The Thyroid Explained: A Primer for Practitioners

Few organs attract as much attention from patients with unexplained symptoms as the thyroid. Fatigue, weight gain, low mood, poor concentration, feeling cold, hair thinning and constipation are all classic features of an underactive thyroid, and they are also among the most common complaints in any practice. Most people with these symptoms do not have thyroid disease, and some who do are missed. A clear understanding of thyroid physiology and testing is essential for navigating between those 2 errors.

How the thyroid system works

The thyroid is regulated by a classical feedback loop. The hypothalamus releases thyrotropin releasing hormone, which stimulates the pituitary to secrete thyroid stimulating hormone, TSH. TSH stimulates the thyroid to produce thyroid hormones. As thyroid hormone levels rise, they suppress TRH and TSH, keeping levels stable.

The thyroid produces mainly thyroxine, T4, with a smaller amount of triiodothyronine, T3. T4 is largely a prohormone. It is converted to the more active T3 in tissues by enzymes called deiodinases, which require selenium. Deiodinases can also convert T4 to reverse T3, an inactive form, and the balance between these pathways shifts with illness, fasting and other stresses. This local conversion allows tissues to fine tune their exposure to active hormone.

In the blood, almost all thyroid hormone is bound to carrier proteins, and only a tiny free fraction is biologically active. This is why free T4 and free T3 are measured, and why conditions that change binding proteins, such as pregnancy and oral oestrogen, affect total hormone levels without necessarily changing thyroid status.

Iodine is the essential raw material. Each molecule of T4 contains 4 iodine atoms and each molecule of T3 contains 3. Both too little and too much iodine can disturb thyroid function.

What thyroid hormones do

Thyroid hormones act on receptors in the nucleus of almost every cell, regulating gene expression. Their effects include setting basal metabolic rate and heat production, influencing heart rate and cardiac output, supporting brain development and function, regulating gut motility, influencing lipid metabolism, and supporting growth, bone turnover and reproductive function. This breadth explains why thyroid dysfunction produces such varied symptoms, and why those symptoms overlap with so many other conditions.

The tests and what they show

Common patterns

TSH rises gradually with age in healthy people, and mild elevations are common in older adults. Acute illness temporarily alters thyroid tests, so results taken during significant illness are best repeated after recovery.

The main conditions

Autoimmune thyroiditis

Hashimoto thyroiditis, in which the immune system gradually damages the thyroid, is the most common cause of hypothyroidism in regions with adequate iodine intake. It is much more common in women and often runs in families alongside other autoimmune conditions. Many people with positive antibodies have normal thyroid function for years; the risk of progression to hypothyroidism rises with antibody levels and TSH.

Iodine deficiency

Globally, iodine deficiency remains an important cause of goitre and hypothyroidism, and during pregnancy it can impair fetal brain development. Salt iodisation has greatly reduced it, but it persists in some regions and groups.

Hyperthyroidism

Graves disease, toxic nodules and thyroiditis are the main causes. Symptoms include weight loss, palpitations, heat intolerance, tremor and anxiety. Hyperthyroidism requires medical diagnosis and treatment; untreated, it carries risks including atrial fibrillation and bone loss.

Thyroid disease in pregnancy

Pregnancy increases thyroid hormone requirements, and both untreated hypothyroidism and hyperthyroidism carry risks. Women treated for hypothyroidism usually need dose adjustments early in pregnancy, which is managed by their doctor.

Nutrition and the thyroid

Iodine intake should be adequate but not excessive. People taking seaweed or kelp supplements can ingest very large amounts of iodine, which may cause thyroid dysfunction in susceptible individuals.

Selenium is needed for thyroid hormone metabolism and antioxidant protection in the gland. Some trials in autoimmune thyroiditis have shown reductions in antibody levels with selenium supplementation, but effects on clinical outcomes are uncertain, and excess selenium carries its own risks.

Iron deficiency can impair thyroid hormone synthesis and is common in women with hypothyroidism.

Gluten matters for people with coeliac disease, which is more common in people with autoimmune thyroid disease. For people without coeliac disease, evidence that gluten free diets improve thyroid outcomes is limited.

Treatment and its practical details

Hypothyroidism is treated with levothyroxine, prescribed and adjusted by a doctor according to TSH and symptoms. Several practical details affect its effectiveness and are areas where any practitioner can help.

• Absorption is reduced by food, coffee, calcium, iron, some antacids and proton pump inhibitors. It is usually taken on an empty stomach, separated from these by the interval the prescriber recommends.
• Consistency matters more than exact timing; changes in routine, brand or formulation can alter levels.
• Biotin supplements can distort thyroid test results and should be stopped for a period before testing, as advised by the laboratory or doctor.
• Some patients remain symptomatic despite normal TSH on treatment. Combination therapy with T3 has been studied; current specialist guidance generally does not recommend it routinely, though it may be considered in selected patients by specialists. Persistent symptoms also warrant looking for other causes.

When symptoms suggest the thyroid but tests are normal

This is one of the most common situations in practice. When TSH and free T4 are normal and there is no pituitary concern, the thyroid is unlikely to explain the symptoms, and the timeline and matrix become the tools for finding what does. Iron deficiency, B12 deficiency, poor sleep, sleep apnoea, depression, perimenopause, insulin resistance and chronic stress are all frequent explanations. Treating people with normal thyroid function with thyroid hormone has not been shown to help and can cause harm.

An illustrative case

A 41 year old woman treated for Hashimoto thyroiditis for 3 years still feels tired. Her TSH varies from test to test. The history reveals that she takes her levothyroxine with her morning coffee and a calcium supplement, and has recently started a hair supplement containing biotin. Her ferritin is low. With her doctor’s guidance, she takes levothyroxine on waking, well before coffee and calcium, pauses the biotin before her next test, and has her iron deficiency treated. Her TSH stabilises and her energy improves. No change of medication was needed; the practical details made the difference.

The practitioner’s role

Thyroid disease requires medical diagnosis and treatment. Practitioners across professions still have much to contribute: recognising symptoms that warrant testing, helping patients take their medicine effectively, identifying supplements and foods that interfere with treatment or testing, addressing nutrient status, and finding the other causes of symptoms that are often mistakenly attributed to the thyroid.