8.6: Thyroid Gland
- Page ID
- 140596
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The thyroid gland produces hormones (T₃, T₄, and calcitonin) that regulate metabolism, growth, development, body temperature, and calcium balance through a tightly controlled feedback system involving the hypothalamus and pituitary gland.
- Describe the location and anatomy of the thyroid gland.
- Identify the hormones produced by the follicular cells and the parafollicular cells of the thyroid.
- Describe the synthesis, secretion, and regulation of the thyroid hormones (T₃ and T₄), emphasizing the hypothalamic–pituitary–thyroid (HPT) feedback loop.
- Explain how thyroid hormones influence metabolism, body temperature, and fetal development.
Thyroid Gland Anatomy
The thyroid gland, a butterfly-shaped organ, is located anterior to the trachea, just inferior to the larynx (Figure \(\PageIndex{1}\)). The medial region, called the isthmus, is flanked by wing-shaped left and right lobes. Each of the thyroid lobes has a pair of parathyroid glands embedded on its posterior surface. The tissue of the thyroid gland is composed mostly of thyroid follicles lined with simple cuboidal epithelium (Figure \(\PageIndex{2}\)). The follicles are made up of a central cavity filled with a sticky fluid called colloid. Surrounded by a wall of epithelial follicle cells, the colloid is the center of thyroid hormone production, and that production is dependent on the essential and unique component of thyroid hormones: iodine.
Thyroid Hormones (T₃ and T₄)
The thyroid gland makes two main hormones — thyroxine (T₄) and triiodothyronine (T₃) — which help control how fast your body uses energy (metabolism). Their production is controlled by thyroid-stimulating hormone (TSH) from the anterior pituitary gland.
Inside the thyroid, follicular cells make a large protein called thyroglobulin, which serves as the “starter material” for these hormones. Iodide (the form of iodine in your blood) is pulled into these cells from the bloodstream through iodide trapping, a process that concentrates iodine about 30-fold inside the gland — iodine is essential for thyroid hormone production.
An enzyme called thyroperoxidase attaches iodine to tyrosine amino acids within thyroglobulin, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT).
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Two DIT molecules combine to make T₄ (thyroxine).
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One MIT and one DIT combine to make T₃ (triiodothyronine).
When the body needs thyroid hormones, the follicular cells break down the stored thyroglobulin, releasing T₃ and T₄ into the bloodstream.
Although most hormone released is T₄, tissues convert much of it into the more active T₃. Think of T₄ as a storage form and T₃ as the active form that actually drives metabolism, growth, and energy use in cells.
As with the steroid hormones, thyroid hormones are lipophillic and can cross the cell membrane and bind to intracellular receptors, which act alone as transcription factors or in association with other factors to modulate DNA transcription.
Control of Thyroid Hormone Release
The production of thyroxine (T₄) and triiodothyronine (T₃) is regulated by a feedback system that begins in the hypothalamus. The hypothalamus releases thyrotropin-releasing hormone (TRH), which stimulates the anterior pituitary gland to secrete thyroid-stimulating hormone (TSH). TSH then acts on the thyroid gland, prompting it to produce and release T₃ and T₄. These hormones increase metabolism, support growth and development, and influence many body systems. When T₃ and T₄ levels rise, they inhibit TRH and TSH release through negative feedback, maintaining hormonal balance.
Thyroid hormones also provide negative feedback to the hypothalamus and anterior pituitary gland. When thyroid levels in the blood are elevated TSH and TRH production is reduced. Excessive TRH can also inhibit the production of further TRH.
The hypothalamus releases TRH (thyrotropin-releasing hormone).
TRH stimulates the anterior pituitary to release TSH (thyroid-stimulating hormone).
TSH travels through the bloodstream to the thyroid gland, stimulating the release of T₃ and T₄.
As you learned in your study of homeostasis, the thyroid system also operates through a negative feedback loop: when levels of T₃ and T₄ rise, they signal the hypothalamus and anterior pituitary to reduce the release of TRH and TSH, helping maintain hormonal balance.
Effects of Thyroid Hormones on the Body
1) Effect on Metabolism
The thyroid hormones T₃ (triiodothyronine) and T₄ (thyroxine) control the body’s overall metabolic rate — how quickly cells use energy from food. They increase the metabolism of carbohydrates, fats, and proteins, helping regulate growth, development, and energy balance. These hormones also enhance the effects of catecholamines (like epinephrine).
2) Effect on Body Temperature
Thyroid hormones also help regulate body temperature by influencing blood vessel dilation and heat production.
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In hyperthyroidism (overactive thyroid), metabolism speeds up — patients often feel hot and may have an elevated body temperature.
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In hypothyroidism (underactive thyroid), metabolism slows down — patients may feel cold and have a lower body temperature.
3) Role in Fetal Development
Thyroid hormones are essential for brain development in the fetus. They guide the growth and organization of nerve cells, supporting normal brain maturation before and after birth. Insufficient thyroid hormone during pregnancy can cause serious developmental problems.
Not Just Thyroxine — Calcitonin: The Thyroid's Other Hormone
The thyroid gland also secretes a hormone called calcitonin that is produced by the parafollicular cells (also called clear cells or C cells) that stud the tissue between distinct follicles (Figure \(\PageIndex{2}\) and Figure \(\PageIndex{3}\)). Calcitonin is released in response to a rise in blood calcium levels.
The hormones secreted by the thyroid gland are summarized in the table below:
| Hormone | Chemical Type | Produced By | Main Target(s) | Primary Function(s) |
|---|---|---|---|---|
| Thyroxine (T₄) | Amine (iodinated tyrosine) | Thyroid follicular cells | Most body cells | Increases metabolic rate; supports growth, development, and energy production. Serves as a storage form that is converted to T₃ in tissues. |
| Triiodothyronine (T₃) | Amine (iodinated tyrosine) | Thyroid follicular cells | Most body cells | Active form of thyroid hormone; regulates metabolism, oxygen use, and heat production; influences heart rate and protein synthesis. |
| Calcitonin | Peptide | Parafollicular (C) cells | Bone (osteoclasts) and kidneys | Lowers blood calcium levels by inhibiting bone resorption and increasing calcium excretion by the kidneys. |