone and thyrotropin. However, things can sometimes go wrong.
Symptoms of hypothyroidism include feeling run down, slow, depressed, sluggish, cold, or tired and having dry skin and hair, constipation, muscle cramps, or weight gain. Women may have an increased menstrual flow. In some cases, goiter may be present with its own symptoms of a tight feeling in your throat; coughing, hoarseness, difficulty swallowing, and difficulty breathing.
Hypothyroidism is most often treated with T4 replacement, in pill form. Some controversy persists regarding the benefits of natural versus synthetic T4.
Symptoms of hyperthyroidism—reflecting a speeding up of metabolism—include nervousness, irritability, increased perspiration, heart racing, hand tremors, anxiety, difficulty sleeping, thinning of your skin, fine brittle hair and weakness in your muscles—especially in the upper arms and thighs. Weight loss can occur (with increased bowel activity), menstrual flow can be lighter, and during early stages, you might feel more energetic. Generally, though, this operating in hyperdrive eventually produces fatigue.
Hyperthyroidism is most often treated with anti-thyroid drugs. Unfortunately, side effects are more common than with T4 replacement.
While the above-described symptoms are surely noticeable, they are by no means definitive. Thus, various blood tests must be utilized to obtain a proper diagnosis.
A high TSH tends to mean hypothyroidism: The thyroid gland is being asked to make more T4 because there isn’t enough T4 in the blood. A separate test can also be run for free T4.
A diagnosis of hyperthyroidism will be confirmed by tests for T4, T3, and TSH. A high level of thyroid hormone in the blood plus a low level of TSH is definitive.
In the past few years, a number of patients—and physicians—have advanced the notion that hypothyroidism is vastly underdiagnosed. “Hypothyroid Mom” Dana Trentini lists the top five reasons doctors fail to diagnose hypothyroidism.
1. Over-reliance on the TSH assay. A “normal” TSH may not tell the full story, and is undeserving of its “gold standard” reputation.
2. The “normal” TSH ranges are outdated and are far too wide.
3. Failure to do full thyroid blood testing. As discussed, thyroid biochemistry is complex, and should be worthy of a more comprehensive panel: TSH, Free T4, Free T3, Reverse T3, Thyroid Peroxidase Antibodies, Thyroglobulin Antibodies.
4. Strict reliance on “normal” lab reference ranges. Your mileage may vary.
5. Failure to recognize warning signs and symptoms. Trentini catalogs 300-plus hypothyroidism symptoms.
I suspect that there are many individuals with superficially normal lab results, who are actually hypothyroid. Fortunately, the technology exists to do something about it.